1016
Current Topic: Review
Vol. 11. No. 11
Eur. J. Clin. Microbial. Infect. Dis., November 1992, p. 1016-1020 0934-9723/92/11 01016-05 $3.00/0
Resistance of Anaerobic Bacteria to Antimicrobial Agents in Spain
E B a q u e r o * , M. Reig
As a consequence of antibiotic consumption, the entire microbial ecosystem attached to man is evolving towards resistance. In Spain, penicillin resistance (MIC > 0.5 mg/i) is found in about 10 % of Peptostreptococcus, Clostridium perfringens and Eubacterium, and in 50 % of Veillonella. Cefoxitin resistance (> 2 rag/l) is present in 10 to 20 % of Peptostreptococcus and Clostridium, and in 50 % of Eubacterium; 2 1 % of Bacteroides (fragilis group) strains are resistant to 16 mg/l. A very low rate of imipenem resistance (> 128 rag/l) is found among Bacteroides (1%), but for 5 % of the isolates MICs of imipenem are 2 to 4 mg/l. Metronidazole resistance (> 8 mg/I) is found in 5 to 10 % ofPeptostreptocoecus, Clostridium and Veillonella, and in less than 1 % of the Bacterotdesfragtlts group. Erythromycin resistance (> 2 rag/l) is present in over two-thirds of the Peptostreptococcus, Veillonella and Fusobacterium isolates, and in 27 % of the Clostridium perfringens strains. Clindamycin resistance (> 4 rag/l) is found in 10 to 20 % of the Peptostreptococcus, Clostridium and Eubacterium isolates, and in 20 % of the Bacterotdes of the fragilis group, this rate being higher (30 %) among faecal isolates.
Antibiotic consumption in the community represents a selective force for resistance that is exerted mainly on the normal microflora. Anaerobic microorganisms, as the dominant components of the normal flora, are thus submitted to periodic selective pressure for antibiotic resistance. During the last 15 years, a sharp evolution towards antibiotic resistance in oropharyngeal and intestinal microbial ecosystems has been observed in Spain. This overview documents the current involvement of anaerobic bacteria in antibiotic resistance at the Ram6n y Cajal Hospital, Madrid. The results of quantitative susceptibility testing of nearly 1,200 isolates (Table 1) are discussed with reference to the data obtained in other Spanish hospitals.
Penicillin Resistance
Peptostreptococcus, one of the most prevalent anaerobic gram-positive organisms isolated from human infections, has been considered fully susDepartment of Microbiology,Hospital Ram6n y Cajal, National Institute of Health, 28034Madrid, Spain.
ceptible to penicillin G. Nevertheless, a consistent percentage (9.5 %) of isolates presenting decreased susceptibility to this antibiotic (MIC > 0.5 mg/1) was detected in our hospital. Almost half of the resistant isolates belonged to the species Peptostreptococcus anaerobius, followed by Peptostreptococcus asaccharolyticus. In 75 % of the cases, the MIC of penicillin for these strains was >_. 8 mg/l. Preliminary work carried out on these Peptostreptococcus anaerobius strains (E Vicente et al., personal communication) suggested the involvement of a penicillin-binding protein (PBP) alteration in the resistant phenotype. It should be borne in mind that Spain has one of the highest rates of PBP-mediated penicillin resistance among Streptococcus pneumoniae and Streptococcus of the viridans group in Europe, probably due to the overconsumption of aminopenicillins (1). Similarly, this selective pressure may have affected Peptostreptococcus as a member of the normal human flora. A few penicillin-resistant strains (Peptostreptococcus
asaccharolyticus, Peptostreptococcus magnus, Peptostreptococcus prevotii) were susceptible to amoxicillin-clavulanic acid (MIC < 4/2 mg/1), but the presence of a putative 15-1actamase could not be demonstrated by conventional methods. Resistance to penicillin among Peptostreptococ-
Vol. 11, 1992
1017
Table 1: Percentage of anaerobic strains exhibiting antibiotic resistance at established breakpoints. Organism
Antibiotic
Peptoslreplococcus spp.
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Clostridiurnperfringens
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Clostridium spp.
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Eubacteriurn spp.
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Baeteroidesfragilis group
Cefoxitin Metronidazole Erythromycin Clindamycin
Veillonella spp.
