Continuous flow cultures of feral (culture FC) and domesticated (culture RPCF) pig gut microflora were established in steady state. Cultures were continuously infused with 25 or 100 μg tylosin/mL and sampled at intervals to assess effects on total culturable anaerobes, Bacteroides and Enterococcus via plating to agar supplemented without or with 100 μg tylosin/mL, the latter to assess bacterial sensitivity to tylosin. Endogenous tylosin-insensitive anaerobes within the cultures, while similar prior to tylosin administration, responded differently during tylosin administration, with concentrations in RPCF cultures becoming enriched more than in FC cultures. Tylosin-insensitive anaerobes in RPCF cultures persisted at increased concentrations after cessation of tylosin administration whereas concentrations in FC cultures decreased slightly. Concentrations of Bacteroides and endogenous Enterococcus recovered on medium without tylosin decreased to near or below detectable levels in FC cultures administered 25 or 100 μg tylosin/mL. Tylosin-insensitive Bacteroides were enriched to >5 log10 CFU/mL in RPCF cultures after 25 μg tylosin/mL but not at 100 μg tylosin/mL. Populations of endogenous tylosin-insensitive Enterococcus were enriched in RPCF but not FC cultures administered 25 or 100 μg tylosin/mL. In cultures administered 100 μg tylosin/mL, an exogenous-sourced E. faecium possessing tylosin resistance maintained itself only in the presence of tylosin. These results indicate that under the conditions of these tests, antibiotic exposure may enrich for antibiotic-insensitive bacteria populations of endogenous or exogenous origin but that the ability of an exogenous tylosin-resistant E. faecium to persist is reduced in the absence of the antibiotic, likely due to exclusion by native flora.
