Bacteroides fragilis is among gut commensals, and its infection is commonly treated by metronidazole. A strain of B. fragilis that overexpresses recA was resistant to metronidazole in comparison to the wild-type strain. Inactivation of recA resulted in the increased sensitivity to metronidazole, and the B. fragilis recA mutants had more double strand breaks. Here, RAD51 is linked to infection.