Penicillin Metronidazole Erythromycin
cus has not been detected in smaller numbers of isolates studied in other Madrid hospitals (2). The rate of resistance of Clostridium perfringens to penicillin (MIC > 0.5 mg/1) was 12.5 %; this rate Was even higher in other Clostridium species (20.5 %). Eubacterium was resistant to penicillin in 9.0 % of cases. A few penicillin-resistant Fusobacterium were detected during the period 19861988; the overall rate of resistance in our series was 7 %. During the period 1989-1990, all Fusobacterium isolates were susceptible to penicillin, both in our study and in that undertaken by the San Carlos Hospital, Madrid (2). On the other hand, a substantial percentage of Veillonella strains showed resistance to 0.5 mg/l of penicillin (57 %), this percentage being lower (18 %) in other Madrid Hospitals (2).
Breakpoint (mg/l)
No. of strains
Percent resistant
0.5 2 8 2 4
350 280 350 350 350
9.5 10.0 5.5 75.0 19.0
0.5 2 16 8 2 4
105 109 105 123 105 105
12.5 20.5 2.0 5.5 37.0 10.5
0.5 2 16 8 2 4
98 62 98 91 98 98
20.5 29.0 9.0 4.5 21.5 19.5
0.5 2 16 8 2 4
89 45 89 68 89 89
9.0 53.5 15.5 19.0 24.5 9.0
470 470 505 511 511
21.5 5.0 0.5 85.0 20.5
21 21 21
57.0 7.5 66.0
16 32 8 2 4 0.5 8 2
Cefoxitin Resistance
In our series the rate of cefoxitin resistance (MIC > 2 mg/l) among Peptostreptococcus strains was 10.5 %. There was a correlation between cefoxitin-resistant and penicillin-resistant strains, with Peptostreptococcus anaerobius accounting for more than one-half (53 %) of the cefoxitinresistant strains. The use of higher breakpoints for gram-positive cocci, such as 16 or 32 mg/l, gives an unrealistic picture of total susceptibility (2). The same may occur when higher breakpoints are used to test cefoxitin against Clostridium. When a breakpoint of 2 mg/1 was applied to Clostridium perfringens strains, the rate of resistance was 2 1 % , much higher than the 2 % obtained with an MIC of > 16 mg/l. The corresponding rates for other members of the genus
1018
Eur. J. Clin. Microbiol. Infect. Dis.
Clostridium were 29 % and 9 %, respectively. In Eubacterium the rate of resistance to cefoxitin was 15.5 % at the higher breakpoint (> 16 rag/l) and 53.5 % at the lower one (> 2 mg/l).
tance) and with the E-test (strains requiring MICs of 2 to 4 mg/l in agar dilution were completely susceptible).
The rate of cefoxitin resistance (MIC > 16 mg/l) in Bacteroides strains of the fragilis group was 21.5 %. Increasing the breakpoint to > 32 mg/l will diminish this rate to 5 %, similar to the 6 % observed in another Madrid hospital (2). A population analysis of the distribution of the Bacteroides isolates with respect to MIC values shows that the mode value for Bacteroidesfragilis is 4 to 8 mg/1. Organisms for which MICs of cefoxitin are > 16 mg/l are clearly out of the distribution of the susceptible strains and should be considered as presumptively resistant. Applying this criterion (resistant ifMIC is > 16 mg/1), cIeardifferences in the rates of cefoxitin resistance are detectable among the different species: Bacteroides fragilis 8 %, Bacteroides vulgatus 9 %, Bacteroides distasonis 5 1 % , Bacteroides ovatus 47 %, Bacteroides thetaiotaomicron 67 % and Bacteroides uniformis 26 %. When the criterion for resistance was established at a breakpoint of > 32 mg/l, the rate of resistance for all Bacteroides species was below 10 %. All Fusobacterium and black-pigmented Bacteroides were susceptible to cefoxitin.
