diarrheaassociated pathogens lactobacilli and cellulolytic bacteria in equine feces responses to antibiotic challenge antibiotics are important to equine medicine but antibioticassociated diarrhea aad can lead to poor performance and even mortality aad is attributed to disruption of the hindgut microbiota which permits proliferation of pathogenic microbes the goal of this study was to evaluate the effects of common antibiotics on cellulolytic bacteria lactobacilli and aadassociated pathogens in the feces of healthy horses fifteen horses were assigned to three treatment groups blocked by age and sex control no antibiotics trimethoprimsulfadiazine po or ceftiofur im fecal samples n8 per horse were taken during dietary adaptation 3 weeks antibiotic challenge 1 week and withdrawal 1 week bacteria were enumerated by serial dilution and viable count cellulolytic bacteria decreased by 99 during administration of either antibiotic p00001 and were still less than controls at the end of the withdrawal period p00001 fecal samples from horses challenged with ceftiofur had 75 fewer lactobacilli than those from control horses at the end of the antibiotic challenge period p005 antibiotic challenged horses also shed more salmonella than control horses p005 antibiotics had no effect on the number of clostridium perfringens isolates there was no detectable clostridium difficile during adaptation or in any control horse c difficile increased p00001 to approximately 104cfug when horses were challenged with antibiotics and were still detectable 1 week after withdrawal these results indicate that antibiotics can disrupt the normal gastrointestinal microbiota and allow proliferation of salmonella spp and c difficile