Mean obligate anaerobic gut microbiota populations (log10 CFU/ml) (a); mean facultative anaerobic gut microbiota populations (log10 CFU/ml) (b); and mean C. difficile total viable counts and spore counts (log10 CFU/ml), cytotoxin titers (relative units, RU), and antimicrobial concentration (mg/liter) (c) in vessel 3 of model OMC. All vertical arrows mark the addition of C. difficile spores to the model, and horizontal arrow marks the period of antibiotic instillation. LF Enterobacteriaceae, lactose-fermenting Enterobacteriaceae; LoD, limit of detection.