Metronidazole Resistance
Imipenem Resistance Imipenem-resistant Bacteroides fragilis strains (MIC >_. 128 mg/1) were detected at a relatively high frequency (1.2 % of the tested isolates) in the Gregorio Marafidn Hospital, Madrid, during the period 1988-1989 (3). These strains were resistant to many other agents, including cefoxitin, piperacillin, ticarcillin-clavulanic acid, ampicillin-sulbactam and clindamycin. Imipenem was a controlled substance in this hospital. The same rate of resistance (1.2 %) among Bacteroidesfragilis was found in 1989-1990 in the San Carlos Hospital, Madrid (2). No resistance to imipenem was found for this group of organisms in other areas of Spain during the period 1981-1987 (4, and G. Prats, Barcelona, personal communication). In our survey we were unable to detect any imipenem-resistant (MIC > 8 mg/l) strains among Bacteroides of the fragilis group. Nevertheless, a small number of isolates (5 %) had imipenem MICs of 2 to 4 mg/l, although they remained cefoxitin-susceptible (MIC < 16 mg/l). Problems of MIC determinations for imipenem were observed with the new Sceptor panel (false resis-
In our series the rate of metronidazole resistance (MIC > 8 mg/l) among Peptostreptococcus strains was 5.5 %. Among Clostridium perfringens, the rate was 5.5 %, and among other species of the genus Clostridium, 4.5 %. Similar or higher figures were found in another Madrid hospital (5). Eubacterium (carefully excluding microaerophilic gram-positive bacteria) had one of the highest rates of metronidazole resistance, 19 %. Metronidazole resistance among Bacteroides of the fragilis group was infrequent (0.5 %) in our series (2 strains: Bacteroides thetaiotaomicron and Bacteroides uniformis). In another Madrid hospital the rate was higher, 2 % during 19861989, involving Bacteroides fragilis, Bacteroides thetaiotaomicron and Bacteroides vulgatus (3). This resistance was not observed in other Madrid hospitals during 198%1990 (2) or in other areas of Spain, at least during the period 1975-1988 (4). In our survey a slightly higher rate of metronidazole resistance (1.9 %) was found among non-fragUis and non-black pigmented Bacteroides, among Bacteroides ureotyticus (9 %) and, unexpectedly, in Veillonellaparvula (7.5 %). No resistance to metronidazole was detected among Fusobacterium or black-pigmented Bacteroides.
Erythromydn Resistance For Peptostreptococcus the rate of erythromycin resistance (> 2 mg/1) was 75 %, Different rates of resistance were found for the different species: Peptostreptococcus micros 27.5 %, Peptostreptococcus anaerobius 39 %, Peptostreptococcus magnus 84 %, Peptostreptococcus prevotii 85 % and Peptostreptococcus asaccharolyticus 92.5 %. Erythromycin susceptibility testing should not be discontinued, as clindamycin-susceptible strains sharing erythromycin resistance may harbour an inducible macrolide-lincosamide resistance mechanism. Constitutively erythromycin-clindamycin resistant mutants can be obtained from inducibly resistant strains (6). A high rate of erythromycin resistance (> 2 mg/l), 37 %, was found for Clostridium perfringens. This rate was lower for the other members of the genus
Voi. 11,1992
Clostridium, 21.5 %. Resistance in Eubacterium reached 24.5 %. An extremely high rate of erythromycin resistance (94 %) was found in Fusobacterium. High rates of resistance have also been found in other surveys in Madrid hospitals (2). It will be of interest to ascertain to what extent this corresponds to acquired resistance, as similar rates were found by American authors in the 1970s. Erythromycin resistance in Veillonella was also unexpectedly high (66 %) in our series. Erythromycin resistance among Bacteroides of the fragilis group reached 85 % in our hospital, while that among black-pigmented Bacteroides was 8 %.
1019
two surveys of 100 faecal specimens from healthy and untreated persons performed in our hospital in 1983 and 1988, rates of resistance in Bacteroides fragilis were 33 % and 27 %, respectively; in Bacteroides vulgatus, 17 % and 42 %; in Bacteroides distasonis, 27 % and 47 %; in Bacteroides ovatus, 34 % and 29 %; and in Bacteroides uniformis, 27 % and 26 %. Despite the very high rate of erythromycin resistance among Fusobacterium isolates, clindamycin resistance remained very low (3 %), which suggested the presence of a mechanism other than reduction of activity of macrolides and lincosamides by 23SrRNA methylase.
