Risk Factors and Outcome Analysis of Acinetobacter baumannii Complex Bacteremia in Critical Patients* Hao-Yuan Lee, MD1,2,3; Chyi-Liang Chen, PhD2,3; Si-Ru Wu, BS3; Chih-Wei Huang, BS3; Cheng-Hsun Chiu, MD, PhD1,2,3
Objectives: Acinetobacter baumannii complex bacteremia has been identified increasingly in critical patients admitted in ICUs. Notably, A. baumannii complex bacteremia has a high mortality rate, yet the risk factors associated with mortality remain unclear and controversial. Design: Retrospective study. Setting: All adult ICUs at a tertiary care medical center. Patients: All patients with A. baumannii complex bacteremia admitted in 2009–2010. Interventions: None. Measurements and Main Results: Risk factors for mortality were analyzed. Bacterial isolates were identified by 16S-23S ribosomal RNA intergenic spacer region sequencing for genospecies and genotyped by pulsed-field gel electrophoresis. Carbapenemase *See also p. 1289. 1 Graduate Institute of Clinical Medical Sciences, Chang Gung University College of Medicine, Taoyuan, Taiwan. 2 Department of Pediatrics, Chang Gung Children’s Hospital, Chang Gung University College of Medicine, Taoyuan, Taiwan. 3 Molecular Infectious Disease Research Center, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Taoyuan, Taiwan. Dr. Lee designed the study, collected and analyzed the data, and wrote the article. Dr. Chen analyzed the data. Ms. Wu and Mr. Huang performed the experiments. Dr. Chiu designed the study, collected and analyzed the data, and wrote the article. All authors read and approved the final article. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (http://journals.lww.com/ ccmjournal). Supported, in part, by grants 100-2314-B-182A-049 and 102-2314-B-182A-023 from National Science Council, Executive Yuan, Taiwan, and by grants CMRPG490052 and CMRPG4C0031 from Chang Gung Memorial Hospital, Taiwan. Dr. Lee, Dr. Chen, Ms. Wu, Mr. Huang, and Dr. Chiu’s institutions received grant support from National Science Council and Chang Gung Memorial Hospital (Taipei, Taiwan). Dr. Lee, Dr. Chen, Ms. Wu, Mr. Huang, and Dr. Chiu received support for article research from National Science Council and Chang Gung Memorial Hospital. For information regarding this article, E-mail: [email protected] Copyright © 2014 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/CCM.0000000000000125
Critical Care Medicine
genes were detected by polymerase chain reaction and sequencing. A total of 298 patients met the inclusion criteria, including 73 (24.5%) infected by imipenem-resistant A. baumannii complex. The overall 30-day mortality was 33.6% (100 of 298). Imipenemresistant A. baumannii complex bacteremia specifically showed a high mortality (69.9%) and was associated with prior use of broad-spectrum antibiotics for more than 5 days for treating ventilator-associated pneumonia before the occurrence of bacteremia. Mortality was associated with inappropriate initial antimicrobial therapy, which was correlated with imipenem-resistant A. baumannii complex but not with any specific genospecies. ISAba1– blaOXA-23–ISAba1 (Tn2006) was found in most (66.7%, 40 of 68) imipenem-resistant A. baumannii (genospecies 2) and also spread beyond species border to all imipenem-resistant genospecies 3 (2), 13TU (2), and 10 (1). Conclusions: For critical patients with A. baumannii complex infection, ventilator-associated pneumonia in particular, the selective pressure from prior use of broad-spectrum antibiotics for 5 days or more increased risk of subsequent imipenem-resistant A. baumannii complex bacteremia. To reduce mortality, rapid identification of imipenem-resistant A. baumannii complex and early initiation of appropriate antimicrobial therapy in these high-risk patients are crucial. (Crit Care Med 2014; 42:1081–1088) Key Words: Acinetobacter baumannii complex; bacteremia; imipenem resistance; outcome; risk factor
A
cinetobacter baumannii bloodstream infection has a high mortality rate of 34.0–43.4% for critical patients treated in ICUs (1). How this infection is acquired and which factors are associated with mortality are not fully understood. Although not without some controversy, risk factors correlated with mortality are reportedly genospecies, inappropriate therapy, carbapenem resistance, disease severity, and the resistance mechanism (2–14). At least three species, A. baumannii (Acinetobacter genospecies 2), Acinetobacter nosocomialis (Acinetobacter genospecies 13TU), and Acinetobacter pittii (Acinetobacter genospecies 3) are invariably being reported as A. baumannii by clinical www.ccmjournal.org
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microbiology laboratories because biochemical methods cannot further differentiate them to the species level. These species form the A. baumannii complex (ABC) (2–4). Clinically, A. baumannii was associated with much higher attributable mortality rate (58.6%), when compared with the other two genospecies (16.7% and 33.3%, respectively) (4). Therefore, molecular methods were needed to accurately identify A. baumannii from other species to predict outcome and arrange further critical care (3, 4). On the other hand, A. baumannii has been found to have a higher resistance to carbapenem, the choice agent for treating this infection (2). To date, no study simultaneously evaluated the clinical features of carbapenem-susceptible ABC infection to avoid the most important confounder, carbapenem resistance, mainly because only a limited number of isolates of non-A. baumannii genospecies were collected in these studies (4). It is possible to carry out this study in Chang Gang Memorial Hospital (CGMH), a 3,500-bed tertiary hospital in Taiwan, because A. baumannii has become the most common pathogen for nosocomial bloodstream infection in ICUs in Chang Gang Memorial Hospital since 2009. This study applied backward root analysis to survey, step by step, independent risk factors associated with mortality due to ABC bacteremia by comparing clinical characteristics, microbiological features, and final outcomes of bacteremic patients stratified by both genospecies and their imipenem susceptibility.
MATERIALS AND METHODS Study Subjects and Inclusion Criteria This study was conducted at CGMH from 2009 to 2010 and was approved by the institutional review board (100-3592B). Charts were reviewed for all ICUs patients with more than or equal to one positive blood culture for A. baumannii and symptoms and
signs of infection. For patients with multiple episodes of bacteremia, only the first episode was included. Patients with incomplete medical records, those who are younger than 18 years, or those with polymicrobial infection were excluded. Bacterial Isolates and Antimicrobial Susceptibility Conventional biochemical tests and 16S-23S ribosomal RNA intergenic spacer region sequencing as described previously were used to characterize bacterial genospecies (15). Antimicrobial susceptibility was determined by the disk diffusion method according to Clinical and Laboratory Standards Institute standards (16). Minimum inhibitory concentration (MIC) of imipenem was further determined by Etest (AB BIODISK, Solna, Sweden). The breakpoint for defining imipenem-susceptible ABC (ISABC) was an MIC less than or equal to 4 mg/L and a breakpoint of more than or equal to 8 mg/L for imipenem-resistant ABC (IRABC), including both intermediately resistant and resistant strains (16). Data Collection and Definitions Clinical data were collected retrospectively. Mortality was defined as bacteremia-attributable death, that is, death before resolution of symptoms and signs of bacteremia and at least one blood culture positive for the ABC (4). Considering “time at risk” for acquiring resistance under antimicrobial selective pressure, prior exposure to antimicrobial agents was defined as at least 5 days of therapy during the 14 days before the isolation of ABC (17, 18). Appropriate antimicrobial therapy was defined as administering patients with at least one antimicrobial agent, except aminoglycoside, susceptible in vitro, within 2 days after bacteremia onset (3). Multidrug resistance (MDR) was defined as resistance to at least three antibiotic classes
Table 1. Comparison of Bacteremia-Attributable Mortality and Its Associated Risk Factors Among Different Genospecies of Acinetobacter baumannii Complex IRABC Genospecies 2 (A. baumannii) (n = 68)
13TU (n = 2)
3 (n = 2)
Mortality
48 (70.0)
1 (50.0)
1 (50.0)
1 (100)
Appropriate therapy (MRF1)
19 (27.9)
0 (0)
1 (50)
0 (0)
Imipenem resistance (MRF2)
68 (100.0)
2 (100)
2 (100)
1 (100)
Multidrug resistance
68 (100.0)
2 (100)
2 (100)
0 (0)
Prior exposure to broad-spectrum antibiotics ≥ 5 d (MRF3)
65 (95.6)
2 (100)
2 (100)
1 (100)
Ventilator-associated pneumonia as infection source (MRF4)
45 (66.2)
2 (100)
2 (100)
1 (100)
3.5 (3–4)
3 (2–4)
4
2 (1–3)
3 (2–4)
1
20 (7–33)
21 (18–23)
27
3 (1–4)
3 (2–3)
Mortality and Its Associated Risk Factors
Culture detecting time (d)
3 (3)
Charlson score
3 (2–5)
Acute Physiology and Chronic Health Evaluation II score Pitt bacteremia score
26 (20–30) 4 (1–4)
10 (n = 1)
3 (2–4)
IRABC = Imipenem-resistant A. baumannii complex, ISABC= Imipenem-susceptible A. baumannii complex, MRF = main risk factor. Data are presented as median value (interquartile range, Q1–Q3) for continuous variables and number of cases (%) for categoric variables.
