The Enterococcus faecalis superoxide dismutase is essential for its tolerance to vancomycin and penicillin

Enterococcus faecalis is a human commensal that has the ability to become a pathogen. Because of its ruggedness, it can persist in the hospital setting and cause serious nosocomial infections. E. faecalis can acquire multiple drug resistance determinants but is also intrinsically tolerant to a number of antibiotics, such as penicillin or vancomycin, meaning that these usually bactericidal drugs only exhibit a bacteriostatic effect. Recently, evidence has been presented that exposure to bactericidal antibiotics induced the production of reactive oxygen species in bacteria. Here, we studied the role of enzymes involved in the oxidative stress response in the survival of E. faecalis after antibiotic treatment. Mutants defective in genes encoding oxidative stress defence activities were tested by time-kill curves for their contribution to antibiotic tolerance in comparison with the E. faecalis JH2-2 wild-type (WT). In killing assays, WT cultures lost 0.2 +/- 0.1 and 1.3 +/- 0.2 log(10) cfu/mL after 24 h of vancomycin or penicillin exposure, respectively. A deletion mutant of the superoxide dismutase gene (delta sodA) exhibited a lack of tolerance as cultures lost 4.1 +/- 0.5 and 4.8 +/- 0.7 log(10) cfu/mL after 24 h of exposure to the same drugs. Complementation of delta sodA re-established the tolerant phenotype. Bacterial killing was an oxygen-dependent process and a model is presented implicating the superoxide anion as the mediator of this killing. As predicted from the model, a mutant defective in peroxidase activities excreted hydrogen peroxide at an elevated rate. SodA is central to the intrinsic ability of E. faecalis to withstand drug-induced killing, and the superoxide anion seems to be the key effector of bacterial death.