926
8. McCracken GH, Ginsburg CM, Clashen JC, Thomas ML" Pharmacokinetics of cefaclor in infants and children. Journal of Antimicrobial Chemotherapy 1978, 4: 515-521. 9. Ginsburg GM, MeCraken GH, Petruska M, Olson K' Pharmacokinetics and bactericidal activity of cefuroxime axetil. Antimicrobial Agents and Chemotherapy 1985, 28: 504-507. 10. Faulkner RD, Yacobi A, Barone JS, Kaplan SA, Silber BM: Pharmacokinetic profile of cefixime in man. Pediatric Infectious Disease Journal 1987, 6: 963-970. 11. Ginsburg CM: Pharmacology of erythromycin in infants and children. Pediatric Infectious Disease Journal 1986, 5: 124-129. 12. Pearson RD, Steigbigel liT, Davis liT, Chapman SW: Method for reliable determination of minimal lethal antibiotic concentrations. Antimicrobial Agents and Chemotherapy 1980, 18: 699-708. 13. Yourassowsky E, Van der Linden MP, Crokaert F, Glupczynski Y: Effect of antibiotic carry-over on bacterial counting by "spiral plating". Journal of Antimicrobial Chemotherapy 1988, 21: 138-140. 14. Yourassowsky E, Van der Linden MP, Lismont MJ, Crokaert F, Glupczynski Y: Rate of bactericidal activity for Branhamella catarrhalis of a new macrolide, CP 62,993, compared with that for amoxicillin-clavulanic acid. Chemotherapy 1988, 34: 191-194. 15. Doern GV, ,lones RN: Antimicrob[al susceptibility testing of Haemophilus influenzae, Branhamella catarrhalis and Neisseria gonorrhoea. Antimicrobial Agents and Chemotherapy 1988, 32: 1747-1753. 16. Wallace RJ, Nash DR, Steingrube VA: Antibiotic susceptibilities and drug resistance in Moraxella (Branhamella) catarrhalis. American Journal of Medicine 1990, 88, Supplement 5A: 46-50. 17. Vogelman B, Craig WA: Kinetics of antimicrobial activity. Journal of Pediatrics 1986, 108: 835--840.
Eur. J. Clin. Microbiol. Infect. Dis.
Error Rates in Cefoperazone and Cefoperazone-Sulbactam Disk Tests with Enterobacteriaceae and
Pseudomonas aeruginosa D.J. H a r d y 1., A . L . B a r r y 2, R C . F u c h s 3, E . H . G e r l a c h 4, J.C. M c L a u g h l i n 5, M.A. Pfaller 6
In a Collaborative study involving five medical centers, 6 % of 2,440 consecutive isolates of Enterobacteriaceae were resistant to cefoperazone; resistance to cefoperazone was reduced to < 1 % by the addition of suibactam. Susceptibility to cefoperazone and cefoperazonesulbactam was accurately predicted by disk diffusion tests. Resistance to cefoperazone, however, was not as reliably detected by disk tests and results of dilution tests were not always consistent. The prevalence of resistance to cefoperazone and/or the ability to detect resistance had a significant influence on very major error rates for individual laboratories.
Previous studies on the correlation of cefoperazone disk test results with M I C s of c e f o p e r a z o n e either alone (1, 2) or in combination with sulbactam (3) were p e r f o r m e d with selected gram-positive and gram-negative clinical isolates and stock strains. T h e strains tested in these studies were at least partly selected to achieve a desired species representation and for IMactamase production. E r r o r rates for disk tests with such skewed populations of microorganisms do not necessarily reflect the error rates that would be expected with the types of strains encountered in the routine clinical setting.
1Department of Microbiology and Immunology, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, New York 14642-8710, USA. 2The Clinical Microbiology Institute, Tualatin, Oregon 97062, USA. 3St. Vincent Hospital and Medical Center, Portland, Oregon 97225, USA. 4St. Francis Regional Medical Center, Wichita, Kansas 67214, USA. s University of New Mexico Medical Center, Albuquerque, New Mexico 87106, USA. 6University of Iowa Medical Center, Iowa City, Iowa 52242, USA.
Vo1.11,1992
In this report we describe the results of a fivelaboratory survey that was designed to document the interpretive error rates when 75 /ag cefoperazone disks and 73/30 lag cefoperazone-sulbactam disks were tested against consecutively isolated Enterobacteriaceae and Pseudomonas aeruginosa. Agar disk test results were compared with those obtained with a standard broth microdilution test using cefoperazone alone and cefoperazone combined with sulbactam in a fixed 2:1 ratio (4, 5).
