A Novel Enterococcus faecalis Heme Transport Regulator (FhtR) Senses Host Heme To Control Its Intracellular Homeostasis

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 (in this report, heme refers to iron protoporphyrin IX regardless of the iron redox state). 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 toxicity, its intracellular levels need to be strictly controlled. Here, we describe a unique transcriptional regulator, FhtR (named FhtR for faecalis heme transport regulator), which manages heme homeostasis by controlling an HrtBA-like efflux pump (named HrtBAEf for the HrtBA from E. faecalis). 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 interaction with heme that induces 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 heme sensing by FhtR 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. IMPORTANCE Enterococcus faecalis, a normal and 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. E. faecalis, which cannot synthesize heme, an essential but toxic metabolite, needs to scavenge this molecule from the host to respire and fight stress generated by oxidants. Here, we report a new mechanism used by E. faecalis to sense heme and trigger the synthesis of a heme efflux pump that balances the amount of heme inside the bacteria. We show in a mouse model that E. faecalis uses this mechanisms within the gastrointestinal tract.

The Many Faces of Enterococcus spp.—Commensal, Probiotic and Opportunistic Pathogen

“…Enterococci have also been isolated from plants such as olives [ 6 ] and are found on plants in the wild [ 7 , 8 ]. Some enterococci species are commensal, can stimulate the immune system, and have a significant influence on the maintenance of intestinal homeostasis [ 9 , 10 ]. Enterococci can be used as a factor to support the immune system in the form of a probiotic (diet supplement or therapeutic application).…”

Deconstructing the role of iron and heme in the structure of healthy human gut microbial communities

“…Iron deprivation did not have detrimental effects on the Enterococcaceae and Burkholderiaceae families, which are low-abundance yet prevalent members of the human gut microbiome and are largely understudied. Most knowledge about the Enterococcaceae focuses on pathogenesis, and specifically on infection by the opportunistic pathogen Enterococcus faecalis (Keogh et al, 2016; Krawczyk et al, 2021; Saillant et al, 2021). Literature on iron responses of the Burkholderiaceae family is almost nonexistent (Hiippala et al, 2016).…”

The unforeseen intracellular lifestyle of Enterococcus faecalis in hepatocytes

“…GFP-expressing OG1RF from the pMV158-GFP plasmid was cultured in BHI with 4 µg/ml tetracycline. 51 E. faecalis strain OG1RF, expressing the luxABCDE ( lux ) operon from Photorhabdus luminescens , was a gift from Dr D. Lechardeur (Micalis Institute, INRAE, Center de Recherche Ile de France – Jouy-en-Josas – Antony) 52 and was cultured in BHI with 20 µg/ml erythromycin.…”