Antibiotic treatment affects commensal bacteria of the gut microbiota, even at sublethal concentrations. Schlomann,Wiles et al. examined the effect of sublethal doses of ciprofloxacin on the dynamics of gut bacterial populations using live imaging of larval zebrafish. They focused on two native zebrafish isolates: fast-growing, planktonic Vibrio cholerae ZWU0020 and slow-growing Enterobacter cloacae ZOR0014, which forms dense bacterial aggregates. Germ-free fish were colonized with either bacterial strain before ciprofloxacin treatment. The antibiotic enhanced aggregation of bacterial cells, which resulted in their increased expulsion from the gut by the mechanical activity of the intestine. Moreover, the effect was more pronounced for slow-growing bacteria. As low concentrations of the antibiotic are often found in the environment, the findings of this study highlight the possibility of gut microbiota perturbations due to environmental antibiotic contaminants.