Materials and Methods. Disk diffusion and broth microdilution susceptibility tests were performed according to procedures described by the National Committee for Clinical Laboratory Standards (6, 7). A common lot of broth microdilution trays was prepared and distributed to each clinical laboratory by Prepared Media Laboratories (USA). MICs of cefoperazone were determined alone and in combination with sulbactam at a fixed ratio of two parts cefoperazone to one part sulbactam. Disk tests were performed with cefoperazone alone (75 tag/disk) and in combination with sulbactam (75 lag cefoperazone and 30~ag sulbactam); commercially prepared disks were distributed to all laboratories from a common source. Each of five. participating laboratories (designated as laboratories A to E) performed broth microdilution susceptibility tests and disk diffusion tests on approximately 500 isolates of Enterobacteriaceae and 50 isolates of Pseudomonas aeruginosa. The organisms were isolated consecutively from clinical specimens in the individual laboratories in the period from November 1990 to January 1991, and were deemed clinically significant by requirements for identification and susceptibility testing. The species and numbers of isolates tested were Citrobacter amalonaticus (7), Citrobacter diversus (20), Citrobacter freundii (69), Enterobacter aerogenes (73), Enterobacter agglomerans (8), Enterobacter cloacae (154), Escherichia coli (1336), Klebsiella oxytoca (79), Klebsiella pneumoniae (366), Morganella morganii (24), Proteus mirabilis (162), Proteus vulgaris (18), Providencia rettgeri (7), Providencia stuartii (8), Pseudomonas aeruginosa (254), Salmonella species (6), Serratia liquefaciens (7), Serratia marcescens (65), Shigella sonnei (14), and other Enterobacteriaceae (17). To ensure quality and comparability of test results among laboratories, each laboratory performed 19 to 24 tests with three standard control strains (Pseudomonas aeruginosa ATCC 27853, Es-
925
cherichia coli ATCC 25922 and Escherichia coli ATCC 35218). An additional twenty reference cross-over strains (10 Escherichia coli, 5 Citrobacter freundii and 5 Klebsiella pneumoniae) were tested by all participating laboratories as described previously (8). Results and Discussion. MICs of cefoperazone alone and with sulbactam combined in a 2:1 ratio were determined for 2,440 consecutive clinical isolates of Enterobacteriaceae. For these isolates, MICs for all strains and MIC90s of cefoperazonesulbactam were generally four- to eight-fold lower than MICs of cefoperazone alone. The addition of sulbactam had the greatest effect on MICs of cefoperazone for isolates of Citrobacter, Enterobacter, Escherichia coli, Morganella morganii, Proteus, Salmonella and Serratia. At the range of concentrations tested, MICs of cefoperazone for Klebsiella and Shigella were minimally affected by the addition of sulbactam. The number of cefoperazone-resistant Enterobacteriaceae, i.e. strainswith cefoperazone MICs > 64 ~tg/ml, was decreased from 153 to 20 (87 % reduction) by the addition of sulbactam. MIC90s of cefoperazone for 254 isolates of Pseudomonas aeruginosa were decreased only twofold by the addition of sulbactam; the number of strains resistant to cefoperazone was decreased from 15 to 9 (40 % reduction) by the addition of sulbactam. Interpretive categories obtained from scattergrams for the combined results of all laboratories and for individual laboratories with Enterobacteriaceae are summarized in Table 1. Six percent of all isolates (153 of 2440) were resistant to cefoperazone in dilution susceptibility tests. The rate of false-susceptible cefoperazone disk test results (very major errors) expressed as a percent of the total population was 2 % (43 of 2440); very major error rates for laboratories A to E were 0.2 % (1 of 495), 4 % (18 of 465), 2 % (10 of 471), 2 % (12 of 504) and 0.4 % (2 of 505), respectively. When expressed as a percent of the resistant population, the rate of false-susceptible cefoperazone disk test results for all laboratories was 28 % (43 of 153); very major error rates for laboratories A to E were 20 % (1 of 5), 43 % (18 of 42), 20 % (10 of 50), 43 % (12 of 28) and 7 % (2 of 28), respectively. False-susceptible cefoperazone disk test results were observed with thirty-two strains of Escherichia coli, four Enterobacter, two Klebsiella pneumoniae, two Proteus mirabitis, two Serratia and one Citrobacter amatonaticus. When cefoperazone disks were tested against Pseudomonas aeruginosa, 1 % of
928
Eur. J. Clin. Microbiol. Infect. Dis.
