A Comparative Study on the Effects of Tylosin on Select Bacteria During Continuous Flow Culture of Mixed Populations of Gut Microflora Derived from a Feral and a Domestic Pig Nicole Ramlachan, Robin C. Anderson, Kathleen Andrews, Roger B. Harvey, and David J. Nisbet Abstract Continuous flow cultures of feral (culture FC) and domesticated (culture RPCF) pig gut microflora were es- tablished in steady state. Cultures were continuously infused with 25 or 100 mg tylosin=mL and sampled at intervals to assess effects on total culturable anaerobes, Bacteroides and Enterococcus via plating to agar sup- plemented without or with 100 mg tylosin=mL, the latter to assess bacterial sensitivity to tylosin. En- dogenous tylosin-insensitive anaerobes within the cultures, while similar prior to tylosin administration, responded differently during tylosin administration, with concentrations in RPCF cultures becoming en- riched 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 mg tylosin=mL. Tylosin- insensitive Bacteroides were enriched to >5 log 10 CFU=mL in RPCF cultures after 25 mg tylosin=mL but not at 100 mg tylosin=mL. Populations of endogenous tylosin-insensitive Enterococcus were enriched in RPCF but not FC cultures administered 25 or 100 mg tylosin=mL. In cultures administered 100 mg 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. Introduction M acrolide antibiotics, such as tylosin, are commonly used in human and veter- inary medicine, primarily to treat infections caused by gram-positive bacteria and also as a feed additive to improve production efficiency in swine (Gaynor and Mankin, 2003). Concern exists that antibiotic resistance elements may be transferred between bacteria in microbial popu- lations inhabiting the gut of domestic animals and humans as well as in the environment (Aubry-Damon et al., 2004; Chee-Sanford et al., 2001; Ku¨ hn et al., 2005; Lee et al., 1993; Manero et al., 2006; Sapkota et al., 2006). Moreover, while it is thought that exposure to low or high con- centrations of antibiotics may differentially af- fect competition between bacteria with varying antibiotic susceptibilities (Fleming et al., 2002), more studies testing the effects of low or high antibiotic exposure on bacterial fitness within mixed populations are warranted. Numerous USDA-ARS, Southern Plains Agricultural Research Center, Food & Feed Safety Research Unit, College Station, Texas. FOODBORNE PATHOGENS AND DISEASE Volume 5, Number 1, 2008 ª Mary Ann Liebert, Inc. DOI: 10.1089=fpd.2007.0022 21