Antimicrobial resistance of Listeria spp. recovered from processed bison

Aims: The current study examined the antimicrobial susceptibility of 86 Listeria spp. isolated from processed bison carcasses. Materials and Methods: Susceptibility to 25 antimicrobial agents was determined using E-test and National Antimicrobial Resistance Monitoring System (NARMS) panels. Most Listeria isolates (88-98%) exhibited resistance to bacitracin, oxacillin, cefotaxime, and fosfomycin. Resistance to tetracycline (18.6%) was also common. Of the 16 tetracycline-resistant Listeria isolates, 15 carried tetM and 2 contained integrase of Tn1545 transposons. Rifampicin and trimethoprim-sulfamethoxazole were the most active antimicrobial agents against Listeria spp., with a MIC90 of 0.38 mu g ml(-1). Ampicillin, erythromycin, penicillin, gentamicin, and tobramycin also exhibited good activity against Listeria spp., with MIC90 not exceeding 1 mu g ml(-1). Differences in resistance among Listeria spp. was displayed, as Listeria innocua strains were more resistant than other Listeria species. Conclusions: The study showed that Listeria monocytogenes strains from bison were susceptible to the antibiotics most commonly used to treat human listeriosis. However, the presence of antimicrobial resistance in L. innocua indicates the potential for transfer of resistance and a conjugative transposon to L. monocytogenes. Significance and Impact of the study: The findings of our study will provide useful information for the development of public health policy in the use of antimicrobials in food animal production.