Table 1: Interpretive agreement between broth microdilution and agar disk diffusion susceptibility tests for cefoperazone and eefoperazone-sulbaetam.
No, (%) of strains in each MIC interpretive category a Cefoperazone Laboratory All Labs
Lab A
Lab B
Lab C
Lab D
Lab E
Disk test result b
R
I
R
47 (2)
I
63 (3)
17 (1)
2 (0.1)
S
43 (2)
22 (1)
Ce foperazone/Sulbactam S
R
I'(< 0.1) 18 (1) 2227 (91)
I
8 (0.3)
4 (0.2)
9 (0.4)
23 (0.9)
19 (0.8)
3 (0.1)
9 (0.4)
2363 (97)
R
3 (0,6)
2 (0.4)
0
0
0
I
1 (0.2)
4 (0.8)
8 (1.6)
0
3 (0.6)
S
1 (0.2)
3 (0.6)
473 (96)
1 (0.2)
1 (0.2)
4 (0.9)
R
0
0
3 (0.6)
0
I
20 (4)
1 (0.2)
1 (0.2)
1 (0.2)
2 (0,4)
S
18 (4)
4 (0.9)
417 (90)
2 (0.4)
3 (0.6)
R
19 (4)
0
1 (0.2)
3 (0.6)
2 (0.4)
I
21 (4.4)
6 (1.3)
2 (0.4)
3 (0.6)
6 (1.3)
S
10 (2)
5 (1)
0
3 (0.6)
407 (86)
S
R
9 (1.8)
0
0
1 (0.2)
0
I
7 (1,4)
3 (0.6)
1 (0.2)
4 (0,8)
3 (0.6)
S
12 (2.4)
8 (1.6)
464 (92)
0
2 (0.4)
R
12 (2.4)
0
0
1 (0.2)
2 (0.4)
I
14 (2.8)
3 (0.6)
6 (1.2)
1 (0.2)
9 (1.8)
S
2 (0.4)
2 (0.4)
466 (92)
0
0
0
0 2 (0.4) 488 (98) 0 0 454 (98) 0 11 (2) 443 (94) 0 3 (0.6) 491 (97) 0 3 (0.6) 487 (97)
a MIC breakpoints are susceptible (S) = ~ 16 ~tg/ml, intermediate (I) = 32 ~g/ml, and resistant (R) = > 64 lag/ml. bDisk diffusion breakpoints are susceptible = >_21 ram, intermediate = 16-20 mm, resistant = _< 15 ram.
Table2: Reproducibility ofcefoperazone susceptibility tests with 6 of 20 reference strains tested in five separate laboratories. Organism & culture no.
Cefoperazone test results
Test method a'b Lab A
Lab B
Lab C
Lab D
Lab E
E. coil, no. 1
MIC Disk
8 S
128 I
256 I
64 I
128 I
E. coli, no. 2
MIC Disk
8 S
256 I
> 256 I
32 S
64 S
E. coli, no. 3
MIC Disk
8 S
64 I
256 I
16 S
32 S
E. coli, no.4
MIC Disk
8 S
16 !
>256 I
8 S
8 S
C. freundii, no. 5
MIC Disk
32 I
256 I
> 256 I
16 S
32 I
K. pneumoniae, no. 6
MIC Disk
16 I
32 I
> 256 I
8 S
16 I
a Broth dilution MICs are expressed in pg/ml. bDisk diffusion results are expressed as susceptible (S), intermediate (I), and resistant (R); see footnote to Table 1 for breakpoint values.
