JOURNAL OF CLINICAL MICROBIOLOGY, Aug. 2007, p. 2758–2759 0095-1137/07/$08.00⫹0 doi:10.1128/JCM.00569-07
Vol. 45, No. 8
Agar Dilution and Disk Diffusion Susceptibility Testing of Campylobacter spp. poultry isolates alone in establishing disk diffusion breakpoints and their subsequent application in human clinical testing requires reconsideration to be consistent with the M23-A2 guidance document (6). In summary, the study by Luangtongkum et al. addresses an important topic and demonstrates the feasibility of a reliable susceptibility-testing alternative to dilution techniques. We suggest that qualified multilaboratory disk diffusion QC studies using a blood-containing medium now be performed, followed by additional testing of human-source isolates. Subsequent MIC/zone diameter scattergrams using validated QC ranges can then be constructed as part of the larger process for the creation of breakpoints per the recommendations of the M23-A2 and M37-A2 documents (6, 8). We invite the authors to participate in the CLSI consensus process to help achieve the goal of establishing appropriate disk testing methods.
We read with interest the article by Luangtongkum et al. concerning susceptibility testing of Campylobacter spp. by dilution and diffusion methods and recommendations for interpretive criteria (3). The Clinical and Laboratory Standards Institute (CLSI) Subcommittees on Antimicrobial Susceptibility Testing and Veterinary Antimicrobial Susceptibility Testing, through their consensus process, have addressed antimicrobial susceptibility testing issues of Campylobacter spp., including method development, types of media, atmospheric conditions, and quality control (QC) ranges, and we would like to alert the authors to these recommendations, realizing that all of the recommendations may not have been available at the time of their studies (1, 2, 7, 9). Using a multilaboratory study protocol as specified by CLSI document M23-A2, an agar dilution reference method establishing QC ranges for five agents during testing of Campylobacter jejuni ATCC 33560 was developed (2, 4, 6, 7, 9). A standardized broth dilution method was subsequently developed and published in CLSI documents M100-S16 and M45-A, with QC ranges established for 13 agents (1, 2, 5). The media of choice for the agar dilution reference method and the standardized broth dilution method were MuellerHinton agar with 5% sheep blood and cation-adjusted Mueller-Hinton broth with 2.5 to 5% lysed horse blood, respectively. During 2005 to 2006, the CLSI convened a working group to recommend methods, QC ranges, and tentative breakpoints for Campylobacter spp. and other infrequently isolated or fastidious organisms (1). Agar and broth dilution methodologies as published were summarized for the testing of Campylobacter spp., along with QC ranges and tentative MIC breakpoints for erythromycin, ciprofloxacin, tetracycline, and doxycycline. Interpretive criteria were based upon epidemiologic “cutoff” values distinguishing wild-type, susceptible populations from those with acquired resistance, since no clinical outcome or pharmacokinetic/pharmacodynamic data were available. Included in the M45-A document was a method for resistance screening by disk diffusion that utilizes Mueller-Hinton agar with 5% sheep blood, the latter condition being consistent with the use of blood-containing media in standardized dilution tests. Isolate-drug combinations showing a no-zone phenomenon correlated completely with MIC determinations indicative of a resistance mechanism. The working group unanimously agreed that interpretation of zones other than 6 mm (no zone) was problematic due to the wide variations in measured zones detected by readers as a result of hazy or film-like growth. While this finding limits somewhat the establishment of reproducible disk diffusion QC ranges, it is critical that such QC ranges be defined prior to the establishment of interpretive criteria. We feel that the modification of breakpoints, especially those from older NCCLS documents as proposed by Luangtongkum et al. (e.g., M100-S12 [10]) requires further consideration in light of the standards-setting consensus process (CLSI) outlined in the M23-A2 and M37-A2 documents. For clarification, the National Antimicrobial Resistance Monitoring System (NARMS) used by the authors as a source of interpretive criteria is not a standards-setting organization, and reference to CLSI documents is made. Importantly, the use of
