In vitro response to chloramphenicol, tetracycline, ampicillin, gentamicin, and beta-lactamase production by halophilic Vibrios from human and environmental sources.

Isolates of Vibrio parahaemolyticus and V. alginolyticus from human and environmental sources were examined for antibiotic susceptibility by the methods of minimal inhibitory concentration (MIC) in broth and agar diffusion. These strains were found to be almost uniformly susceptible to chloramphenicol and tetracycline within attainable serum levels. The relationships of zone sizes to MICs for these two antimicrobial agents and ampicillin conformed essentially to those obtained by standard methods with gram-negative rods. Most strains were resistant to ampicillin and exhibited beta-lactamase activity, which accounted for this resistance. Nine of 30 V. alginolyticus strains from environmental sources were ampicillin resistant but did not produce measurable amounts of beta-lactamase. Three strains exhibited multiresistance to high concentrations of chloramphenicol, tetracycline, and ampicillin, which suggests the presence of plasmids. Although the great majority of vibrios appeared to be susceptible to gentamicin by agar diffusion, susceptibility could not be measured by MIC because the added NaCl, required for growth by the halophilic vibrios, diminished gentamicin activity.