Vo1.11,1992
all strains (3 of 254) or 20 % (3 of 15) of the resistant subpopulation produced very major errors (data not shown). For the combination of cefoperazone-sulbactam against Enterobacteriaceae, less than 1 % of all isolates (20 of 2438) were resistant in dilution susceptibility tests. The very major error rate of cefoperazone-sulbactam disk tests expressed as a percent of the total population was 0.1% (3 of 2438); very major error rates for laboratories A to E were 0.2 % (1 of 495), 0.4 % (2 of 465), 0 % (0 of 471), 0 % (0 of 504) and 0 % (0 of 503), respectively. When expressed as a percent of the resistant population, the rate of false-susceptible cefoperazone-sulbactam disk test results was 15 % (3 of 20); very major error rates for laboratories A to E were 100 % (1 of 1), 33 % (2 of 6), 0 % (0 of 6), 0 % (0 of 5) and 0 % (0 of 2), respectively. False-susceptible disk test results were observed with two strains of Escherichia coli and one strain of Enterobacter aerogenes. No very major errors were observed in cefoperazone-sulbactam disk tests with Pseudomonas aeruginosa (data not shown). Major error rates, i.e. false-resistant disk test results, in cefoperazone and cefoperazone-sulbactam disk tests with Enterobacteriaceae did not differ and were < 0.1%. The minor error rate in cefoperazone disk tests, however, was 4 % while that in cefoperazone-sulbactam was 2 %. No major errors and 5 % minor errors were observed in both cefoperazone and cefoperazone-sulbactam disk tests with Pseudomonas aeruginosa (data not shown). In 108 replicate tests, MICs of cefoperazone and cefoperazone-sulbactam for Escherichia coli ATCC 25922 were _21 m m to >_22 mm, 2 23 ram, _>24 m m or _>25 mm, very m a j o r errors expressed as a p e r c e n t a g e of the total population tested would be r e d u c e d f r o m 1,8 % to 1.3 %, 0.8 %, 0.5 % and 0.4 %, respectively. While minimizing very m a j o r error rates, these changes in susceptibility breakpoints would increase minor errors f r o m 4,3 % to 5.2 %, 7.8 %, 9.3 % and 11.3 %, respectively. T h e merit of such changes in disk test criteria is worth further consideration but no such changes are r e c o m m e n d e d at this time.
References 1. ThornsberryC, BarryAL, Jones RN, Baker CN, Badal RE: Tentative interpretive standards for agar disk diffusion antimicrobial susceptibility testing of cefoperazone. Journal of Clinical Microbiology 1982, 15: 769-776. 2. Jones RN, Gaven TL, Barry AL, Thornsberry C, Gibbs DL: Cefoperazone disk diffusion susceptibility test: confirmation of the tentative interpretive criteria, cross-resistance, and determination af quality control performance limits. Journal of Clinical Microbiology 1982, 15: 777-786.
Eur. J. Clin. Microbiol. Infect. Dis.
3. Jones RN, Barry AL, Thornsberry C, Wilson HW: The cefoperazone sulbactam combination: in vitro qualities including beta-lactamase stability, antimicrobial activity, and interpretive criteria for disk diffusion tests. American Journal of Clinical Pathology 1985, 84: 496-504. 4. Jones RN, Barry AL, Packer RR, Gregory WW, Thornsberry C: In vitro antimicrobial spectrum, occurrence of synergy, and recommendations for dilution susceptibility testing concentrations of the cefoperazone-sulbactam combination. Journal of Clinical Microbiology 1987, 25: 1725-1729. 5. BarryAL, Jones RL, The CollaborativeAntimicrobial Susceptibility Testing Group: Criteria for disk susceptibility tests and quality control guidelines for the cefoperazone-sulbactam combination. Journal of Clinical Microbiology 1988, 26: 13-17. 6. National Committee for Clinical Laboratory Standards: Performance standards for antimicrobial disk susceptibility tests. Approved standard M2-A2. NCCLS, Villanova, PA, 1990. 7. National Committee for Clinical Laboratory Standards:Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically. Approved Standard M7-A2. NCCLS, Villanova, PA, 1990. 8. Barry AL, Fuchs PC, Gerlach EH, Hardy DJ, MeLaughlin JC, Pfaller MA: Tiearcillin and ticarcillinclavulanic acid susceptibility tests: error rates for disk tests with consecutively isolated membe1~ of the family Enterobacteriaceae. Antimicrobial Agents and Chemotherapy 1992, 36: 137-143.
A Simple Medium for the Primary Isolation of Haemophilus ducreyi Y. D a n g o r * , S.D. Miller, H . J . K o o r n h o f , R.C. Ballard
Two simple, inexpensive media containing gonococcal agar-base, supplemented with 5 % Fildes' extract and either chocolated or unchocolated horse blood (GC-FHBC or GC-FHB) were compared with the standard gonococcal agarbased (GC-HgS) and Mueller-Hinton agar-based media (MH-HB) normally used for primary isolation of Haemophilus ducreyi from presumptive chancroid lesions. Overall, Haemophilus ducreyi was recovered from 162 of 178 (91%) samples from primary chancroid lesions. As a single isolation medium GC-HgS proved the most sensitive Emergent Pathogen Research Unit, School of Pathology, University of the Witwatersrand and the South African Institute for Medical Research, PO Box 1038, Johannesburg 2000, Republic of South Africa.
