Abstract

Enterococcus faecalis is a commensal Gram-positive pathogen found in the intestines of mammals, and is also a leading cause of severe infections occurring mainly among antibiotic-treated dysbiotic hospitalized patients. Like most intestinal bacteria, E. faecalis does not synthesize heme. Nevertheless, environmental heme can improve E. faecalis fitness by activating respiration metabolism and a catalase that limits hydrogen peroxide stress. Since free heme also generates redox toxicity, its intracellular levels need to be strictly controlled. Here, we describe a unique transcriptional regulator, FhtR, (Faecalis heme transport Regulator), which manages heme homeostasis by controlling an HrtBA-like efflux pump (named HrtBAEf). We show that FhtR, by managing intracellular heme concentration, regulates the functional expression of the heme dependent catalase A (KatA), thus participating in heme detoxification. The biochemical features of FhtR binding to DNA, and its interactions with heme that induce efflux, are characterized. The FhtR-HrtBAEf system is shown to be relevant in a mouse intestinal model. We further show that FhtR senses heme from blood and hemoglobin but also from crossfeeding by Escherichia coli. These findings bring to light the central role of FhtR heme sensing in response to heme fluctuations within the gastrointestinal tract, which allow this pathogen to limit heme toxicity while ensuring expression of an oxidative defense system. ImportanceEnterococcus faecalis, a normal, harmless colonizer of the human intestinal flora can cause severe infectious diseases in immunocompromised patients, particularly those that have been heavily treated with antibiotics. Therefore, it is important to understand the factors that promote its resistance and its virulence. Here, we report a new mechanism used by E. faecalis to detect the concentration of heme, an essential but toxic metabolite that is present in the intestine. E. faecalis needs to scavenge this molecule to respire and fight stress generated by oxydants. Heme sensing triggers the synthesis of a heme efflux pump that balances the amount of heme inside the bacteria. With this mechanism, E. faecalis can use heme without suffering from its toxicity.