Postantibiotic and physiological effects of tilmicosin, tylosin, and apramycin at subminimal and suprainhibitory concentrations on some swine and bovine respiratory tract pathogens.

The antimicrobial activity of tilmicosin, tylosin, and apramycin on some important gram-negative swine and bovine pathogens namely, Pasteurella multocida, Pasteurella haemolytica, Bordetella bronchiseptica, and Actinobacillus pleuropneumoniae were studied in vitro. The effect of minimal inhibitory concentrations (MICs) and sub-MICs (1/4, 1/2 MIC) on bacterial growth was evaluated. The presence of tilmicosin, tylosin and apramycin in the medium decreased the rate of growth of the bacterial strains tested using drug concentrations as low as 1/4 MIC. The postantibiotic effect (PAE) which is the suppression of optimal bacterial growth that persists after a short exposure (2 h) of microorganisms to an antibiotic was studied by exposure of bacteria to drugs at 1/4, 1/2, 1, 4 and 8 times MIC. The duration of PAEs increased with rising concentration for all drugs tested but at concentrations below the MIC, tilmicosin showed more significant PAEs than tylosin or apramycin against P. multocida and A. pleuropneumoniae. Tilmicosin and tylosin caused PAEs of up to 8 h when used at 8 times the MIC, while apramycin caused PAEs of up to 5 h when used at this concentration. Sub-MICs of either tilmicosin, tylosin, or apramycin had no effect on P. multocida dermonecrotic toxin production. However sub-MICs of tylosin, or apramycin significantly reduced the haemolytic activity of A. pleuropneumoniae and affected the capsular material production of this isolate and of one isolate of P. multocida (type A). The in vitro effect of tilmicosin, tylosin, and apramycin (even when used at sub-MIC levels) on growth, production of capsular material, and haemolytic activity might impair the virulence of some of the microorganisms studied. In addition to the effects of these drugs on some putative virulence factors, we suggest that the strong PAEs caused by tilmicosin, tylosin, and apramycin may also contribute to the in vivo efficacy of these drugs.