Clindamycin Resistance
The clindamycin resistance rate (MIC > 4 mg/1) for Peptostreptococcus was 19 %. In accordance with the hypothesis that clindamycin resistance develops from inducible erythromycin resistance, the Peptostreptococcus species exhibiting increased resistance to clindamycin were those with the highest rates of erythromycin resistance: Pep-
tostreptococcus magnus, Peptostreptococcus prevotii and Peptostreptococcus asaccharolyticus. A total of 10.5 % of Clostridium perfringens isolates and 19.5 % of other Clostridium strains were resistant to clindamycin. In Eubacterium the rate of clindamycin resistance was 9 %. The overall resistance rate (MIC > 4 mg/l) for Bacteroides of the fragilis group was 20.5 %. Resistance rates varied for the different species, from 13.5 % for Bacteroides fragilis, 20 % for Bacteroides uniformis, 26.5 % for Bacteroides vulgatus, 29 % for Bacteroides thetaiotaomicron, 3 4 % for Bacteroides distasonis to 36.1% for Bacteroides ovatus. During the last 12 years, rates of resistance to clindamycin have increased from 0 % (before 1979) to 30 % (1991) in our hospital. Several peaks in resistance rates have been observed, some of them correlating with a higher rate of isolation in clinical samples of some highly resistant species such as Bacteroides distasonis. Similar or higher peaks of clindamycin resistance (45 %) have been found in other Madrid hospitals (3, 8) among Bacteroides of thefragilis group. Geographical differences are clear: in the Universitary Hospital of Salamanca, the findings were different from those of the Madrid institutions, with an overall resistance rate maintained below 8 % in recent years (4). Faecal isolates of Bacteroides have been implicated as a reservoir of clindamycin resistance. In
Resistance to Other A g e n t s
Resistance to amoxicillin-clavulanic acid (MIC > 4/8 mg/1) was found in a limited number (1.5 %) of Peptostreptococcus strains, mostly among penicillin-resistant isolates of Peptostreptococcus anaerobius. Similarly, a low rate of amoxicillinclavulanic acid resistance (3.5 %) was detected among Bacteroides of the fragilis group. The lower rates were found among Bacteroides fragilis, Bacteroides vulgatus and Bacteroides ovatus (less than 1.5 %), and the highest among Bacteroides thetaiotaomicron (7.5 %) and Bacteroides distasonis (10 %). The more resistant strains generally exhibited resistance to piperacillin (> 128 mg/l), but resistance to cefoxitin was not unusually increased. Chloramphenicol resistance (MIC > 8 mg/1) was detected in 3 % of our Peptostreptococcus isolates, particularly among Peptostreptococcus magnus. Despite the observed presence in Spain of a transferable plasmid encoding high-level chloramphenicol resistance in Bacteroides in 1985 (9), resistance remains extremely infrequent (a single strain among 411 isolates tested).
References
1. Baquero F, Loza E: Penicillin resistance in Spain. Infectious Diseases in Clinical Practice 1992, 1: 147-148. 2. BetrifiC, Campos E, Picazo J J: Susceptibilitypatterns of anaerobic bacteria isolated over a ten-month period. Revista Espafiola de Quimioterapia 1991, 4: 237-239. 3. Pel~iez MT, Cercenado E, Rodrlguez-Crrixems M, Bouza E: Resistance of anaerobic bacteria to antimicrobial agents. Reviews of Infectious Diseases 1991,
13: 183.
1020
4. Garcla-Roddguez JA, Garc/a-S~nchez JE: Evolution of antimicrobial susceptibility in isolates of the Bacteroides fragilis group in Spain. Reviews of Infectious Diseases 1990, 12, Supplement 2: 142-151. 5. Betri6 C, Moreno JJ, Cuadros JA, Garc/a G, Cruceyra A, Picazo JJ: Sensibilidad de Bacteroides grupo fragilis a nuevos agentes 15-1act~imicos. Revista Espafiola de MicrobioIogfa Clfnica 1989, 4: 652657. 6. Reig M, Moreno A, Baquero F: Resistance of Peptostreptococcus spp. to macrolides and lincosamides: inducible and constitutive phenotypes. Antimicrobial Agents and Chemotherapy 1992, 36: 662--664.
Eur. J. Clin. Microbiol. Infect. Dis.
7. l~negoid SM: Antimicrobial agents susceptibility of anaerobic bacteria. In: Finegold SM (ed): Anaerobic bacteria in human diseases. Academic Press, New York, 1977, p. 513-531. 8. Betrifi C, Campos E, Cabronero C, Roddguez.Avial C, Picazo JJ: Susceptibility of species of the Bacteroidesfragilis group to 10 antimicrobial agents. Antimicrobial Agents and Chemotherapy 1990, 34: 671673. 9. Martlnez.Sufirez JV, Baquero F, Rcig M, P6rez-Dfaz JC: Transferable plasmid-linked chloramphenicol acetyltransferase conferring high level resistance in Bacteroides uniformb. Antimicrobial Agents and Chemotherapy 1985, 28: 113-117.
