In vitro activities of tulathromycin and ceftiofur combined with other antimicrobial agents using bovine Pasteurella multocida and Mannheimia haemolytica isolates.

The purpose of this study was to determine the activities of two antibacterial agents used in the treatment of bovine respiratory infections—tulathromycin, a macrolide, and ceftiofur, a third-generation cephalosporin—alone, in combination with each other, and in combination with each of seven additional antibiotics (tilmicosin, florfenicol, enrofloxacin, danofloxacin, ampicillin, tetracycline, and penicillin G) against bovine Pasteurella multocida (n = 60) and Mannheimia haemolytica (n = 10) isolates for determination of synergy, antagonism, or indifference. Of 458 organism–drug combinations, 160 combinations of tulathromycin and 209 combinations of ceftiofur with eight antimicrobial drugs were indifferent. One combination was antagonistic (ceftiofur + florfenicol against one isolate of P. multocida). Time–kill studies showed loss of cidality for ceftiofur when combined with florfenicol at 1× the minimal inhibitory concentration. Overall, the in vitro data demonstrated that tulathromycin and ceftiofur, in combination with each other or seven other antimicrobial agents, primarily produce an indifferent response with no occurrences of synergism and rare occurrences of antagonism.