REFERENCES 1. Clinical and Laboratory Standards Institute. 2006. Methods for antimicrobial dilution and disk susceptibility testing of infrequently isolated or fastidious bacteria; approved guideline. M45-A. Clinical and Laboratory Standards Institute, Wayne, PA. 2. Clinical and Laboratory Standards Institute. 2006. Performance standards for antimicrobial susceptibility testing; 16th informational supplement. M100-S16. Clinical and Laboratory Standards Institute, Wayne, PA. 3. Luangtongkum, T., T. Y. Morishita, A. B. El-Tayeb, A. J. Ison, and Q. Zhang. 2007. Comparison of antimicrobial susceptibility testing of Campylobacter spp. by the agar dilution and the agar disk diffusion methods. J. Clin. Microbiol. 45:590–594. 4. McDermott, P. F., S. M. Bodeis, F. M. Aarestrup, S. Brown, M. Traczewski, P. Fedorka-Cray, M. Wallace, I. A. Critchley, C. Thornsberry, S. Graff, R. Flamm, J. Beyer, D. Shortridge, L. J. Piddock, V. Ricci, M. M. Johnson, R. N. Jones, B. Reller, S. Mirrett, J. Aldrobi, R. Rennie, C. Brosnikoff, L. Turnbull, G. Stein, S. Schooley, R. A. Hanson, and R. D. Walker. 2004. Development of a standardized susceptibility test for campylobacter with qualitycontrol ranges for ciprofloxacin, doxycycline, erythromycin, gentamicin, and meropenem. Microb. Drug Resist. 10:124–131. 5. McDermott, P. F., S. M. Bodeis-Jones, T. R. Fritsche, R. N. Jones, R. D. Walker, and the Campylobacter Susceptibility Testing Group. 2005. Broth microdilution susceptibility testing of Campylobacter jejuni and the determination of quality control ranges for fourteen antimicrobial agents. J. Clin. Microbiol. 43:6136–6138. 6. National Committee for Clinical Laboratory Standards. 2001. Development of in vitro susceptibility testing criteria and quality controls parameters; approved guideline, 2nd ed. M23-A2. National Committee for Clinical Laboratory Standards, Wayne, PA. 7. National Committee for Clinical Laboratory Standards. 2002. Performance standards for antimicrobial disk and dilution susceptibility tests for bacteria isolated from animals. Approved standard M31-A2, 2nd ed. National Committee for Clinical Laboratory Standards, Wayne, PA. 8. National Committee for Clinical Laboratory Standards. 2002. Development of in vitro susceptibility testing criteria and quality control parameters for veterinary antimicrobial agents; approved guideline, 2nd ed. M37-A2. National Committee for Clinical Laboratory Standards, Wayne, PA. 9. National Committee for Clinical Laboratory Standards. 2004. Performance standards for antimicrobial disk and dilution susceptibility tests for bacteria isolated from animals; informational supplement. M31-S1. National Committee for Clinical Laboratory Standards, Wayne, PA. 10. National Committee for Clinical Laboratory Standards. 2002. Performance standards for antimicrobial susceptibility testing; 12th informational supplement. M100-S12. National Committee for Clinical Laboratory Standards, Wayne, PA.
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LETTER TO THE EDITOR Thomas R. Fritsche* JMI Laboratories 345 Beaver Kreek Centre, Suite A North Liberty, Iowa 52317 Patrick F. McDermott U.S. Food and Drug Administration Center for Veterinary Medicine Laurel, Maryland 20708 Thomas R. Shryock Elanco Animal Health Greenfield, Indiana 46140 Robert D. Walker Mesa State College Grand Junction, Colorado 81501
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Author’s Reply On behalf of my coauthors, I would like to respond to the foregoing letter by Drs. Thomas R. Fritsche, Patrick F. McDermott, Thomas R. Shryock, and Robert Walker. I would like to thank them for their insightful comments and would be happy to collaborate on the CLSI consensus process to help establish appropriate disk testing methods for Campylobacter. Teresa Y. Morishita College of Veterinary Medicine Western University of Health Sciences 309 E. Second Street Pomona, California 91766-1854 E-mail:
[email protected]
