Competitive exclusion of a glycopeptide-resistant Enterococcus faecium in the presence of vancomycin but not equivalent concentrations of tylosin or gentamicin.

The effect of subtherapeutic concentrations of antibiotics (10.0 and 40.0 microg/mL of vancomycin, gentamicin, and tylosin) on the efficacy of a mixed anaerobe culture of chicken microflora (CCF) was studied in a continuous-flow fermentation system. Efficacy of CCF posttreatment was assessed by challenge with glycopeptide-resistant Enterococcus faecium (GRE) at 6.0 log10 cfu/mL. Bacterial enumeration of endogenous CCF isolates, volatile fatty acid (VFA) analysis, and challenge with GRE indicated that CCF efficacy was affected by all antibiotic treatments. Although CCF treated with 10.0 microg/mL of vancomycin eliminated GRE13 at a rate of 0.61 log10 cfu/ mL per day, it was unable to eliminate E. coli, a gram-negative challenge organism. All other antibiotic treatments allowed GRE persistence at approximately 2.0 to 6.5 log10 cfu/mL. All antibiotic-treated cultures had decreased concentrations of acetic and propionic acids. Our data suggest that low concentrations of antimicrobials may adversely affect the microbial ecology of gut microflora with respect to its ability to exclude exogenous bacteria. Moreover, gentamicin had an adverse effect on the inhibitory stringency of CCF even though it showed little anti-anaerobic activity against CCF strict anaerobes in pure culture. Verification of the results in live animals will be necessary to determine if antimicrobial treatment could compromise the effectiveness of normal microflora to serve as a natural host defense against infection.