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(3). Culture detecting time was the interval (days) from culture sampling to reporting. Ventilator-associated pneumonia (VAP) was defined as pneumonia with pulmonary infiltrates on chest radiographs, purulent tracheal secretions not less than 48 hours after intubation, and the start of mechanical ventilation (19, 20). Multistage Risk Factor Analysis A multistage model was proposed for critical patients that covered the span from the appearance of ABC infection to mortality. The most important risk factor (MRF1) associated with mortality due to ABC was the first one to be analyzed by multivariate logistic analysis. The second main risk factor (MRF2) correlated with MRF1 was then analyzed by the same method. In this manner, all main risk factors would be identified as the MRF1 in each stage moving backward from death to infection onset. Polymerase Chain Reaction and Sequencing Primers specific for resistance genes (blaOXA-23, blaOXA-24, blaOXA-40, blaOXA-54, blaOXA-58, blaOXA-51, blaADC, blaIMP-1, blaIMP-2, blaVIM-1, blaVIM-2, and blaNDM-1), insertion sequences (ISAba1, ISAba2, ISAba3, ISAba4, and IS1008), and the AbaR4-type islands were designed, and polymerase chain reaction amplification and sequence determination were performed as described previously (21–25). Pulsed-Field Gel Electrophoresis Isolates were analyzed by pulsed-field gel electrophoresis (PFGE) using methods as described elsewhere (26). Fragment patterns obtained were interpreted as described by Tenover et al (27).
Statistical Analysis Data were recorded and entered into a database. Analyses were performed using SPSS software, v. 17.0 (SPSS, Chicago, IL). The Student t test, the chi-square test, or Fisher exact test was used when appropriate to compare proportions. Variables with a p value of less than 0.2 in the univariate analysis were added in a forward stepwise manner and selected to create the final model for multivariable analysis. All statistical analyses were two-sided, and significance was set at a p value of less than 0.05.
RESULTS Clinical Presentations Among Different Genospecies of ISABC After screening with the inclusion criteria, 298 patients were enrolled and their infected isolates were phenotypically identified to five genospecies (Table 1). Clinical presentations of patients with ISABC bacteremia were compared (Supplemental Table 1, Supplemental Digital Content 1, http://links.lww. com/CCM/A819). Patients with Acinetobacter baylyi, genospecies 13TU, or genospecies 3 bacteremia had significantly higher Charlson scores (p < 0.001) than scores of those with A. baumannii bacteremia. Clinical Presentations of Different Genospecies of IRABC No significant difference in Charlson scores, Acute Physiology and Chronic Health Evaluation (APACHE) II scores, or Pitt bacteremia scores existed for patients infected by different genospecies of IRABC (Table 1). No significant difference in Charlson scores existed between patients infected by
ISABC Genospecies 2 (A. baumannii) (n = 151)
13TU (n = 31)
3 (n = 37)
Acinetobacter baylyi (n = 6)
37 (24.5)
6 (19.4)
6 (16.2)
0 (0)
< 0.001
101 (66.9)
24 (77.4)
28 (75.7)
5 (83.3)
< 0.001
0 (0)
0 (0)
0 (0)
0 (0)
< 0.001
21 (13.9)
0 (0)
1 (2.7)
0 (0)
< 0.001
21 (13.2)
6 (19.4)
7 (18.9)
0 (0)
< 0.001
39 (25.8)
6 (19.4)
4 (10.8)
1 (16.7)
< 0.001
3 (2–3)
2 (2–3)
3 (3–5)
2.5 (2–3)
0.751
3 (1–5)
4 (3–7)
4 (3–5)
6 (4–10)
19 (16–24)
17 (13–23)
17 (14–19)
15 (12–17)
< 0.001
4 (1–4)
3 (2–4)
3 (1–4)
3 (2–3)
< 0.001
Critical Care Medicine
p (IRABC vs I SABC)
0.386
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i mipenem-resistant (IRAB) and imipenem-susceptible A. baumannii (ISAB), except that patients infected by IRAB had more renal disease (p < 0.001). Patients with IRAB infection had the highest APACHE II score, Pitt bacteremia score, and C-reactive protein level, suggesting a more clinically severe condition. VAP was the most common source of bacteremia for those with IRAB or IRABC infection (Table 1). Clinical Outcomes Overall 30-day mortality was 33.6% (100 of 298). Patients infected by IRAB had a higher 30-day mortality rate (70.0%) than those infected by ISAB (24.5%) (p < 0.001) (Table 1).
Nearly 75% of patients with ISAB bacteremia survived through day 30, compared with only 30% with IRAB infection (p < 0.001, by log-rank test) (Fig. 1A). No significant difference existed in 30-day mortality among patients infected by ISAB (24.5%), imipenem-susceptible genospecies 3 (16.2%), and imipenem-susceptible genospecies 13TU (19.4%) (p > 0.05) or by Kaplan-Meier estimate of survival curves (p > 0.05 by log-rank test) (Fig. 1, B and C). Similarly, mortality rates among patients infected by IRAB and the other three imipenem-resistant genospecies did not differ significantly. Notably, A. baumannii had a higher imipenem resistance than the other genospecies (p < 0.001).
Figure 1. The Kaplan-Meier estimate of survival curves of patients with Acinetobacter baumannii complex bacteremia. A, The Kaplan-Meier survival curve of patients infected by imipenem-susceptible A. baumannii (genospecies 2, solid line) compared with those by imipenem-resistant A. baumannii (dotted line) (p < 0.001, by log-rank test). B, The Kaplan-Meier survival curve of patients infected by imipenem-susceptible genospecies 13TU (dotted line) compared with those by imipenem-susceptible A. baumannii (solid line) (p = 0.927, by log-rank test). C, The Kaplan-Meier survival curve of patients infected by imipenem-susceptible genospecies 3 (dotted line) compared with those by imipenem-susceptible A. baumannii (solid line) (p = 0.495, by log-rank test). D, The Kaplan-Meier survival curve of patients who received appropriate initial antimicrobial therapy (solid line) compared with those without appropriate initial antimicrobial therapy (dotted line) (p < 0.001, by log-rank test).