8. McCracken GH, Ginsburg CM, Clashen JC, Thomas ML" Pharmacokinetics of cefaclor in infants and children. Journal of Antimicrobial Chemotherapy 1978, 4: 515-521. 9. Ginsburg GM, MeCraken GH, Petruska M, Olson K' Pharmacokinetics and bactericidal activity of cefuroxime axetil. Antimicrobial Agents and Chemotherapy 1985, 28: 504-507. 10. Faulkner RD, Yacobi A, Barone JS, Kaplan SA, Silber BM: Pharmacokinetic profile of cefixime in man. Pediatric Infectious Disease Journal 1987, 6: 963-970. 11. Ginsburg CM: Pharmacology of erythromycin in infants and children. Pediatric Infectious Disease Journal 1986, 5: 124-129. 12. Pearson RD, Steigbigel liT, Davis liT, Chapman SW: Method for reliable determination of minimal lethal antibiotic concentrations. Antimicrobial Agents and Chemotherapy 1980, 18: 699-708. 13. Yourassowsky E, Van der Linden MP, Crokaert F, Glupczynski Y: Effect of antibiotic carry-over on bacterial counting by "spiral plating". Journal of Antimicrobial Chemotherapy 1988, 21: 138-140. 14. Yourassowsky E, Van der Linden MP, Lismont MJ, Crokaert F, Glupczynski Y: Rate of bactericidal activity for Branhamella catarrhalis of a new macrolide, CP 62,993, compared with that for amoxicillin-clavulanic acid. Chemotherapy 1988, 34: 191-194. 15. Doern GV, ,lones RN: Antimicrob[al susceptibility testing of Haemophilus influenzae, Branhamella catarrhalis and Neisseria gonorrhoea. Antimicrobial Agents and Chemotherapy 1988, 32: 1747-1753. 16. Wallace RJ, Nash DR, Steingrube VA: Antibiotic susceptibilities and drug resistance in Moraxella (Branhamella) catarrhalis. American Journal of Medicine 1990, 88, Supplement 5A: 46-50. 17. Vogelman B, Craig WA: Kinetics of antimicrobial activity. Journal of Pediatrics 1986, 108: 835--840.
Eur. J. Clin. Microbiol. Infect. Dis.
Error Rates in Cefoperazone and Cefoperazone-Sulbactam Disk Tests with Enterobacteriaceae and
Pseudomonas aeruginosa D.J. H a r d y 1., A . L . B a r r y 2, R C . F u c h s 3, E . H . G e r l a c h 4, J.C. M c L a u g h l i n 5, M.A. Pfaller 6
In a Collaborative study involving five medical centers, 6 % of 2,440 consecutive isolates of Enterobacteriaceae were resistant to cefoperazone; resistance to cefoperazone was reduced to < 1 % by the addition of suibactam. Susceptibility to cefoperazone and cefoperazonesulbactam was accurately predicted by disk diffusion tests. Resistance to cefoperazone, however, was not as reliably detected by disk tests and results of dilution tests were not always consistent. The prevalence of resistance to cefoperazone and/or the ability to detect resistance had a significant influence on very major error rates for individual laboratories.
Previous studies on the correlation of cefoperazone disk test results with M I C s of c e f o p e r a z o n e either alone (1, 2) or in combination with sulbactam (3) were p e r f o r m e d with selected gram-positive and gram-negative clinical isolates and stock strains. T h e strains tested in these studies were at least partly selected to achieve a desired species representation and for IMactamase production. E r r o r rates for disk tests with such skewed populations of microorganisms do not necessarily reflect the error rates that would be expected with the types of strains encountered in the routine clinical setting.
1Department of Microbiology and Immunology, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, New York 14642-8710, USA. 2The Clinical Microbiology Institute, Tualatin, Oregon 97062, USA. 3St. Vincent Hospital and Medical Center, Portland, Oregon 97225, USA. 4St. Francis Regional Medical Center, Wichita, Kansas 67214, USA. s University of New Mexico Medical Center, Albuquerque, New Mexico 87106, USA. 6University of Iowa Medical Center, Iowa City, Iowa 52242, USA.
Vo1.11,1992
In this report we describe the results of a fivelaboratory survey that was designed to document the interpretive error rates when 75 /ag cefoperazone disks and 73/30 lag cefoperazone-sulbactam disks were tested against consecutively isolated Enterobacteriaceae and Pseudomonas aeruginosa. Agar disk test results were compared with those obtained with a standard broth microdilution test using cefoperazone alone and cefoperazone combined with sulbactam in a fixed 2:1 ratio (4, 5).
