Antimicrobial susceptibility of equine isolates of Actinobacillus spp. andidentification of beta-lactamases in some strains

A total number of 149 Actinobacillus strains isolated from clinical samples (73 strains) and from the oral cavity of healthy horses (76 strains) were tested for their susceptibility to 17 antimicrobial substances. The antibio grams were generally very similar between the various strains and no differ ences could be clearly correlated to either phenotype or source of isolates . However, when tested against penicillin, ampicillin, trimethoprim-sulfa, and streptomycin, small groups of strains with what appeared to be acquired resistance could be identified. Eight of the penicillin-resistant strains were found to produce beta-lactamase. The beta-lactamases appeared to be bo und tightly to the cell wall, thereby frustrating further characterization by isoelectric focusing. Plasmids of approximately 3 kb were found in four out of seven beta-lactamase-producing strains submitted to plasmid analysis .