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Antimicrobial Susceptibility of IRABC and ISABC Except for colistin, IRAB were more resistant to all antibiotics than ISAB (Table 2). In fact, except for colistin and tigecycline, all IRAB isolates were multidrug resistant and more than 88% of IRAB were resistant to all antibiotics tested. On the contrary, all imipenem-susceptible isolates showed more than 80% susceptibility to all other antibiotics tested. Risk Factor Analysis for Mortality Risk factors for 30-day mortality were analyzed by comparing characteristics of nonsurvivors and survivors (Supplemental Table 2, Supplemental Digital Content 2, http://links.lww. com/CCM/A820). Inappropriate initial therapy, MRF1, had the highest odds ratio among independent risk factors of mortality. Nearly 80% of patients with appropriate antimicrobial therapy survived through day 30, compared with only 40% without (p < 0.001, by log-rank test) (Fig. 1D). For IRABC infection, patients administered appropriate therapy (early therapy in 2 d) by colistin alone, tigecycline alone, or combination of colistin/tigecycline had a lower mortality (37.5%, 20%, 20%) than those given inappropriate therapy (late therapy) with the same antibiotics (88.2%, 88.9%, 100%) (p = 0.017, 0.023, 0.048) (Table 3). Multivariate analysis of risk factors for inappropriate therapy was further performed. Infection by imipenem-resistant pathogen (adjusted odds ratio [AOR], 4.72; 95% CI, 2.48–8.99; p < 0.001), higher Pitt bacteremia score, and catheter-related
infection or urinary tract infection as the primary source were independent risk factors for inappropriate therapy. Infection by imipenem-resistant organisms had the highest odds ratio (MRF2). Risk factors for acquiring IRABC were also identified by comparing characteristics of IRABC and ISABC infections. After multivariate analysis, prior use of carbapenems, extended-spectrum cephalosporins, fluoroquinolones, or piperacillin/tazobactam for at least 5 days (MRF3) was the main independent risk factor for acquiring IRABC infection (Supplemental Table 3, Supplemental Digital Content 3, http://links.lww.com/CCM/A821). Main indication for prescribing these antibiotics was VAP (50 of 70, 71.4%). As MRF4, VAP as infection source was correlated with broad-spectrum antibiotics use for at least 5 days prior to infection (AOR, 3.874; 95% CI, 2.342–6.408; p < 0.001). At least one culture positive for IRABC with the same resistance pattern was found prior to (n = 62, 1–120 d before) or at the same time as (n = 9) IRABC bacteremia. Risk Analysis From Acquiring ABC to Mortality in Critical Patients According to the above multistage risk factor analysis by backward root method, mortality due to IRABC stemmed from inappropriate initial therapy. Table 4 shows the five stages from development of IRABC bacteremia to mortality in backward root analysis.
Comparison in Antibiotic Resistance Among Different Genospecies of Imipenem-Resistant Acinetobacter baumannii Complex and Imipenem-Susceptible A. baumannii Complex Table 2.
IRABC
Antibiotics
Genospecies 2 (A. baumannii) (n = 68)
13TU (n = 2)
ISABC 3 (n = 2)
10 (n = 1)
Genospecies 2 (A. baumannii) 13TU 3 (n = 151) (n = 31) (n = 37)
Acinetobacter baylyi (n = 6)
p (IRABC vs I SABC)
Multidrug resistance
68 (100)
2 (100) 2 (100)
0 (0)
21 (13.9)
0 (0)
1 (2.7)
0 (0)
< 0.001
Amikacin
67 (98.5)
2 (100) 2 (100)
0 (0)
13 (8.6)
0 (0)
0 (0)
0 (0)
< 0.001
Ceftazidime
68 (100)
2 (100) 2 (100)
0 (0)
21 (13.9)
1 (2.7)
1 (3.2)
0 (0)
< 0.001
Ciprofloxacin
68 (100)
0 (0)
0 (0)
10 (6.6)
2 (5.4)
0 (0)
1 (16.7)
< 0.001
Cefepime
67 (98.5)
2 (100) 2 (100)
0 (0)
17 (11.3)
1 (2.7)
0 (0)
0 (0)
< 0.001
Gentamicin
68 (100)
0 (0)
1 (100)
19 (12.6)
6 (16.2) 0 (0)
0 (0)
< 0.001
Piperacillintazobactam
68 (100)
2 (100) 1 (50)
0 (0)
22 (14.6)
2 (5.4)
0 (0)
0 (0)
< 0.001
Imipenem
68 (100)
2 (100) 2 (100)
1 (100)
0 (0)
0 (0)
0 (0)
0 (0)
< 0.001
Meropenem
68 (100)
2 (100) 2 (100)
1 (100)
0 (0)
0 (0)
0 (0)
0 (0)
< 0.001
Unasyn
60 (88.2)
0 (0)
0 (0)
0 (0)
7 (4.6)
1 (2.7)
0 (0)
0 (0)
< 0.001
Tigecycline
16 (23.5)
0 (0)
0 (0)
0 (0)
1 (0.7)
0 (0)
0 (0)
0 (0)
< 0.001
0 (0)
0 (0)
0 (0)
2 (1.3)
0 (0)
0 (0)
0 (0)
1.000
Colistin
0 (0)
0 (0) 2 (100)
IRABC = Imipenem-resistant A. baumannii complex, ISABC= Imipenem-susceptible A. baumannii complex. Data are presented as number of cases (%).
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Table 3. Comparison of Mortality of Patients With Imipenem-Resistant Acinetobacter baumannii Complex Infection Administered Appropriate Therapy With Those Administered Inappropriate Therapy Effectivea or Ineffectiveb Antimicrobial Therapy for Imipenem-Resistant A. baumannii Complex
Mortality of Appropriate herapy (Effective Therapy T Given in 2 d) (n/Total n) (%)
Mortality of Inappropriate Therapy (Late or Ineffective Therapy) (n/Total n) (%)
p
Colistin
3/8 (37.5)
15/17 (88.2)
0.017
Tigecycline
1/5 (20.0)
8/9 (88.9)
0.023
Colistin + Tigecycline
1/5 (20.0)
5/5 (100.0)
0.048
Sulbactam
0/1 (0)
3/4 (75.0)
0.400
Ciprofloxacin
0/1 (0)
1/2 (50.0)
1.000
b
Imipenem
—
11/12 (91.7)
b
Cefepime
—
3/4 (75.0)
Dashes signify that no patient in that group received such antibiotics. a Effective antimicrobial therapy was defined as giving patients with at least one antimicrobial agent, except aminoglycoside, susceptible in vitro, within 2 d after the onset of bacteremia. Late or ineffective therapy was considered as inappropriate therapy. b Imipenem and cefepime were ineffective antibiotics for these patients with imipenem-resistant A. baumannii complex infections.
Imipenem-Resistance Genes All A. baumannii isolates carried the blaOXA-51-like. The upstream ISAba1 was found in 12 isolates with blaOXA-51-like and in 47 IRAB isolates with blaOXA-23, including 40 with Tn2006 (ISAba1– blaOXA-23–ISAba1) and seven with Tn2008 (ISAba1–blaOXA-23) (Supplemental Table 4, Supplemental Digital Content 4, http://links.lww.com/CCM/A822). Tn2006 in an AbaR4-type resistance island was found in most IRAB isolates (40 of 68, 58.8%) and all imipenem-resistant genospecies 3 (2), genospecies 13TU (2), and genospecies 10 (1). PFGE Patterns of IRAB Based on the criteria by Tenover et al (27), 68 IRAB isolates showed 26 different PFGE patterns (Supplemental Table 2, Supplemental Digital Content 2, http://links.lww.com/CCM/ A820). In total, 35 IRAB isolates (51.47%) belonged to PFGE type 1, the major IRAB clone in Taiwan. Notably, ISAba1– blaOXA-23 (Tn2006 or Tn2008) was found in 69.2% of the genotypes (18 of 26) and was more widely distributed than ISAba1–blaOXA-51 (34.6%, 9 of 26) (p = 0.025) (Supplemental Table 5, Supplemental Digital Content 5, http://links.lww.com/
CCM/A823). In contrast to the clonal nature of IRAB, all ISAB isolates were diverse and each had a different PFGE genotype. Most isolates belonging to PFGE type 1 carried Tn2006 (25 of 35, 71.4%). These isolates were derived from patients with VAP (30 of 35, 85.7%) and for whom the mortality rate was high (25 of 35, 71.4%).
DISCUSSION According to this comprehensive analysis of any ABC infection in critical patients, the selective pressure from prior use of broad-spectrum antibiotics for 5 days or more increased risk of subsequent IRABC bacteremia. Of IRABC isolates, only colistin and tigecycline were effective. Shortening the time for confirmation of IRABC and initiating appropriate antibiotics to treat the infection is crucial for critical patients in this scenario. In this study, mortality due to ABC bacteremia correlated with imipenem resistance but not with specific genospecies. Many differences between IRABC and ISABC were observed. First, IRABC had much higher MDR (98.6%) than ISABC (9.8%). Second, more patients with IRABC infection
Backward Root Analysis for Major Risk Factor at Different Stages From the Development of Imipenem-Resistant Acinetobacter baumannii Complex Bacteremia to Mortality and Proposed Solution for Each Stage Table 4.
Stages
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
MRF or outcome
Ventilator-associated pneumonia (MRF4)
Prior exposure to broad-spectrum antibiotics ≥ 5 d (MRF3)
IRABC (MRF2)
Inappropriate therapy (MRF1)
Mortality due to A. baumannii complex bacteremia
Recommended intervention
Early detection of IRABC
Antibiotic stewardship
Detection of IRABC bacteremia
Use of effective antibiotics in 2d
MRF = main risk factor, IRABC = imipenem-resistant A. baumannii complex.