Current Topic: Review
Vol. 11. No. 11
Eur. J. Clin. Microbial. Infect. Dis., November 1992, p. 1016-1020 0934-9723/92/11 01016-05 $3.00/0
Resistance of Anaerobic Bacteria to Antimicrobial Agents in Spain
E B a q u e r o * , M. Reig
As a consequence of antibiotic consumption, the entire microbial ecosystem attached to man is evolving towards resistance. In Spain, penicillin resistance (MIC > 0.5 mg/i) is found in about 10 % of Peptostreptococcus, Clostridium perfringens and Eubacterium, and in 50 % of Veillonella. Cefoxitin resistance (> 2 rag/l) is present in 10 to 20 % of Peptostreptococcus and Clostridium, and in 50 % of Eubacterium; 2 1 % of Bacteroides (fragilis group) strains are resistant to 16 mg/l. A very low rate of imipenem resistance (> 128 rag/l) is found among Bacteroides (1%), but for 5 % of the isolates MICs of imipenem are 2 to 4 mg/l. Metronidazole resistance (> 8 mg/I) is found in 5 to 10 % ofPeptostreptocoecus, Clostridium and Veillonella, and in less than 1 % of the Bacterotdesfragtlts group. Erythromycin resistance (> 2 rag/l) is present in over two-thirds of the Peptostreptococcus, Veillonella and Fusobacterium isolates, and in 27 % of the Clostridium perfringens strains. Clindamycin resistance (> 4 rag/l) is found in 10 to 20 % of the Peptostreptococcus, Clostridium and Eubacterium isolates, and in 20 % of the Bacterotdes of the fragilis group, this rate being higher (30 %) among faecal isolates.
Antibiotic consumption in the community represents a selective force for resistance that is exerted mainly on the normal microflora. Anaerobic microorganisms, as the dominant components of the normal flora, are thus submitted to periodic selective pressure for antibiotic resistance. During the last 15 years, a sharp evolution towards antibiotic resistance in oropharyngeal and intestinal microbial ecosystems has been observed in Spain. This overview documents the current involvement of anaerobic bacteria in antibiotic resistance at the Ram6n y Cajal Hospital, Madrid. The results of quantitative susceptibility testing of nearly 1,200 isolates (Table 1) are discussed with reference to the data obtained in other Spanish hospitals.
Penicillin Resistance
Peptostreptococcus, one of the most prevalent anaerobic gram-positive organisms isolated from human infections, has been considered fully susDepartment of Microbiology,Hospital Ram6n y Cajal, National Institute of Health, 28034Madrid, Spain.
ceptible to penicillin G. Nevertheless, a consistent percentage (9.5 %) of isolates presenting decreased susceptibility to this antibiotic (MIC > 0.5 mg/1) was detected in our hospital. Almost half of the resistant isolates belonged to the species Peptostreptococcus anaerobius, followed by Peptostreptococcus asaccharolyticus. In 75 % of the cases, the MIC of penicillin for these strains was >_. 8 mg/l. Preliminary work carried out on these Peptostreptococcus anaerobius strains (E Vicente et al., personal communication) suggested the involvement of a penicillin-binding protein (PBP) alteration in the resistant phenotype. It should be borne in mind that Spain has one of the highest rates of PBP-mediated penicillin resistance among Streptococcus pneumoniae and Streptococcus of the viridans group in Europe, probably due to the overconsumption of aminopenicillins (1). Similarly, this selective pressure may have affected Peptostreptococcus as a member of the normal human flora. A few penicillin-resistant strains (Peptostreptococcus
asaccharolyticus, Peptostreptococcus magnus, Peptostreptococcus prevotii) were susceptible to amoxicillin-clavulanic acid (MIC < 4/2 mg/1), but the presence of a putative 15-1actamase could not be demonstrated by conventional methods. Resistance to penicillin among Peptostreptococ-
Vol. 11, 1992
1017
Table 1: Percentage of anaerobic strains exhibiting antibiotic resistance at established breakpoints. Organism
Antibiotic
Peptoslreplococcus spp.
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Clostridiurnperfringens
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Clostridium spp.
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Eubacteriurn spp.
Penicillin Cefoxitin Metronidazole Erythromycin Clindamycin
Baeteroidesfragilis group
Cefoxitin Metronidazole Erythromycin Clindamycin
Veillonella spp.
