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.
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.


METHODS
217 typical and atypical A salmonicida isolates obtained from 20 states and 12 countries.


METHODS
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
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.
Miller
Ron A
RA
FDA, Center for Veterinary Medicine, Office of Research, Division of Animal Research, 8401 Muirkirk Rd, Laurel, MD 20708,USA.
Reimschuessel
Renate
R
D048409:Aeromonas salmonicida / Q000187:drug effects*; D000890:Anti-Infective Agents / Q000633:toxicity*; D017756:Aquaculture / Q000379:methods*; D017931:DNA Primers; D052978:Disk Diffusion Antimicrobial Tests; D004352:Drug Resistance, Microbial; D008826:Microbial Sensitivity Tests; D010093:Oxolinic Acid / Q000633:toxicity*; D010118:Oxytetracycline / Q000633:toxicity*; D016133:Polymerase Chain Reaction; D011743:Pyrimidines / Q000633:toxicity*; D013045:Species Specificity; D013839:Thiamphenicol / Q000031:analogs & derivatives* / Q000633:toxicity
D000890:Anti-Infective Agents; D017931:DNA Primers; D011743:Pyrimidines; C035534:florfenicol; D013839:Thiamphenicol; D010093:Oxolinic Acid; C100125:ormetoprim; D010118:Oxytetracycline