A clinically reflective model of the human colon was used to investigate the effects of the broad-spectrum antibiotic omadacycline on the gut microbiome and the subsequent potential to induce simulated infection (CDI). Triple-stage chemostat gut models were inoculated with pooled human fecal slurry from healthy volunteers (age, ≥60 years). Models were challenged twice with 10 CFU spores (PCR ribotype 027). Omadacycline effects were assessed in a single gut model. Observations were confirmed in a parallel study with omadacycline and moxifloxacin. Antibiotic instillation was performed once daily for 7 days. The models were observed for 3 weeks postantibiotic challenge. Gut microbiota populations and total viable and spore counts were enumerated daily by culture. Cytotoxin titers and antibiotic concentrations were also measured. Gut microbiota populations were stable before antibiotic challenge. Moxifloxacin instillation caused an ∼4 log CFU/ml decline in enterococci and group populations and an ∼3 log CFU/ml decline in bifidobacteria and lactobacilli, followed by simulated CDI (vegetative cell proliferation and detectable toxin). In both models, omadacycline instillation decreased populations of bifidobacteria (∼8 log CFU/ml), group populations (7 to 8 log CFU/ml), lactobacilli (2 to 6 log CFU/ml), and enterococci (4 to 6 log CFU/ml). Despite these microbial shifts, there was no evidence of bacteria germination or toxin production. In contrast to moxifloxacin, omadacycline exposure did not facilitate simulated CDI, suggesting this antibiotic may have a low propensity to induce CDI in the clinical setting.