Materials and Methods. Disk diffusion and broth microdilution susceptibility tests were performed according to procedures described by the National Committee for Clinical Laboratory Standards (6, 7). A common lot of broth microdilution trays was prepared and distributed to each clinical laboratory by Prepared Media Laboratories (USA). MICs of cefoperazone were determined alone and in combination with sulbactam at a fixed ratio of two parts cefoperazone to one part sulbactam. Disk tests were performed with cefoperazone alone (75 tag/disk) and in combination with sulbactam (75 lag cefoperazone and 30~ag sulbactam); commercially prepared disks were distributed to all laboratories from a common source. Each of five. participating laboratories (designated as laboratories A to E) performed broth microdilution susceptibility tests and disk diffusion tests on approximately 500 isolates of Enterobacteriaceae and 50 isolates of Pseudomonas aeruginosa. The organisms were isolated consecutively from clinical specimens in the individual laboratories in the period from November 1990 to January 1991, and were deemed clinically significant by requirements for identification and susceptibility testing. The species and numbers of isolates tested were Citrobacter amalonaticus (7), Citrobacter diversus (20), Citrobacter freundii (69), Enterobacter aerogenes (73), Enterobacter agglomerans (8), Enterobacter cloacae (154), Escherichia coli (1336), Klebsiella oxytoca (79), Klebsiella pneumoniae (366), Morganella morganii (24), Proteus mirabilis (162), Proteus vulgaris (18), Providencia rettgeri (7), Providencia stuartii (8), Pseudomonas aeruginosa (254), Salmonella species (6), Serratia liquefaciens (7), Serratia marcescens (65), Shigella sonnei (14), and other Enterobacteriaceae (17). To ensure quality and comparability of test results among laboratories, each laboratory performed 19 to 24 tests with three standard control strains (Pseudomonas aeruginosa ATCC 27853, Es-
925
cherichia coli ATCC 25922 and Escherichia coli ATCC 35218). An additional twenty reference cross-over strains (10 Escherichia coli, 5 Citrobacter freundii and 5 Klebsiella pneumoniae) were tested by all participating laboratories as described previously (8). Results and Discussion. MICs of cefoperazone alone and with sulbactam combined in a 2:1 ratio were determined for 2,440 consecutive clinical isolates of Enterobacteriaceae. For these isolates, MICs for all strains and MIC90s of cefoperazonesulbactam were generally four- to eight-fold lower than MICs of cefoperazone alone. The addition of sulbactam had the greatest effect on MICs of cefoperazone for isolates of Citrobacter, Enterobacter, Escherichia coli, Morganella morganii, Proteus, Salmonella and Serratia. At the range of concentrations tested, MICs of cefoperazone for Klebsiella and Shigella were minimally affected by the addition of sulbactam. The number of cefoperazone-resistant Enterobacteriaceae, i.e. strainswith cefoperazone MICs > 64 ~tg/ml, was decreased from 153 to 20 (87 % reduction) by the addition of sulbactam. MIC90s of cefoperazone for 254 isolates of Pseudomonas aeruginosa were decreased only twofold by the addition of sulbactam; the number of strains resistant to cefoperazone was decreased from 15 to 9 (40 % reduction) by the addition of sulbactam. Interpretive categories obtained from scattergrams for the combined results of all laboratories and for individual laboratories with Enterobacteriaceae are summarized in Table 1. Six percent of all isolates (153 of 2440) were resistant to cefoperazone in dilution susceptibility tests. The rate of false-susceptible cefoperazone disk test results (very major errors) expressed as a percent of the total population was 2 % (43 of 2440); very major error rates for laboratories A to E were 0.2 % (1 of 495), 4 % (18 of 465), 2 % (10 of 471), 2 % (12 of 504) and 0.4 % (2 of 505), respectively. When expressed as a percent of the resistant population, the rate of false-susceptible cefoperazone disk test results for all laboratories was 28 % (43 of 153); very major error rates for laboratories A to E were 20 % (1 of 5), 43 % (18 of 42), 20 % (10 of 50), 43 % (12 of 28) and 7 % (2 of 28), respectively. False-susceptible cefoperazone disk test results were observed with thirty-two strains of Escherichia coli, four Enterobacter, two Klebsiella pneumoniae, two Proteus mirabitis, two Serratia and one Citrobacter amatonaticus. When cefoperazone disks were tested against Pseudomonas aeruginosa, 1 % of
928
Eur. J. Clin. Microbiol. Infect. Dis.