Penicillin Metronidazole Erythromycin
cus has not been detected in smaller numbers of isolates studied in other Madrid hospitals (2). The rate of resistance of Clostridium perfringens to penicillin (MIC > 0.5 mg/1) was 12.5 %; this rate Was even higher in other Clostridium species (20.5 %). Eubacterium was resistant to penicillin in 9.0 % of cases. A few penicillin-resistant Fusobacterium were detected during the period 19861988; the overall rate of resistance in our series was 7 %. During the period 1989-1990, all Fusobacterium isolates were susceptible to penicillin, both in our study and in that undertaken by the San Carlos Hospital, Madrid (2). On the other hand, a substantial percentage of Veillonella strains showed resistance to 0.5 mg/l of penicillin (57 %), this percentage being lower (18 %) in other Madrid Hospitals (2).
Breakpoint (mg/l)
No. of strains
Percent resistant
0.5 2 8 2 4
350 280 350 350 350
9.5 10.0 5.5 75.0 19.0
0.5 2 16 8 2 4
105 109 105 123 105 105
12.5 20.5 2.0 5.5 37.0 10.5
0.5 2 16 8 2 4
98 62 98 91 98 98
20.5 29.0 9.0 4.5 21.5 19.5
0.5 2 16 8 2 4
89 45 89 68 89 89
9.0 53.5 15.5 19.0 24.5 9.0
470 470 505 511 511
21.5 5.0 0.5 85.0 20.5
21 21 21
57.0 7.5 66.0
16 32 8 2 4 0.5 8 2
Cefoxitin Resistance
In our series the rate of cefoxitin resistance (MIC > 2 mg/l) among Peptostreptococcus strains was 10.5 %. There was a correlation between cefoxitin-resistant and penicillin-resistant strains, with Peptostreptococcus anaerobius accounting for more than one-half (53 %) of the cefoxitinresistant strains. The use of higher breakpoints for gram-positive cocci, such as 16 or 32 mg/l, gives an unrealistic picture of total susceptibility (2). The same may occur when higher breakpoints are used to test cefoxitin against Clostridium. When a breakpoint of 2 mg/1 was applied to Clostridium perfringens strains, the rate of resistance was 2 1 % , much higher than the 2 % obtained with an MIC of > 16 mg/l. The corresponding rates for other members of the genus
1018
Eur. J. Clin. Microbiol. Infect. Dis.
Clostridium were 29 % and 9 %, respectively. In Eubacterium the rate of resistance to cefoxitin was 15.5 % at the higher breakpoint (> 16 rag/l) and 53.5 % at the lower one (> 2 mg/l).
tance) and with the E-test (strains requiring MICs of 2 to 4 mg/l in agar dilution were completely susceptible).
The rate of cefoxitin resistance (MIC > 16 mg/l) in Bacteroides strains of the fragilis group was 21.5 %. Increasing the breakpoint to > 32 mg/l will diminish this rate to 5 %, similar to the 6 % observed in another Madrid hospital (2). A population analysis of the distribution of the Bacteroides isolates with respect to MIC values shows that the mode value for Bacteroidesfragilis is 4 to 8 mg/1. Organisms for which MICs of cefoxitin are > 16 mg/l are clearly out of the distribution of the susceptible strains and should be considered as presumptively resistant. Applying this criterion (resistant ifMIC is > 16 mg/1), cIeardifferences in the rates of cefoxitin resistance are detectable among the different species: Bacteroides fragilis 8 %, Bacteroides vulgatus 9 %, Bacteroides distasonis 5 1 % , Bacteroides ovatus 47 %, Bacteroides thetaiotaomicron 67 % and Bacteroides uniformis 26 %. When the criterion for resistance was established at a breakpoint of > 32 mg/l, the rate of resistance for all Bacteroides species was below 10 %. All Fusobacterium and black-pigmented Bacteroides were susceptible to cefoxitin.