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received inappropriate initial therapy (72.6%) than patients with ISABC did (29.8%). Third and more importantly, mortality attributable to IRABC bacteremia (69.9%) was much higher than that for ISABC (21.8%) and for all nosocomial bloodstream infections (25.7–47.9%) reported in U.S. ICUs (1). Some earlier studies also showed that imipenem resistance was associated with mortality by bacteremia, VAP, or other infections caused by A. baumannii (7, 18, 28), whereas others did not (4, 14). Fourth, the main primary source of IRABC bacteremia was VAP (82.2%), which was not the case in ISABC bacteremia (22.2%). Giving appropriate initial antimicrobial therapy is critical for treatment of septic patients (28) and was associated with a low mortality rate for patients with A. baumannii bacteremia (8, 10, 12). Use of in vitro effective antibiotics was reported as an independent factor favoring rapid bacterial clearance (29). All of IRABCs except genospecies 10 were MDR. Mean time needed to identify IRABC by culture was 3 days, longer than the maximum time (2 d) defined in this study as initiation of appropriate therapy. Without guidance by microbiological data, only 27.4% of patients infected by IRABC were given effective antibiotics, either colistin or tigecycline, within 2 days, presumably resulting in a high 30-day morality rate of 69.9% for these patients. The most important risk factor for mortality for IRABC bacteremia was no appropriate therapy given within 2 days. This study found that most IRABC bacteremia had a preceding infection caused by ABC with the same resistance pattern, for which extended-spectrum antibiotics had been given. This is consistent with a previous assertion that prior exposure to imipenem or third-generation cephalosporins increased risk of acquiring IRAB infection (9, 18, 30). This study has three important clinical microbiological findings. First, our results were different from those in previous studies, which showed that infection with genospecies 2 (A. baumannii) was the most important independent predictor of mortality due to ABC (3). In this large-scale clinical study, no specific genospecies was an independent risk factor for mortality. Genospecies 2 (A. baumannii) generated a high mortality due to high rate of imipenem resistance resulting from high rate of inappropriate initial therapy. Second, two new members of ABC were identified, A. baylyi and genospecies 10; both were misidentified as ABC by routine biochemical testing. Clinical isolates of A. baylyi and genospecies 10 were reported before (31, 32), but their identification as ABC has never been described. Third, this study found that ISAba1–blaOXA-23–ISAba1 (Tn2006) located in an AbaR4-type resistance island had spread in most A. baumannii but had also spread beyond the species border to genospecies 3, 13TU, and 10. Although the AbaR4-type resistance island was first found in two isolates of genospecies 13TU from Korea and Thailand (33), the resistance island had never been identified in genospecies 3 or 10. Our study along with previous studies from Taiwan confirmed that the major resistance mechanism of IRABC in Taiwan has changed from ISAba1–blaOXA-51-like to ISAba1–blaOXA-23 since 2009 (34, 35). The wide dissemination Critical Care Medicine
of Tn2006 in an AbaR4-type resistance island was supported by the finding that most IRAB contained the specific genetic determinant irrespective of their PFGE patterns. This is to date the most comprehensive study of ABC bacteremia. Genospecies did not influence patients’ outcomes. Mortality due to ABC bacteremia was mainly associated with inappropriate initial therapy, which was due to imipenem resistance and also MDR of IRABC. Appearance of IRABC bacteremia was associated with prior use of broad-spectrum antibiotics prescribed to treat the preceding VAP. To reduce mortality from IRABC bacteremia, a rapid diagnostic method for detecting IRABC in blood or deep airway specimens should be developed for patients with VAP who have been given broad-spectrum antibiotics. These patients are at increased risk for IRABC bacteremia. To save lives, effective antibiotics should be given to these patients in 2 days.
ACKNOWLEDGMENTS We thank professor Chee-Jen Chang and Ms. Hsiao-Jung Tseng in Clinical Informatics and Medical Statistics Research Center, Chang Gung Memorial Hospital, for validating and confirming all the statistics in this work. We thank professor Tzu-Lan Wu for providing bacterial isolates for this study.
REFERENCES
1. Wisplinghoff H, Bischoff T, Tallent SM, et al: Nosocomial bloodstream infections in US hospitals: Analysis of 24,179 cases from a prospective nationwide surveillance study. Clin Infect Dis 2004; 39:309–317 2. Peleg AY, Seifert H, Paterson DL: Acinetobacter baumannii: Emergence of a successful pathogen. Clin Microbiol Rev 2008; 21:538–582 3. Lee YT, Kuo SC, Yang SP, et al: Impact of appropriate antimicrobial therapy on mortality associated with Acinetobacter baumannii bacteremia: Relation to severity of infection. Clin Infect Dis 2012; 55:209–215 4. Chuang YC, Sheng WH, Li SY, et al: Influence of genospecies of Acinetobacter baumannii complex on clinical outcomes of patients with Acinetobacter bacteremia. Clin Infect Dis 2011; 52:352–360 5. Kuo SC, Lee YT, Yang SP, et al: Evaluation of the effect of appropriate antimicrobial therapy on mortality associated with Acinetobacter nosocomialis bacteraemia. Clin Microbiol Infect 2013; 19:634–639 6. Chuang YC, Chang SC, Wang WK: High and increasing Oxa-51 DNA load predict mortality in Acinetobacter baumannii bacteremia: Implication for pathogenesis and evaluation of therapy. PLoS One 2010; 5:e14133 7. Kwon KT, Oh WS, Song JH, et al: Impact of imipenem resistance on mortality in patients with Acinetobacter bacteraemia. J Antimicrob Chemother 2007; 59:525–530 8. Erbay A, Idil A, Gözel MG, et al: Impact of early appropriate antimicrobial therapy on survival in Acinetobacter baumannii bloodstream infections. Int J Antimicrob Agents 2009; 34:575–579 9. Sheng WH, Liao CH, Lauderdale TL, et al: A multicenter study of risk factors and outcome of hospitalized patients with infections due to carbapenem-resistant Acinetobacter baumannii. Int J Infect Dis 2010; 14:e764–e769 10. Falagas ME, Kasiakou SK, Rafailidis PI, et al: Comparison of mortality of patients with Acinetobacter baumannii bacteraemia receiving appropriate and inappropriate empirical therapy. J Antimicrob Chemother 2006; 57:1251–1254 11. Lee YC, Huang YT, Tan CK, et al: Acinetobacter baumannii and Acinetobacter genospecies 13TU and 3 bacteraemia: Comparison www.ccmjournal.org
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Lee et al of clinical features, prognostic factors and outcomes. J Antimicrob Chemother 2011; 66:1839–1846 12. Falagas ME, Rafailidis PI: Attributable mortality of Acinetobacter baumannii: No longer a controversial issue. Crit Care 2007; 11:134 13. Chiang MC, Kuo SC, Chen SJ, et al: Clinical characteristics and outcomes of bacteremia due to different genomic species of Acinetobacter baumannii complex in patients with solid tumors. Infection 2012; 40:19–26 14. Routsi C, Pratikaki M, Platsouka E, et al: Carbapenem-resistant versus carbapenem-susceptible Acinetobacter baumannii bacteremia in a Greek intensive care unit: Risk factors, clinical features and outcomes. Infection 2010; 38:173–180 15. Chang HC, Wei YF, Dijkshoorn L, et al: Species-level identification of isolates of the Acinetobacter calcoaceticus-Acinetobacter baumannii complex by sequence analysis of the 16S-23S rRNA gene spacer region. J Clin Microbiol 2005; 43:1632–1639 16. Clinical and Laboratory Standards Institute: Performance Standards for Antimicrobial Susceptibility Testing: 20th Informational Supplement. Document M100-S20. Wayne, PA, CLSI, 2010 17. Ye JJ, Huang CT, Shie SS, et al: Multidrug resistant Acinetobacter baumannii: Risk factors for appearance of imipenem resistant strains on patients formerly with susceptible strains. PLoS One 2010; 5:e9947 18. Lee HY, Chang RC, Su LH, et al: Wide spread of Tn2006 in an AbaR4-type resistance island among carbapenem-resistant Acinetobacter baumannii clinical isolates in Taiwan. Int J Antimicrob Agents 2012; 40:163–167 19. Bonten MJ, Kollef MH, Hall JB: Risk factors for ventilator-associated pneumonia: From epidemiology to patient management. Clin Infect Dis 2004; 38:1141–1149 20. Bekaert M, Timsit JF, Vansteelandt S, et al; Outcomerea Study Group: Attributable mortality of ventilator-associated pneumonia: A reappraisal using causal analysis. Am J Respir Crit Care Med 2011; 184:1133–1139 21. Corvec S, Poirel L, Naas T, et al: Genetics and expression of the c arbapenem-hydrolyzing oxacillinase gene blaOXA-23 in Acinetobacter baumannii. Antimicrob Agents Chemother 2007; 51: 1530–1533 22. Chen TL, Wu RC, Shaio MF, et al: Acquisition of a plasmid-borne blaOXA-58 gene with an upstream IS1008 insertion conferring a high level of carbapenem resistance to Acinetobacter baumannii. Antimicrob Agents Chemother 2008; 52:2573–2580 23. Lee HY, Chen CL, Wang SB, et al: Imipenem heteroresistance induced by imipenem in multidrug-resistant Acinetobacter baumannii:
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Mechanism and clinical implications. Int J Antimicrob Agents 2011; 37:302–308 24. Turton JF, Baddal B, Perry C: Use of the accessory genome for characterization and typing of Acinetobacter baumannii. J Clin Microbiol 2011; 49:1260–1266 25. Hamidian M, Hall RM: AbaR4 replaces AbaR3 in a carbapenem-resistant Acinetobacter baumannii isolate belonging to global clone 1 from an Australian hospital. J Antimicrob Chemother 2011; 66:2484–2491 26. Peleg AY, Potoski BA, Rea R, et al: Acinetobacter baumannii bloodstream infection while receiving tigecycline: A cautionary report. J Antimicrob Chemother 2007; 59:128–131 27. Tenover FC, Arbeit RD, Goering RV, et al: Interpreting chromosomal DNA restriction patterns produced by pulsed-field gel electrophoresis: Criteria for bacterial strain typing. J Clin Microbiol 1995; 33:2233–2239 28. Kumar A, Roberts D, Wood KE, et al: Duration of hypotension before initiation of effective antimicrobial therapy is the critical determinant of survival in human septic shock. Crit Care Med 2006; 34:1589–1596 29. Chuang YC, Chang SC, Wang WK: Using the rate of bacterial clearance determined by real-time polymerase chain reaction as a timely surrogate marker to evaluate the appropriateness of antibiotic usage in critical patients with Acinetobacter baumannii bacteremia. Crit Care Med 2012; 40:2273–2280 30. Lee SO, Kim NJ, Choi SH, et al: Risk factors for acquisition of imipenem-resistant Acinetobacter baumannii: A case-control study. Antimicrob Agents Chemother 2004; 48:224–228 31. Zhou Z, Du X, Wang L, et al: Clinical carbapenem-resistant Acinetobacter baylyi strain coharboring blaSIM-1 and blaOXA-23 from China. Antimicrob Agents Chemother 2011; 55:5347–5349 32. Bonnin RA, Ocampo-Sosa AA, Poirel L, et al: Biochemical and genetic characterization of carbapenem-hydrolyzing β-lactamase OXA-229 from Acinetobacter bereziniae. Antimicrob Agents Chemother 2012; 56:3923–3927 33. Kim DH, Choi JY, Jung SI, et al: AbaR4-type resistance island including the blaOXA-58 gene in Acinetobacter nosocomialis isolates. Antimicrob Agents Chemother 2012; 56:4548–4549 34. Lee YT, Huang LY, Chiang DH, et al: Differences in phenotypic and genotypic characteristics among imipenem-non-susceptible Acinetobacter isolates belonging to different genomic species in Taiwan. Int J Antimicrob Agents 2009; 34:580–584 35. Chen TL, Lee YT, Kuo SC, et al: Emergence and distribution of plasmids bearing the blaOXA-51-like gene with an upstream ISAba1 in carbapenem-resistant Acinetobacter baumannii isolates in Taiwan. Antimicrob Agents Chemother 2010; 54:4575–4581
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Objectives: Acinetobacter baumannii complex bacteremia has been identified increasingly in critical patients admitted in ICUs. Notably, A. baumannii complex bacteremia has a high mortality rate, yet the risk factors associated with mortality remain unclear and controversial. Design: Retrospective study. Setting: All adult ICUs at a tertiary care medical center. Patients: All patients with A. baumannii complex bacteremia admitted in 2009–2010. Interventions: None. Measurements and Main Results: Risk factors for mortality were analyzed. Bacterial isolates were identified by 16S-23S ribosomal RNA intergenic spacer region sequencing for genospecies and genotyped by pulsed-field gel electrophoresis. Carbapenemase *See also p. 1289. 1 Graduate Institute of Clinical Medical Sciences, Chang Gung University College of Medicine, Taoyuan, Taiwan. 2 Department of Pediatrics, Chang Gung Children’s Hospital, Chang Gung University College of Medicine, Taoyuan, Taiwan. 3 Molecular Infectious Disease Research Center, Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Taoyuan, Taiwan. Dr. Lee designed the study, collected and analyzed the data, and wrote the article. Dr. Chen analyzed the data. Ms. Wu and Mr. Huang performed the experiments. Dr. Chiu designed the study, collected and analyzed the data, and wrote the article. All authors read and approved the final article. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (http://journals.lww.com/ ccmjournal). Supported, in part, by grants 100-2314-B-182A-049 and 102-2314-B-182A-023 from National Science Council, Executive Yuan, Taiwan, and by grants CMRPG490052 and CMRPG4C0031 from Chang Gung Memorial Hospital, Taiwan. Dr. Lee, Dr. Chen, Ms. Wu, Mr. Huang, and Dr. Chiu’s institutions received grant support from National Science Council and Chang Gung Memorial Hospital (Taipei, Taiwan). Dr. Lee, Dr. Chen, Ms. Wu, Mr. Huang, and Dr. Chiu received support for article research from National Science Council and Chang Gung Memorial Hospital. For information regarding this article, E-mail: [email protected] Copyright © 2014 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/CCM.0000000000000125
Critical Care Medicine
genes were detected by polymerase chain reaction and sequencing. A total of 298 patients met the inclusion criteria, including 73 (24.5%) infected by imipenem-resistant A. baumannii complex. The overall 30-day mortality was 33.6% (100 of 298). Imipenemresistant A. baumannii complex bacteremia specifically showed a high mortality (69.9%) and was associated with prior use of broad-spectrum antibiotics for more than 5 days for treating ventilator-associated pneumonia before the occurrence of bacteremia. Mortality was associated with inappropriate initial antimicrobial therapy, which was correlated with imipenem-resistant A. baumannii complex but not with any specific genospecies. ISAba1– blaOXA-23–ISAba1 (Tn2006) was found in most (66.7%, 40 of 68) imipenem-resistant A. baumannii (genospecies 2) and also spread beyond species border to all imipenem-resistant genospecies 3 (2), 13TU (2), and 10 (1). Conclusions: For critical patients with A. baumannii complex infection, ventilator-associated pneumonia in particular, the selective pressure from prior use of broad-spectrum antibiotics for 5 days or more increased risk of subsequent imipenem-resistant A. baumannii complex bacteremia. To reduce mortality, rapid identification of imipenem-resistant A. baumannii complex and early initiation of appropriate antimicrobial therapy in these high-risk patients are crucial. (Crit Care Med 2014; 42:1081–1088) Key Words: Acinetobacter baumannii complex; bacteremia; imipenem resistance; outcome; risk factor
A
cinetobacter baumannii bloodstream infection has a high mortality rate of 34.0–43.4% for critical patients treated in ICUs (1). How this infection is acquired and which factors are associated with mortality are not fully understood. Although not without some controversy, risk factors correlated with mortality are reportedly genospecies, inappropriate therapy, carbapenem resistance, disease severity, and the resistance mechanism (2–14). At least three species, A. baumannii (Acinetobacter genospecies 2), Acinetobacter nosocomialis (Acinetobacter genospecies 13TU), and Acinetobacter pittii (Acinetobacter genospecies 3) are invariably being reported as A. baumannii by clinical www.ccmjournal.org
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microbiology laboratories because biochemical methods cannot further differentiate them to the species level. These species form the A. baumannii complex (ABC) (2–4). Clinically, A. baumannii was associated with much higher attributable mortality rate (58.6%), when compared with the other two genospecies (16.7% and 33.3%, respectively) (4). Therefore, molecular methods were needed to accurately identify A. baumannii from other species to predict outcome and arrange further critical care (3, 4). On the other hand, A. baumannii has been found to have a higher resistance to carbapenem, the choice agent for treating this infection (2). To date, no study simultaneously evaluated the clinical features of carbapenem-susceptible ABC infection to avoid the most important confounder, carbapenem resistance, mainly because only a limited number of isolates of non-A. baumannii genospecies were collected in these studies (4). It is possible to carry out this study in Chang Gang Memorial Hospital (CGMH), a 3,500-bed tertiary hospital in Taiwan, because A. baumannii has become the most common pathogen for nosocomial bloodstream infection in ICUs in Chang Gang Memorial Hospital since 2009. This study applied backward root analysis to survey, step by step, independent risk factors associated with mortality due to ABC bacteremia by comparing clinical characteristics, microbiological features, and final outcomes of bacteremic patients stratified by both genospecies and their imipenem susceptibility.