Table 1: Interpretive agreement between broth microdilution and agar disk diffusion susceptibility tests for cefoperazone and eefoperazone-sulbaetam.
No, (%) of strains in each MIC interpretive category a Cefoperazone Laboratory All Labs
Lab A
Lab B
Lab C
Lab D
Lab E
Disk test result b
R
I
R
47 (2)
I
63 (3)
17 (1)
2 (0.1)
S
43 (2)
22 (1)
Ce foperazone/Sulbactam S
R
I'(< 0.1) 18 (1) 2227 (91)
I
8 (0.3)
4 (0.2)
9 (0.4)
23 (0.9)
19 (0.8)
3 (0.1)
9 (0.4)
2363 (97)
R
3 (0,6)
2 (0.4)
0
0
0
I
1 (0.2)
4 (0.8)
8 (1.6)
0
3 (0.6)
S
1 (0.2)
3 (0.6)
473 (96)
1 (0.2)
1 (0.2)
4 (0.9)
R
0
0
3 (0.6)
0
I
20 (4)
1 (0.2)
1 (0.2)
1 (0.2)
2 (0,4)
S
18 (4)
4 (0.9)
417 (90)
2 (0.4)
3 (0.6)
R
19 (4)
0
1 (0.2)
3 (0.6)
2 (0.4)
I
21 (4.4)
6 (1.3)
2 (0.4)
3 (0.6)
6 (1.3)
S
10 (2)
5 (1)
0
3 (0.6)
407 (86)
S
R
9 (1.8)
0
0
1 (0.2)
0
I
7 (1,4)
3 (0.6)
1 (0.2)
4 (0,8)
3 (0.6)
S
12 (2.4)
8 (1.6)
464 (92)
0
2 (0.4)
R
12 (2.4)
0
0
1 (0.2)
2 (0.4)
I
14 (2.8)
3 (0.6)
6 (1.2)
1 (0.2)
9 (1.8)
S
2 (0.4)
2 (0.4)
466 (92)
0
0
0
0 2 (0.4) 488 (98) 0 0 454 (98) 0 11 (2) 443 (94) 0 3 (0.6) 491 (97) 0 3 (0.6) 487 (97)
a MIC breakpoints are susceptible (S) = ~ 16 ~tg/ml, intermediate (I) = 32 ~g/ml, and resistant (R) = > 64 lag/ml. bDisk diffusion breakpoints are susceptible = >_21 ram, intermediate = 16-20 mm, resistant = _< 15 ram.
Table2: Reproducibility ofcefoperazone susceptibility tests with 6 of 20 reference strains tested in five separate laboratories. Organism & culture no.
Cefoperazone test results
Test method a'b Lab A
Lab B
Lab C
Lab D
Lab E
E. coil, no. 1
MIC Disk
8 S
128 I
256 I
64 I
128 I
E. coli, no. 2
MIC Disk
8 S
256 I
> 256 I
32 S
64 S
E. coli, no. 3
MIC Disk
8 S
64 I
256 I
16 S
32 S
E. coli, no.4
MIC Disk
8 S
16 !
>256 I
8 S
8 S
C. freundii, no. 5
MIC Disk
32 I
256 I
> 256 I
16 S
32 I
K. pneumoniae, no. 6
MIC Disk
16 I
32 I
> 256 I
8 S
16 I
a Broth dilution MICs are expressed in pg/ml. bDisk diffusion results are expressed as susceptible (S), intermediate (I), and resistant (R); see footnote to Table 1 for breakpoint values.
