Mutant prevention and minimum inhibitory concentration drug values for enrofloxacin, ceftiofur, florfenicol, tilmicosin and tulathromycin tested against swine pathogens Actinobacillus pleuropneumoniae, Pasteurella multocida and Streptococcus suis.

Actinobacillus pleuropneumoniae, Pasteurella multocida and Streptococcus suis are prevalent bacterial causes of swine infections. Morbidity, mortality and positively impacting the financial burden of infection occurs with appropriate antimicrobial therapy. Increasing antimicrobial resistance complicates drug therapy and resistance prevention is now a necessity to optimize therapy and prolong drug life. Mutant bacterial cells are said to arise spontaneously in bacterial densities of 107-109 or greater colony forming units/ml. Antibiotic drug concentration inhibiting growth of the least susceptible cell in these high density populations has been termed the mutant prevention concentration (MPC). In this study MPC and minimum inhibitory concentration (MIC) values of ceftiofur, enrofloxacin, florfenicol, tilmicosin and tulathromycin were determined against the swine pathogens A. pleuropneumoniae, P.multocida and S. suis. The following MIC90/MPC90 values (mg/L) for 67 A. pleuropneumoniae and 73 P. multocida strains respectively were as follows: A. pleuropneumoniae 0.031/0.5, ≤0.016/0.5, 0.5/2, 4/32, 2/32; P. multocida 0.004/0.25, 0.016/0.125, 0.5/0.5, 8/16, 0.5/1. For 33 S. suis strains, MIC90 values (mg/L) respectively were as follows: 1, 0.25, 4, ≥8 and ≥8. A total of 16 S. suis strains with MIC values of 0.063-0.5 mg/L to ceftiofur and 0.25-0.5 mg/L to enrofloxacin were tested by MPC; MPC values respectively were 0.5 and 1 mg/L respectively. MPC concentrations provide a dosing target which may serve to reduce amplification of bacterial subpopulations with reduced antimicrobial susceptibility. Drug potency based on MIC90 values was ceftiofur > enrofloxacin >florfenicol = tulathromycin > tilmicosin; based on MPC90 values was enrofloxacin > ceftiofur > tulathromycin > florfenicol ≥ tilmicosin.