Metronidazole Resistance
Imipenem Resistance Imipenem-resistant Bacteroides fragilis strains (MIC >_. 128 mg/1) were detected at a relatively high frequency (1.2 % of the tested isolates) in the Gregorio Marafidn Hospital, Madrid, during the period 1988-1989 (3). These strains were resistant to many other agents, including cefoxitin, piperacillin, ticarcillin-clavulanic acid, ampicillin-sulbactam and clindamycin. Imipenem was a controlled substance in this hospital. The same rate of resistance (1.2 %) among Bacteroidesfragilis was found in 1989-1990 in the San Carlos Hospital, Madrid (2). No resistance to imipenem was found for this group of organisms in other areas of Spain during the period 1981-1987 (4, and G. Prats, Barcelona, personal communication). In our survey we were unable to detect any imipenem-resistant (MIC > 8 mg/l) strains among Bacteroides of the fragilis group. Nevertheless, a small number of isolates (5 %) had imipenem MICs of 2 to 4 mg/l, although they remained cefoxitin-susceptible (MIC < 16 mg/l). Problems of MIC determinations for imipenem were observed with the new Sceptor panel (false resis-
In our series the rate of metronidazole resistance (MIC > 8 mg/l) among Peptostreptococcus strains was 5.5 %. Among Clostridium perfringens, the rate was 5.5 %, and among other species of the genus Clostridium, 4.5 %. Similar or higher figures were found in another Madrid hospital (5). Eubacterium (carefully excluding microaerophilic gram-positive bacteria) had one of the highest rates of metronidazole resistance, 19 %. Metronidazole resistance among Bacteroides of the fragilis group was infrequent (0.5 %) in our series (2 strains: Bacteroides thetaiotaomicron and Bacteroides uniformis). In another Madrid hospital the rate was higher, 2 % during 19861989, involving Bacteroides fragilis, Bacteroides thetaiotaomicron and Bacteroides vulgatus (3). This resistance was not observed in other Madrid hospitals during 198%1990 (2) or in other areas of Spain, at least during the period 1975-1988 (4). In our survey a slightly higher rate of metronidazole resistance (1.9 %) was found among non-fragUis and non-black pigmented Bacteroides, among Bacteroides ureotyticus (9 %) and, unexpectedly, in Veillonellaparvula (7.5 %). No resistance to metronidazole was detected among Fusobacterium or black-pigmented Bacteroides.
Erythromydn Resistance For Peptostreptococcus the rate of erythromycin resistance (> 2 mg/1) was 75 %, Different rates of resistance were found for the different species: Peptostreptococcus micros 27.5 %, Peptostreptococcus anaerobius 39 %, Peptostreptococcus magnus 84 %, Peptostreptococcus prevotii 85 % and Peptostreptococcus asaccharolyticus 92.5 %. Erythromycin susceptibility testing should not be discontinued, as clindamycin-susceptible strains sharing erythromycin resistance may harbour an inducible macrolide-lincosamide resistance mechanism. Constitutively erythromycin-clindamycin resistant mutants can be obtained from inducibly resistant strains (6). A high rate of erythromycin resistance (> 2 mg/l), 37 %, was found for Clostridium perfringens. This rate was lower for the other members of the genus
Voi. 11,1992
Clostridium, 21.5 %. Resistance in Eubacterium reached 24.5 %. An extremely high rate of erythromycin resistance (94 %) was found in Fusobacterium. High rates of resistance have also been found in other surveys in Madrid hospitals (2). It will be of interest to ascertain to what extent this corresponds to acquired resistance, as similar rates were found by American authors in the 1970s. Erythromycin resistance in Veillonella was also unexpectedly high (66 %) in our series. Erythromycin resistance among Bacteroides of the fragilis group reached 85 % in our hospital, while that among black-pigmented Bacteroides was 8 %.
1019
two surveys of 100 faecal specimens from healthy and untreated persons performed in our hospital in 1983 and 1988, rates of resistance in Bacteroides fragilis were 33 % and 27 %, respectively; in Bacteroides vulgatus, 17 % and 42 %; in Bacteroides distasonis, 27 % and 47 %; in Bacteroides ovatus, 34 % and 29 %; and in Bacteroides uniformis, 27 % and 26 %. Despite the very high rate of erythromycin resistance among Fusobacterium isolates, clindamycin resistance remained very low (3 %), which suggested the presence of a mechanism other than reduction of activity of macrolides and lincosamides by 23SrRNA methylase.
