In vitro susceptibilities of 126 clinical isolates of Yersinia enterocolitica to 21 beta-lactam antibiotics.

The MICs of 21 beta-lactam antibiotics were measured against 126 clinically significant and epidemiologically unrelated Yersinia enterocolitica isolates. The most active antimicrobial agents tested (geometric means of MICs) were ceftriaxone, cefotaxime, ceftizoxime, and cefmenoxime (0.06 to 0.08 micrograms/ml). Mezlocillin (1.36 micrograms/ml) and piperacillin (1.57 micrograms/ml) were the most active penicillins. Aztreonam and imipenem had MICs of 0.44 and 0.24 micrograms/ml. All isolates exhibited some degree of resistance against ampicillin (MICs, greater than or equal to 4 micrograms/ml) and cephalothin (MICs, greater than or equal to 8 micrograms/ml). Cephalosporinase or penicillinase activities were expressed by all isolates. A principal component analysis of MIC data separated the major serotypes (O3, O9, O8, and O5,27). The MCBs of 14 newer beta-lactams were measured against 10 clinical isolates. On the basis of the ratio of MBC to MIC (expressed in dilution factors), a real bactericidal activity was only observed for imipenem (MBC/MIC = 1). For the other newer beta-lactams, the ratios ranged from 4 to 8.