Vo1.11,1992
all strains (3 of 254) or 20 % (3 of 15) of the resistant subpopulation produced very major errors (data not shown). For the combination of cefoperazone-sulbactam against Enterobacteriaceae, less than 1 % of all isolates (20 of 2438) were resistant in dilution susceptibility tests. The very major error rate of cefoperazone-sulbactam disk tests expressed as a percent of the total population was 0.1% (3 of 2438); very major error rates for laboratories A to E were 0.2 % (1 of 495), 0.4 % (2 of 465), 0 % (0 of 471), 0 % (0 of 504) and 0 % (0 of 503), respectively. When expressed as a percent of the resistant population, the rate of false-susceptible cefoperazone-sulbactam disk test results was 15 % (3 of 20); very major error rates for laboratories A to E were 100 % (1 of 1), 33 % (2 of 6), 0 % (0 of 6), 0 % (0 of 5) and 0 % (0 of 2), respectively. False-susceptible disk test results were observed with two strains of Escherichia coli and one strain of Enterobacter aerogenes. No very major errors were observed in cefoperazone-sulbactam disk tests with Pseudomonas aeruginosa (data not shown). Major error rates, i.e. false-resistant disk test results, in cefoperazone and cefoperazone-sulbactam disk tests with Enterobacteriaceae did not differ and were < 0.1%. The minor error rate in cefoperazone disk tests, however, was 4 % while that in cefoperazone-sulbactam was 2 %. No major errors and 5 % minor errors were observed in both cefoperazone and cefoperazone-sulbactam disk tests with Pseudomonas aeruginosa (data not shown). In 108 replicate tests, MICs of cefoperazone and cefoperazone-sulbactam for Escherichia coli ATCC 25922 were _21 m m to >_22 mm, 2 23 ram, _>24 m m or _>25 mm, very m a j o r errors expressed as a p e r c e n t a g e of the total population tested would be r e d u c e d f r o m 1,8 % to 1.3 %, 0.8 %, 0.5 % and 0.4 %, respectively. While minimizing very m a j o r error rates, these changes in susceptibility breakpoints would increase minor errors f r o m 4,3 % to 5.2 %, 7.8 %, 9.3 % and 11.3 %, respectively. T h e merit of such changes in disk test criteria is worth further consideration but no such changes are r e c o m m e n d e d at this time.
References 1. ThornsberryC, BarryAL, Jones RN, Baker CN, Badal RE: Tentative interpretive standards for agar disk diffusion antimicrobial susceptibility testing of cefoperazone. Journal of Clinical Microbiology 1982, 15: 769-776. 2. Jones RN, Gaven TL, Barry AL, Thornsberry C, Gibbs DL: Cefoperazone disk diffusion susceptibility test: confirmation of the tentative interpretive criteria, cross-resistance, and determination af quality control performance limits. Journal of Clinical Microbiology 1982, 15: 777-786.
Eur. J. Clin. Microbiol. Infect. Dis.
3. Jones RN, Barry AL, Thornsberry C, Wilson HW: The cefoperazone sulbactam combination: in vitro qualities including beta-lactamase stability, antimicrobial activity, and interpretive criteria for disk diffusion tests. American Journal of Clinical Pathology 1985, 84: 496-504. 4. Jones RN, Barry AL, Packer RR, Gregory WW, Thornsberry C: In vitro antimicrobial spectrum, occurrence of synergy, and recommendations for dilution susceptibility testing concentrations of the cefoperazone-sulbactam combination. Journal of Clinical Microbiology 1987, 25: 1725-1729. 5. BarryAL, Jones RL, The CollaborativeAntimicrobial Susceptibility Testing Group: Criteria for disk susceptibility tests and quality control guidelines for the cefoperazone-sulbactam combination. Journal of Clinical Microbiology 1988, 26: 13-17. 6. National Committee for Clinical Laboratory Standards: Performance standards for antimicrobial disk susceptibility tests. Approved standard M2-A2. NCCLS, Villanova, PA, 1990. 7. National Committee for Clinical Laboratory Standards:Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically. Approved Standard M7-A2. NCCLS, Villanova, PA, 1990. 8. Barry AL, Fuchs PC, Gerlach EH, Hardy DJ, MeLaughlin JC, Pfaller MA: Tiearcillin and ticarcillinclavulanic acid susceptibility tests: error rates for disk tests with consecutively isolated membe1~ of the family Enterobacteriaceae. Antimicrobial Agents and Chemotherapy 1992, 36: 137-143.
A Simple Medium for the Primary Isolation of Haemophilus ducreyi Y. D a n g o r * , S.D. Miller, H . J . K o o r n h o f , R.C. Ballard
Two simple, inexpensive media containing gonococcal agar-base, supplemented with 5 % Fildes' extract and either chocolated or unchocolated horse blood (GC-FHBC or GC-FHB) were compared with the standard gonococcal agarbased (GC-HgS) and Mueller-Hinton agar-based media (MH-HB) normally used for primary isolation of Haemophilus ducreyi from presumptive chancroid lesions. Overall, Haemophilus ducreyi was recovered from 162 of 178 (91%) samples from primary chancroid lesions. As a single isolation medium GC-HgS proved the most sensitive Emergent Pathogen Research Unit, School of Pathology, University of the Witwatersrand and the South African Institute for Medical Research, PO Box 1038, Johannesburg 2000, Republic of South Africa.
