Possible cross-resistance and horizontal gene transfer associated with halquinol in Escherichia coli and Salmonella enterica isolated from pigs

Abstract

The aims of this study were to determine the possible cross-resistance and horizontal gene transfer associated with halquinol in Escherichia coli and Salmonella spp. from pigs. Two of each E. coli and Salmonella isolates exhibiting low halquinol MIC (8 and 16 μg/ml, respectively) and susceptible to antibiotics tested were selected for in vitro stepwise-halquinol exposure experiment. Another E. coli (n=19) and Salmonella (n=6) isolates with high halquinol MICs (16-64 and 32-64 μg/ml, respectively) containing plasmids were used for transferability testing of halquinol resistance-encoding gene(s) using filter mating method. In vitro exposure to subinhibitory concentrations of halquinol promoted neither resistance to halquinol nor cross-resistance to antibiotics in E. coli. In contrast, spontaneous halquinol-resistant mutants of Salmonella were obtained with cross-resistance to trimethoprim. Horizontal transfer of halquinol resistance determinants was not detected in either E. coli or Salmonella. These findings demonstrate the possible species-specific responses to halquinol exposure and emphasize the importance of prudent antimicrobial use to limit resistance development and spread.