MATERIALS AND METHODS Study Subjects and Inclusion Criteria This study was conducted at CGMH from 2009 to 2010 and was approved by the institutional review board (100-3592B). Charts were reviewed for all ICUs patients with more than or equal to one positive blood culture for A. baumannii and symptoms and
signs of infection. For patients with multiple episodes of bacteremia, only the first episode was included. Patients with incomplete medical records, those who are younger than 18 years, or those with polymicrobial infection were excluded. Bacterial Isolates and Antimicrobial Susceptibility Conventional biochemical tests and 16S-23S ribosomal RNA intergenic spacer region sequencing as described previously were used to characterize bacterial genospecies (15). Antimicrobial susceptibility was determined by the disk diffusion method according to Clinical and Laboratory Standards Institute standards (16). Minimum inhibitory concentration (MIC) of imipenem was further determined by Etest (AB BIODISK, Solna, Sweden). The breakpoint for defining imipenem-susceptible ABC (ISABC) was an MIC less than or equal to 4 mg/L and a breakpoint of more than or equal to 8 mg/L for imipenem-resistant ABC (IRABC), including both intermediately resistant and resistant strains (16). Data Collection and Definitions Clinical data were collected retrospectively. Mortality was defined as bacteremia-attributable death, that is, death before resolution of symptoms and signs of bacteremia and at least one blood culture positive for the ABC (4). Considering “time at risk” for acquiring resistance under antimicrobial selective pressure, prior exposure to antimicrobial agents was defined as at least 5 days of therapy during the 14 days before the isolation of ABC (17, 18). Appropriate antimicrobial therapy was defined as administering patients with at least one antimicrobial agent, except aminoglycoside, susceptible in vitro, within 2 days after bacteremia onset (3). Multidrug resistance (MDR) was defined as resistance to at least three antibiotic classes
Table 1. Comparison of Bacteremia-Attributable Mortality and Its Associated Risk Factors Among Different Genospecies of Acinetobacter baumannii Complex IRABC Genospecies 2 (A. baumannii) (n = 68)
13TU (n = 2)
3 (n = 2)
Mortality
48 (70.0)
1 (50.0)
1 (50.0)
1 (100)
Appropriate therapy (MRF1)
19 (27.9)
0 (0)
1 (50)
0 (0)
Imipenem resistance (MRF2)
68 (100.0)
2 (100)
2 (100)
1 (100)
Multidrug resistance
68 (100.0)
2 (100)
2 (100)
0 (0)
Prior exposure to broad-spectrum antibiotics ≥ 5 d (MRF3)
65 (95.6)
2 (100)
2 (100)
1 (100)
Ventilator-associated pneumonia as infection source (MRF4)
45 (66.2)
2 (100)
2 (100)
1 (100)
3.5 (3–4)
3 (2–4)
4
2 (1–3)
3 (2–4)
1
20 (7–33)
21 (18–23)
27
3 (1–4)
3 (2–3)
Mortality and Its Associated Risk Factors
Culture detecting time (d)
3 (3)
Charlson score
3 (2–5)
Acute Physiology and Chronic Health Evaluation II score Pitt bacteremia score
26 (20–30) 4 (1–4)
10 (n = 1)
3 (2–4)
IRABC = Imipenem-resistant A. baumannii complex, ISABC= Imipenem-susceptible A. baumannii complex, MRF = main risk factor. Data are presented as median value (interquartile range, Q1–Q3) for continuous variables and number of cases (%) for categoric variables.
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Clinical Investigations
(3). Culture detecting time was the interval (days) from culture sampling to reporting. Ventilator-associated pneumonia (VAP) was defined as pneumonia with pulmonary infiltrates on chest radiographs, purulent tracheal secretions not less than 48 hours after intubation, and the start of mechanical ventilation (19, 20). Multistage Risk Factor Analysis A multistage model was proposed for critical patients that covered the span from the appearance of ABC infection to mortality. The most important risk factor (MRF1) associated with mortality due to ABC was the first one to be analyzed by multivariate logistic analysis. The second main risk factor (MRF2) correlated with MRF1 was then analyzed by the same method. In this manner, all main risk factors would be identified as the MRF1 in each stage moving backward from death to infection onset. Polymerase Chain Reaction and Sequencing Primers specific for resistance genes (blaOXA-23, blaOXA-24, blaOXA-40, blaOXA-54, blaOXA-58, blaOXA-51, blaADC, blaIMP-1, blaIMP-2, blaVIM-1, blaVIM-2, and blaNDM-1), insertion sequences (ISAba1, ISAba2, ISAba3, ISAba4, and IS1008), and the AbaR4-type islands were designed, and polymerase chain reaction amplification and sequence determination were performed as described previously (21–25). Pulsed-Field Gel Electrophoresis Isolates were analyzed by pulsed-field gel electrophoresis (PFGE) using methods as described elsewhere (26). Fragment patterns obtained were interpreted as described by Tenover et al (27).
Statistical Analysis Data were recorded and entered into a database. Analyses were performed using SPSS software, v. 17.0 (SPSS, Chicago, IL). The Student t test, the chi-square test, or Fisher exact test was used when appropriate to compare proportions. Variables with a p value of less than 0.2 in the univariate analysis were added in a forward stepwise manner and selected to create the final model for multivariable analysis. All statistical analyses were two-sided, and significance was set at a p value of less than 0.05.
RESULTS Clinical Presentations Among Different Genospecies of ISABC After screening with the inclusion criteria, 298 patients were enrolled and their infected isolates were phenotypically identified to five genospecies (Table 1). Clinical presentations of patients with ISABC bacteremia were compared (Supplemental Table 1, Supplemental Digital Content 1, http://links.lww. com/CCM/A819). Patients with Acinetobacter baylyi, genospecies 13TU, or genospecies 3 bacteremia had significantly higher Charlson scores (p < 0.001) than scores of those with A. baumannii bacteremia. Clinical Presentations of Different Genospecies of IRABC No significant difference in Charlson scores, Acute Physiology and Chronic Health Evaluation (APACHE) II scores, or Pitt bacteremia scores existed for patients infected by different genospecies of IRABC (Table 1). No significant difference in Charlson scores existed between patients infected by
ISABC Genospecies 2 (A. baumannii) (n = 151)
13TU (n = 31)
3 (n = 37)
Acinetobacter baylyi (n = 6)
37 (24.5)
6 (19.4)
6 (16.2)
0 (0)
< 0.001
101 (66.9)
24 (77.4)
28 (75.7)
5 (83.3)
< 0.001
0 (0)
0 (0)
0 (0)
0 (0)
< 0.001
21 (13.9)
0 (0)
1 (2.7)
0 (0)
< 0.001
21 (13.2)
6 (19.4)
7 (18.9)
0 (0)
< 0.001
39 (25.8)
6 (19.4)
4 (10.8)
1 (16.7)
< 0.001
3 (2–3)
2 (2–3)
3 (3–5)
2.5 (2–3)
0.751
3 (1–5)
4 (3–7)
4 (3–5)
6 (4–10)
19 (16–24)
17 (13–23)
17 (14–19)
15 (12–17)
< 0.001
4 (1–4)
3 (2–4)
3 (1–4)
3 (2–3)
< 0.001
Critical Care Medicine
p (IRABC vs I SABC)
0.386
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i mipenem-resistant (IRAB) and imipenem-susceptible A. baumannii (ISAB), except that patients infected by IRAB had more renal disease (p < 0.001). Patients with IRAB infection had the highest APACHE II score, Pitt bacteremia score, and C-reactive protein level, suggesting a more clinically severe condition. VAP was the most common source of bacteremia for those with IRAB or IRABC infection (Table 1). Clinical Outcomes Overall 30-day mortality was 33.6% (100 of 298). Patients infected by IRAB had a higher 30-day mortality rate (70.0%) than those infected by ISAB (24.5%) (p < 0.001) (Table 1).
