In vivo fitness of B. theta and B. vulgatus is regulated by MAFF. (A) Experimental scheme for cohousing of SPF-Abx mice inoculated with B. theta strains after antibiotic treatment. Ery^r, erythromycin resistant; Crm^r, chloramphenicol resistant. (B) CFU measurements of each B. theta strain in cohouse1 in C. The squares with dotted lines indicate the bacteria detected from the same mice. Each plot represents the result obtained from independent animals (n = 8 mice for each group). (C) The log₁₀ ratio of invaded/initial strains in feces was determined by selective plating on day 7 of cohousing (n = 8 mice). The squares with dotted lines indicate the samples collected from the same group of the cohousing experiments. Statistical analyses in B and C were performed with unpaired Student’s t test. *P < 0.05 and ****P < 0.0001. Error bars represent SEM.