Clindamycin Resistance
The clindamycin resistance rate (MIC > 4 mg/1) for Peptostreptococcus was 19 %. In accordance with the hypothesis that clindamycin resistance develops from inducible erythromycin resistance, the Peptostreptococcus species exhibiting increased resistance to clindamycin were those with the highest rates of erythromycin resistance: Pep-
tostreptococcus magnus, Peptostreptococcus prevotii and Peptostreptococcus asaccharolyticus. A total of 10.5 % of Clostridium perfringens isolates and 19.5 % of other Clostridium strains were resistant to clindamycin. In Eubacterium the rate of clindamycin resistance was 9 %. The overall resistance rate (MIC > 4 mg/l) for Bacteroides of the fragilis group was 20.5 %. Resistance rates varied for the different species, from 13.5 % for Bacteroides fragilis, 20 % for Bacteroides uniformis, 26.5 % for Bacteroides vulgatus, 29 % for Bacteroides thetaiotaomicron, 3 4 % for Bacteroides distasonis to 36.1% for Bacteroides ovatus. During the last 12 years, rates of resistance to clindamycin have increased from 0 % (before 1979) to 30 % (1991) in our hospital. Several peaks in resistance rates have been observed, some of them correlating with a higher rate of isolation in clinical samples of some highly resistant species such as Bacteroides distasonis. Similar or higher peaks of clindamycin resistance (45 %) have been found in other Madrid hospitals (3, 8) among Bacteroides of thefragilis group. Geographical differences are clear: in the Universitary Hospital of Salamanca, the findings were different from those of the Madrid institutions, with an overall resistance rate maintained below 8 % in recent years (4). Faecal isolates of Bacteroides have been implicated as a reservoir of clindamycin resistance. In
Resistance to Other A g e n t s
Resistance to amoxicillin-clavulanic acid (MIC > 4/8 mg/1) was found in a limited number (1.5 %) of Peptostreptococcus strains, mostly among penicillin-resistant isolates of Peptostreptococcus anaerobius. Similarly, a low rate of amoxicillinclavulanic acid resistance (3.5 %) was detected among Bacteroides of the fragilis group. The lower rates were found among Bacteroides fragilis, Bacteroides vulgatus and Bacteroides ovatus (less than 1.5 %), and the highest among Bacteroides thetaiotaomicron (7.5 %) and Bacteroides distasonis (10 %). The more resistant strains generally exhibited resistance to piperacillin (> 128 mg/l), but resistance to cefoxitin was not unusually increased. Chloramphenicol resistance (MIC > 8 mg/1) was detected in 3 % of our Peptostreptococcus isolates, particularly among Peptostreptococcus magnus. Despite the observed presence in Spain of a transferable plasmid encoding high-level chloramphenicol resistance in Bacteroides in 1985 (9), resistance remains extremely infrequent (a single strain among 411 isolates tested).
References
1. Baquero F, Loza E: Penicillin resistance in Spain. Infectious Diseases in Clinical Practice 1992, 1: 147-148. 2. BetrifiC, Campos E, Picazo J J: Susceptibilitypatterns of anaerobic bacteria isolated over a ten-month period. Revista Espafiola de Quimioterapia 1991, 4: 237-239. 3. Pel~iez MT, Cercenado E, Rodrlguez-Crrixems M, Bouza E: Resistance of anaerobic bacteria to antimicrobial agents. Reviews of Infectious Diseases 1991,
13: 183.
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4. Garcla-Roddguez JA, Garc/a-S~nchez JE: Evolution of antimicrobial susceptibility in isolates of the Bacteroides fragilis group in Spain. Reviews of Infectious Diseases 1990, 12, Supplement 2: 142-151. 5. Betri6 C, Moreno JJ, Cuadros JA, Garc/a G, Cruceyra A, Picazo JJ: Sensibilidad de Bacteroides grupo fragilis a nuevos agentes 15-1act~imicos. Revista Espafiola de MicrobioIogfa Clfnica 1989, 4: 652657. 6. Reig M, Moreno A, Baquero F: Resistance of Peptostreptococcus spp. to macrolides and lincosamides: inducible and constitutive phenotypes. Antimicrobial Agents and Chemotherapy 1992, 36: 662--664.
Eur. J. Clin. Microbiol. Infect. Dis.
7. l~negoid SM: Antimicrobial agents susceptibility of anaerobic bacteria. In: Finegold SM (ed): Anaerobic bacteria in human diseases. Academic Press, New York, 1977, p. 513-531. 8. Betrifi C, Campos E, Cabronero C, Roddguez.Avial C, Picazo JJ: Susceptibility of species of the Bacteroidesfragilis group to 10 antimicrobial agents. Antimicrobial Agents and Chemotherapy 1990, 34: 671673. 9. Martlnez.Sufirez JV, Baquero F, Rcig M, P6rez-Dfaz JC: Transferable plasmid-linked chloramphenicol acetyltransferase conferring high level resistance in Bacteroides uniformb. Antimicrobial Agents and Chemotherapy 1985, 28: 113-117.
