Antimicrobial susceptibility of Arcanobacterium pyogenes isolated from cattle and pigs.

The minimum inhibitory concentrations (MICs) of 18 antimicrobial agents were determined for 49 Arcanobacterium pyogenes isolates (42 bovine isolates and 7 porcine isolates). Benzylpenicillin and ampicillin were the most active antibiotics, with MIC ranges of < or = 0.0125-0.05 microgram/ml for both bovine and porcine isolates. All isolates were susceptible to penicillins and cephems. MICs for 90% of the isolates of dihydrostreptomycin, gentamicin and oxytetracycline for bovine isolates were > 100 micrograms/ml, 1.56 micrograms/ml and 25 micrograms/ml, respectively. More resistance to dihydrostreptomycin appeared among porcine isolates (85.7%) than among bovine isolates (52.4%). Resistance to gentamicin occurred in only 3 (7.1%) of the bovine isolates. Resistance to oxytetracycline also appeared more frequent among porcine isolates (85.7%) than among bovine isolates (57.1%). All bovine isolates were susceptible to erythromycin, tilmocosin and lincomycin, but two porcine isolates (28.6%) were simultaneously resistant to these antibiotics. Tiamulin was as active as tilmicosin, with an MIC for 50% of the isolates (MIC50) of 0.05 microgram/ml for both bovine and porcine isolates. The MIC50s of chloramphenicol and its derivatives florfenicol and thiamphenicol were all 1.56 micrograms/ml. The fluoroquinolones enrofloxacin and ofloxacin were not so active as penicillins and macrolides, with MIC50s of 0.78 microgram/ml and 1.56 micrograms/ml, respectively, for both bovine and porcine isolates.