Nearly 75% of patients with ISAB bacteremia survived through day 30, compared with only 30% with IRAB infection (p < 0.001, by log-rank test) (Fig. 1A). No significant difference existed in 30-day mortality among patients infected by ISAB (24.5%), imipenem-susceptible genospecies 3 (16.2%), and imipenem-susceptible genospecies 13TU (19.4%) (p > 0.05) or by Kaplan-Meier estimate of survival curves (p > 0.05 by log-rank test) (Fig. 1, B and C). Similarly, mortality rates among patients infected by IRAB and the other three imipenem-resistant genospecies did not differ significantly. Notably, A. baumannii had a higher imipenem resistance than the other genospecies (p < 0.001).
Figure 1. The Kaplan-Meier estimate of survival curves of patients with Acinetobacter baumannii complex bacteremia. A, The Kaplan-Meier survival curve of patients infected by imipenem-susceptible A. baumannii (genospecies 2, solid line) compared with those by imipenem-resistant A. baumannii (dotted line) (p < 0.001, by log-rank test). B, The Kaplan-Meier survival curve of patients infected by imipenem-susceptible genospecies 13TU (dotted line) compared with those by imipenem-susceptible A. baumannii (solid line) (p = 0.927, by log-rank test). C, The Kaplan-Meier survival curve of patients infected by imipenem-susceptible genospecies 3 (dotted line) compared with those by imipenem-susceptible A. baumannii (solid line) (p = 0.495, by log-rank test). D, The Kaplan-Meier survival curve of patients who received appropriate initial antimicrobial therapy (solid line) compared with those without appropriate initial antimicrobial therapy (dotted line) (p < 0.001, by log-rank test).
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Clinical Investigations
Antimicrobial Susceptibility of IRABC and ISABC Except for colistin, IRAB were more resistant to all antibiotics than ISAB (Table 2). In fact, except for colistin and tigecycline, all IRAB isolates were multidrug resistant and more than 88% of IRAB were resistant to all antibiotics tested. On the contrary, all imipenem-susceptible isolates showed more than 80% susceptibility to all other antibiotics tested. Risk Factor Analysis for Mortality Risk factors for 30-day mortality were analyzed by comparing characteristics of nonsurvivors and survivors (Supplemental Table 2, Supplemental Digital Content 2, http://links.lww. com/CCM/A820). Inappropriate initial therapy, MRF1, had the highest odds ratio among independent risk factors of mortality. Nearly 80% of patients with appropriate antimicrobial therapy survived through day 30, compared with only 40% without (p < 0.001, by log-rank test) (Fig. 1D). For IRABC infection, patients administered appropriate therapy (early therapy in 2 d) by colistin alone, tigecycline alone, or combination of colistin/tigecycline had a lower mortality (37.5%, 20%, 20%) than those given inappropriate therapy (late therapy) with the same antibiotics (88.2%, 88.9%, 100%) (p = 0.017, 0.023, 0.048) (Table 3). Multivariate analysis of risk factors for inappropriate therapy was further performed. Infection by imipenem-resistant pathogen (adjusted odds ratio [AOR], 4.72; 95% CI, 2.48–8.99; p < 0.001), higher Pitt bacteremia score, and catheter-related
infection or urinary tract infection as the primary source were independent risk factors for inappropriate therapy. Infection by imipenem-resistant organisms had the highest odds ratio (MRF2). Risk factors for acquiring IRABC were also identified by comparing characteristics of IRABC and ISABC infections. After multivariate analysis, prior use of carbapenems, extended-spectrum cephalosporins, fluoroquinolones, or piperacillin/tazobactam for at least 5 days (MRF3) was the main independent risk factor for acquiring IRABC infection (Supplemental Table 3, Supplemental Digital Content 3, http://links.lww.com/CCM/A821). Main indication for prescribing these antibiotics was VAP (50 of 70, 71.4%). As MRF4, VAP as infection source was correlated with broad-spectrum antibiotics use for at least 5 days prior to infection (AOR, 3.874; 95% CI, 2.342–6.408; p < 0.001). At least one culture positive for IRABC with the same resistance pattern was found prior to (n = 62, 1–120 d before) or at the same time as (n = 9) IRABC bacteremia. Risk Analysis From Acquiring ABC to Mortality in Critical Patients According to the above multistage risk factor analysis by backward root method, mortality due to IRABC stemmed from inappropriate initial therapy. Table 4 shows the five stages from development of IRABC bacteremia to mortality in backward root analysis.
Comparison in Antibiotic Resistance Among Different Genospecies of Imipenem-Resistant Acinetobacter baumannii Complex and Imipenem-Susceptible A. baumannii Complex Table 2.
IRABC
Antibiotics
Genospecies 2 (A. baumannii) (n = 68)
13TU (n = 2)
ISABC 3 (n = 2)
10 (n = 1)
Genospecies 2 (A. baumannii) 13TU 3 (n = 151) (n = 31) (n = 37)
Acinetobacter baylyi (n = 6)
p (IRABC vs I SABC)
Multidrug resistance
68 (100)
2 (100) 2 (100)
0 (0)
21 (13.9)
0 (0)
1 (2.7)
0 (0)
< 0.001
Amikacin
67 (98.5)
2 (100) 2 (100)
0 (0)
13 (8.6)
0 (0)
0 (0)
0 (0)
< 0.001
Ceftazidime
68 (100)
2 (100) 2 (100)
0 (0)
21 (13.9)
1 (2.7)
1 (3.2)
0 (0)
< 0.001
Ciprofloxacin
68 (100)
0 (0)
0 (0)
10 (6.6)
2 (5.4)
0 (0)
1 (16.7)
< 0.001
Cefepime
67 (98.5)
2 (100) 2 (100)
0 (0)
17 (11.3)
1 (2.7)
0 (0)
0 (0)
< 0.001
Gentamicin
68 (100)
0 (0)
1 (100)
19 (12.6)
6 (16.2) 0 (0)
0 (0)
< 0.001
Piperacillintazobactam
68 (100)
2 (100) 1 (50)
0 (0)
22 (14.6)
2 (5.4)
0 (0)
0 (0)
< 0.001
Imipenem
68 (100)
2 (100) 2 (100)
1 (100)
0 (0)
0 (0)
0 (0)
0 (0)
< 0.001
Meropenem
68 (100)
2 (100) 2 (100)
1 (100)
0 (0)
0 (0)
0 (0)
0 (0)
< 0.001
Unasyn
60 (88.2)
0 (0)
0 (0)
0 (0)
7 (4.6)
1 (2.7)
0 (0)
0 (0)
< 0.001
Tigecycline
16 (23.5)
0 (0)
0 (0)
0 (0)
1 (0.7)
0 (0)
0 (0)
0 (0)
< 0.001
0 (0)
0 (0)
0 (0)
2 (1.3)
0 (0)
0 (0)
0 (0)
1.000
Colistin
0 (0)
0 (0) 2 (100)
IRABC = Imipenem-resistant A. baumannii complex, ISABC= Imipenem-susceptible A. baumannii complex. Data are presented as number of cases (%).
