Gallocin A, an atypical two-peptide bacteriocin secreted by Streptococcus gallolyticus subsp. gallolyticus
Streptococcus gallolyticus subsp. gallolyticus (SGG) is an opportunistic gut pathogen associated with colorectal cancer. We previously showed that colonization of the murine colon by SGG in tumoral conditions was strongly enhanced by the production of gallocin A, a two-peptide bacteriocin. Here, we aimed at characterizing the mechanisms of its action and resistance. Using a genetic approach, we demonstrated that gallocin A is composed of two peptides, GllA1 and GllA2, which are inactive alone and act together to kill "target" bacteria. We showed that gallocin A can kill phylogenetically close relatives. Importantly, we demonstrated that gallocin A peptides can insert into membranes and permeabilize lipid bilayer vesicles. Next, we showed that the third gene of the gallocin A operon named GIP, is necessary and sufficient to confer immunity to gallocin A. Structural modelling of GllA1 and GllA2 mature peptides suggested that both peptides form alpha-helical hairpins stabilized by intramolecular disulfide bridges. The presence of a disulfide bond in GllA1 and GllA2 was confirmed experimentally. Addition of disulfide reducing agents abrogated gallocin A activity. Likewise, deletion of a gene encoding a surface protein with a thioredoxin-like domain impaired gallocin A ability to kill Enterococcus faecalis. Structural modelling of GIP revealed a hairpin-like structure strongly resembling that of the GllA1 and GllA2 mature peptides, suggesting a mechanism of immunity by competition with GllA1/2. Finally, identification of other class IIb bacteriocins exhibiting a similar alpha-helical hairpin fold stabilized with an intramolecular disulfide bridge suggests the existence of a new subclass of class IIb bacteriocins. IMPORTANCEStreptococcus gallolyticus subsp. gallolyticus (SGG), previously named Streptococcus bovis biotype I, is an opportunistic pathogen responsible for invasive infections (septicemia, endocarditis) in elderly people and often associated with asymptomatic colon tumors. SGG is one of the first bacteria to be associated with the occurrence of colorectal cancer in humans. Previously, we showed that tumor-associated conditions in the colon provide to SGG with the ideal environment to proliferate at the expense of phylogenetically and metabolically closely related commensal bacteria such as enterococci (Aymeric et al., 2017). SGG takes advantage of CRC-associated conditions to outcompete and substitute commensal members of the gut microbiota using a specific bacteriocin named gallocin and renamed gallocin A recently following the discovery of gallocin D in a peculiar SGG isolate. Here, we showed that gallocin A is a two-peptide bacteriocin and that both GllA1 and GllA2 peptides are required for antimicrobial activity. Gallocin A was shown to permeabilize bacterial membranes and to kill phylogenetically closely related bacteria such as most streptococci, lactococci and enterococci, probably through membrane pore formation. GllA1 and GllA2 secreted peptides are unusually long (42 and 60 amino acids long) and with very few charged amino acids compared to well-known class IIb bacteriocins. In silico modelling revealed that both GllA1 and GllA2 exhibit a similar hairpin-like conformation stabilized by an intramolecular disulfide bond. We also showed that the GIP immunity peptide also forms a hairpin like structure like GllA1/GllA2. Thus, we hypothesize that GIP blocks the formation of the GllA1/GllA2 complex by interacting with GllA1 or GllA2. Gallocin A may constitute the first class IIb bacteriocin displaying disulfide bridges important for its structure and activity and the founding member of a subtype of class IIb bacteriocins.