Submitted to: National Antimicrobial Resistance Monitoring System Scientific Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 6/27/2002
Publication Date: N/A
Technical Abstract: Background: The use of sub-therapeutic antimicrobials in food animal production is considered a primary factor in the prevalence of antimicrobial resistance among enteric bacteria, which may serve as both a reservoir of resistance genes and a vehicle for their dissemination. A fundamental understanding of the changes in gut microbial ecology that accompany antimicrobial treatment is needed for development of effective intervention strategies to prevent the spread of antimicrobial resistance. Methods: Mixed cultures of enteric bacteria maintained in anaerobic continuous-flow chemostats were used as models of the microbial ecology of the gastrointestinal tract. Cultures were treated with varying concentrations of either vancomycin or tylosin, then challenged with 7 log10 (CFU/ml) of either a vancomycin resistant enterococci (VRE) or Escherichia coli O157:H7, respectively. The populations of exogenous VRE and E. coli, and endogenous bacterial species including Enterococcus faecalis, were monitored by colony counts on selective media. Results: Untreated mixed-cultures of enteric bacteria excluded the exogenous VRE and E. coli, completely clearing these organisms from the chemostat at a rate of approximately 1 log10 (CFU/ml) per day. Treatment with 0.01 ug/ml or 0.1 ug/ml of vancomycin had no effect on the concentration of endogenous E. faecalis (MIC for vancomycin of 0.5 ug/ml), but did allow the VRE to persist in the culture for more than 24 days at a concentration of 2 log10 (CFU/ml). No transfer of vancomycin resistance genes to E. faecalis was observed during this time. Treatment with 20 ug/ml tylosin, a concentration comparable to that used for growth promotion in feed, resulted in elimination of 26 of the 29 initial bacterial species, leaving a stable culture of E. faecalis, Lactobacillus salivarus, and an unspeciated Veillonella. The tylosin treated culture was unable to exclude endogenous E. coli, which persisted at approximately 6 log10 (CFU/ml). Conclusions: Our data, obtained in continuous flow chemostats, suggest that low concentrations of antimicrobials adversely affect the microbial ecology of the GI tract with respect to its ability to exclude exogenous bacteria. Verification of the results in live animals will be necessary to determine if tylosin treatment could compromise the effectiveness of normal microflora to serve as a natural host defense against infection.