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Table 3. Comparison of Mortality of Patients With Imipenem-Resistant Acinetobacter baumannii Complex Infection Administered Appropriate Therapy With Those Administered Inappropriate Therapy Effectivea or Ineffectiveb Antimicrobial Therapy for Imipenem-Resistant A. baumannii Complex
Mortality of Appropriate herapy (Effective Therapy T Given in 2 d) (n/Total n) (%)
Mortality of Inappropriate Therapy (Late or Ineffective Therapy) (n/Total n) (%)
p
Colistin
3/8 (37.5)
15/17 (88.2)
0.017
Tigecycline
1/5 (20.0)
8/9 (88.9)
0.023
Colistin + Tigecycline
1/5 (20.0)
5/5 (100.0)
0.048
Sulbactam
0/1 (0)
3/4 (75.0)
0.400
Ciprofloxacin
0/1 (0)
1/2 (50.0)
1.000
b
Imipenem
—
11/12 (91.7)
b
Cefepime
—
3/4 (75.0)
Dashes signify that no patient in that group received such antibiotics. a Effective antimicrobial therapy was defined as giving patients with at least one antimicrobial agent, except aminoglycoside, susceptible in vitro, within 2 d after the onset of bacteremia. Late or ineffective therapy was considered as inappropriate therapy. b Imipenem and cefepime were ineffective antibiotics for these patients with imipenem-resistant A. baumannii complex infections.
Imipenem-Resistance Genes All A. baumannii isolates carried the blaOXA-51-like. The upstream ISAba1 was found in 12 isolates with blaOXA-51-like and in 47 IRAB isolates with blaOXA-23, including 40 with Tn2006 (ISAba1– blaOXA-23–ISAba1) and seven with Tn2008 (ISAba1–blaOXA-23) (Supplemental Table 4, Supplemental Digital Content 4, http://links.lww.com/CCM/A822). Tn2006 in an AbaR4-type resistance island was found in most IRAB isolates (40 of 68, 58.8%) and all imipenem-resistant genospecies 3 (2), genospecies 13TU (2), and genospecies 10 (1). PFGE Patterns of IRAB Based on the criteria by Tenover et al (27), 68 IRAB isolates showed 26 different PFGE patterns (Supplemental Table 2, Supplemental Digital Content 2, http://links.lww.com/CCM/ A820). In total, 35 IRAB isolates (51.47%) belonged to PFGE type 1, the major IRAB clone in Taiwan. Notably, ISAba1– blaOXA-23 (Tn2006 or Tn2008) was found in 69.2% of the genotypes (18 of 26) and was more widely distributed than ISAba1–blaOXA-51 (34.6%, 9 of 26) (p = 0.025) (Supplemental Table 5, Supplemental Digital Content 5, http://links.lww.com/
CCM/A823). In contrast to the clonal nature of IRAB, all ISAB isolates were diverse and each had a different PFGE genotype. Most isolates belonging to PFGE type 1 carried Tn2006 (25 of 35, 71.4%). These isolates were derived from patients with VAP (30 of 35, 85.7%) and for whom the mortality rate was high (25 of 35, 71.4%).
DISCUSSION According to this comprehensive analysis of any ABC infection in critical patients, the selective pressure from prior use of broad-spectrum antibiotics for 5 days or more increased risk of subsequent IRABC bacteremia. Of IRABC isolates, only colistin and tigecycline were effective. Shortening the time for confirmation of IRABC and initiating appropriate antibiotics to treat the infection is crucial for critical patients in this scenario. In this study, mortality due to ABC bacteremia correlated with imipenem resistance but not with specific genospecies. Many differences between IRABC and ISABC were observed. First, IRABC had much higher MDR (98.6%) than ISABC (9.8%). Second, more patients with IRABC infection
Backward Root Analysis for Major Risk Factor at Different Stages From the Development of Imipenem-Resistant Acinetobacter baumannii Complex Bacteremia to Mortality and Proposed Solution for Each Stage Table 4.
Stages
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
MRF or outcome
Ventilator-associated pneumonia (MRF4)
Prior exposure to broad-spectrum antibiotics ≥ 5 d (MRF3)
IRABC (MRF2)
Inappropriate therapy (MRF1)
Mortality due to A. baumannii complex bacteremia
Recommended intervention
Early detection of IRABC
Antibiotic stewardship
Detection of IRABC bacteremia
Use of effective antibiotics in 2d
MRF = main risk factor, IRABC = imipenem-resistant A. baumannii complex.
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Clinical Investigations
received inappropriate initial therapy (72.6%) than patients with ISABC did (29.8%). Third and more importantly, mortality attributable to IRABC bacteremia (69.9%) was much higher than that for ISABC (21.8%) and for all nosocomial bloodstream infections (25.7–47.9%) reported in U.S. ICUs (1). Some earlier studies also showed that imipenem resistance was associated with mortality by bacteremia, VAP, or other infections caused by A. baumannii (7, 18, 28), whereas others did not (4, 14). Fourth, the main primary source of IRABC bacteremia was VAP (82.2%), which was not the case in ISABC bacteremia (22.2%). Giving appropriate initial antimicrobial therapy is critical for treatment of septic patients (28) and was associated with a low mortality rate for patients with A. baumannii bacteremia (8, 10, 12). Use of in vitro effective antibiotics was reported as an independent factor favoring rapid bacterial clearance (29). All of IRABCs except genospecies 10 were MDR. Mean time needed to identify IRABC by culture was 3 days, longer than the maximum time (2 d) defined in this study as initiation of appropriate therapy. Without guidance by microbiological data, only 27.4% of patients infected by IRABC were given effective antibiotics, either colistin or tigecycline, within 2 days, presumably resulting in a high 30-day morality rate of 69.9% for these patients. The most important risk factor for mortality for IRABC bacteremia was no appropriate therapy given within 2 days. This study found that most IRABC bacteremia had a preceding infection caused by ABC with the same resistance pattern, for which extended-spectrum antibiotics had been given. This is consistent with a previous assertion that prior exposure to imipenem or third-generation cephalosporins increased risk of acquiring IRAB infection (9, 18, 30). This study has three important clinical microbiological findings. First, our results were different from those in previous studies, which showed that infection with genospecies 2 (A. baumannii) was the most important independent predictor of mortality due to ABC (3). In this large-scale clinical study, no specific genospecies was an independent risk factor for mortality. Genospecies 2 (A. baumannii) generated a high mortality due to high rate of imipenem resistance resulting from high rate of inappropriate initial therapy. Second, two new members of ABC were identified, A. baylyi and genospecies 10; both were misidentified as ABC by routine biochemical testing. Clinical isolates of A. baylyi and genospecies 10 were reported before (31, 32), but their identification as ABC has never been described. Third, this study found that ISAba1–blaOXA-23–ISAba1 (Tn2006) located in an AbaR4-type resistance island had spread in most A. baumannii but had also spread beyond the species border to genospecies 3, 13TU, and 10. Although the AbaR4-type resistance island was first found in two isolates of genospecies 13TU from Korea and Thailand (33), the resistance island had never been identified in genospecies 3 or 10. Our study along with previous studies from Taiwan confirmed that the major resistance mechanism of IRABC in Taiwan has changed from ISAba1–blaOXA-51-like to ISAba1–blaOXA-23 since 2009 (34, 35). The wide dissemination Critical Care Medicine
of Tn2006 in an AbaR4-type resistance island was supported by the finding that most IRAB contained the specific genetic determinant irrespective of their PFGE patterns. This is to date the most comprehensive study of ABC bacteremia. Genospecies did not influence patients’ outcomes. Mortality due to ABC bacteremia was mainly associated with inappropriate initial therapy, which was due to imipenem resistance and also MDR of IRABC. Appearance of IRABC bacteremia was associated with prior use of broad-spectrum antibiotics prescribed to treat the preceding VAP. To reduce mortality from IRABC bacteremia, a rapid diagnostic method for detecting IRABC in blood or deep airway specimens should be developed for patients with VAP who have been given broad-spectrum antibiotics. These patients are at increased risk for IRABC bacteremia. To save lives, effective antibiotics should be given to these patients in 2 days.
ACKNOWLEDGMENTS We thank professor Chee-Jen Chang and Ms. Hsiao-Jung Tseng in Clinical Informatics and Medical Statistics Research Center, Chang Gung Memorial Hospital, for validating and confirming all the statistics in this work. We thank professor Tzu-Lan Wu for providing bacterial isolates for this study.
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