THE INVITRO SUSCEPTIBILITY OF AEROMONAS-SALMONICIDA AND OTHER FISH-PATHOGENIC BACTERIA TO 29 ANTIMICROBIAL AGENTS

The minimum inhibitory concentrations (MICs) of 29 antimicrobial agents were determined for some representative Aeromonas salmonicida, Yersinia ruckeri and Vibrio isolates. Eleven of them, including three widely used in clinical practice, were further tested against 28-36 recent field isolates of A. salmonicida. The fluoroquinolones were inhibitory at very low concentrations with 50% of A. salmonicida isolates having a MIC of enrofloxacin less-than-or-equal-to 0.02-mu-g ml-1. The distribution of MICs to the fluoroquinolones and to oxolinic acid were similar indicating possible cross resistance. Amoxycillin, florfenicol, minocycline, rifampicin and the potentiated sulphonamides had a single peak MIC distribution with values of less-than-or-equal-to 1.25-mu-g ml-1. Minimum inhibitory concentrations of the fluoroquinolones were four or more times greater at 10-degrees-C compared with 22-degrees-C; this was not so with amoxycillin, florfenicol or the sulphachlorpyridazine-trimethoprim (S-TP) combination. A. salmonicida was rapidly killed at 22-degrees-C by the fluoroquinolones and S-TP whereas oxolinic acid and amoxycillin killed within 24 h. Florfenicol was bacteriostatic. The therapeutic potential of some of these antibiotics is considered.