ARS | Publication request: Transfer of tylosin resistance between Enterococcus spp. during continuous-flow culture of domestic or feral porcine gut microflora in vitro.

Submitted to: Agriculture, Food and Analytical Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2011
Publication Date: N/A

Interpretive Summary: Public health agencies are concerned that the continued agricultural use of antibiotics for growth promotion may contribute to the emergence and proliferation of antimicrobial-resistant microbial populations within the gut of farm animals. In order to test if farm-raised pigs harbor gut bacteria more able to acquire antibiotic resistance than gut bacteria from wild pigs, we grew mixed populations of domesticated and wild pig gut bacteria in specialized growth chambers that simulated their respective gut environments. Once the separate mixed populations were allowed to achieve equilibrium, they were inoculated with a particular bacterium possessing resistance to a common antibiotic called tylosin. Because tylosin is commonly fed to farm-raised pigs, only the mixed bacteria population from the farm-raised pig would have had prior exposure to this antibiotic. We then administered tylosin to the separate bacteria populations and found that there was little if any difference in the effect that the tylosin treatment had on the separate bacterial populations, with resistance being transferred from the inoculated bacterium to bacteria native within both populations. These results indicate that under the conditions of our study, bacteria from the farm-raised pig were no more likely than those from wild pigs to acquire resistance to this antibiotic. Ultimately, this research will help farmers and ranchers develop the most prudent ways to use antimicrobials to help prevent disease and produce safe meat and milk for the American consumer.

Technical Abstract: Mixed populations of domesticated and feral pig gut microbes (RPCF and FC, respectively) were grown in continuous culture to investigate the effects of tylosin on antimicrobial resistance. Cultures established in steady state were inoculated with 9.7 log10 colony forming units (CFU) of a tylosin-resistant Enterococcus faecium and allowed 7 days to re-establish equilibrium before administration of 100 µg tylosin ml-1. Total culturable anaerobes recovered on non-antibiotic supplemented medium, thus inclusive of tylosin-sensitive and -insensitive bacteria, ranged from 7.15 to 9.20 log10 CFU ml-1 throughout 8 days of tylosin administration and 6 subsequent days without tylosin administration. Recovery of total anaerobes on tylosin-supplemented medium revealed that populations of total tylosin-insensitive anaerobes ranged from 6.30 to 9.02 log10 CFU ml-1 during the experiment. Concentrations of the introduced tylosin-resistant E. faecium decreased to near minimum detectable levels (1.3 log10 CFU ml-1) in the cultures before initiation of tylosin administration and then increased to 6.80 ± 0.28 and 8.30 ± 0.43 log10 CFU ml-1 in RPCF and FC cultures, respectively, and remained higher than day 0 concentrations for the remainder of the experiment. Endogenous tylosin-insensitive enterococcus were undetectable before administration of tylosin, but tylosin-resistant E. faecalis and E. hirea found to have acquired an ermB gene of expected size and sequence of that contained in the introduced E. faecium were enriched to 7.74 ± 0.37 and 3.85 ± 4.03 after initiation of tylosin administration. These results demonstrate the acquisition, propagation, and persistence of tylosin resistance in mixed populations of domestic and feral swine gut microflora.