Editorial Type:
Antimicrobials

Epidemiologic cutoff values for antimicrobial agents against Aeromonas salmonicida isolates determined by frequency distributions of minimal inhibitory concentration and diameter of zone of inhibition data

Ron A. Miller FDA, Center for Veterinary Medicine, Office of Research, Division of Animal Research, 8401 Muirkirk Rd, Laurel, MD 20708.

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Renate Reimschuessel FDA, Center for Veterinary Medicine, Office of Research, Division of Animal Research, 8401 Muirkirk Rd, Laurel, MD 20708.

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 VMD, PhD
DOI:
https://doi.org/10.2460/ajvr.67.11.1837
Volume/Issue:
Volume 67: Issue 11
Online Publication Date:
01 Nov 2006
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Abstract

Objective—To develop epidemiologic cutoff values by use of frequency distributions for susceptibility to 4 antimicrobial agents when tested against a representative population of a major aquaculture pathogen, Aeromonas salmonicida.

Sample Population—217 typical and atypical A salmonicida isolates obtained from 20 states and 12 countries.

Procedures—Species identification of A salmonicida isolates was confirmed by detection of specific nucleotide sequences by use of a PCR assay. Minimal inhibitory concentration (MIC) and diameter of the zone of inhibition for oxytetracycline, ormetoprim-sulfadimethoxine, oxolinic acid, and florfenicol were determined for each isolate in accordance with standardized antimicrobial susceptibility testing methods that have been approved by the Clinical and Laboratory Standards Institute for bacterial isolates from aquatic animals. Susceptibility data were tabulated in a scattergram and analyzed by use of error rate bounding.

Results—Susceptibility tests for oxytetracycline, ormetoprim-sulfadimethoxine, and oxolinic acid revealed 2 distinct populations of bacteria. Isolates tested against florfenicol clustered into a single population. Oxolinic acid susceptibility data revealed higher MICs in the non–United States A salmonicida isolates. Slow-growing (atypical) A salmonicida isolates were generally more susceptible than typical isolates for all antimicrobials, except oxolinic acid.

Conclusions and Clinical Relevance—Use of frequency distributions of susceptibility results to develop epidemiologic cutoff values appears to be applicable to aquatic isolates. Frequency distributions of susceptibility results for A salmonicida revealed clear divisions between isolate susceptibilities. This type of data, considered in conjunction with pharmacokinetic and efficacy data, may be useful for developing clinical breakpoints for use in aquaculture.

Abstract

Objective—To develop epidemiologic cutoff values by use of frequency distributions for susceptibility to 4 antimicrobial agents when tested against a representative population of a major aquaculture pathogen, Aeromonas salmonicida.

Sample Population—217 typical and atypical A salmonicida isolates obtained from 20 states and 12 countries.

Procedures—Species identification of A salmonicida isolates was confirmed by detection of specific nucleotide sequences by use of a PCR assay. Minimal inhibitory concentration (MIC) and diameter of the zone of inhibition for oxytetracycline, ormetoprim-sulfadimethoxine, oxolinic acid, and florfenicol were determined for each isolate in accordance with standardized antimicrobial susceptibility testing methods that have been approved by the Clinical and Laboratory Standards Institute for bacterial isolates from aquatic animals. Susceptibility data were tabulated in a scattergram and analyzed by use of error rate bounding.

Results—Susceptibility tests for oxytetracycline, ormetoprim-sulfadimethoxine, and oxolinic acid revealed 2 distinct populations of bacteria. Isolates tested against florfenicol clustered into a single population. Oxolinic acid susceptibility data revealed higher MICs in the non–United States A salmonicida isolates. Slow-growing (atypical) A salmonicida isolates were generally more susceptible than typical isolates for all antimicrobials, except oxolinic acid.

Conclusions and Clinical Relevance—Use of frequency distributions of susceptibility results to develop epidemiologic cutoff values appears to be applicable to aquatic isolates. Frequency distributions of susceptibility results for A salmonicida revealed clear divisions between isolate susceptibilities. This type of data, considered in conjunction with pharmacokinetic and efficacy data, may be useful for developing clinical breakpoints for use in aquaculture.

Contributor Notes

Supported by the Oak Ridge Associated Universities through a grant sponsored by the FDA.

Presented in part at the National Foundation for Infectious Diseases Annual Conference on Antimicrobial Resistance, Bethesda, Md, June 2006, and at the FDA Science Forum, Washington, DC, April 2006.

The authors thank Drs. Ana Baya, Rocco Cipriano, Joy Evered, Hui-Min Hsu, Sharon Landin, Lindsay Oaks, Scott Lapatra, Jessica Boyd, David Bruno, Sarah Burr, Maria Figueras, Bjarnhei Gu∂ˇdmundsdóttir, Vera Lund, Sarah Maurice, Michael Sinyakov, and Satu Viljamaa-Dirks for provision of bacterial isolates.

Address correspondence to Mr. Miller.
Cover American Journal of Veterinary Research
American Journal of Veterinary Research
Publication Date:
01 Nov 2006
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