Mutation of the Thiol-Disulfide Oxidoreductase SdbA Activates the CiaRH Two-Component System, Leading to Bacteriocin Expression Shutdown in Streptococcus gordonii

Abstract

Streptococcus gordonii is a commensal inhabitant of the human oral cavity. To maintain its presence as a major component of oral biofilms, S. gordonii secretes inhibitory molecules such as hydrogen peroxide and bacteriocins to inhibit competitors. S. gordonii produces two nonmodified bacteriocins (i.e., Sth 1 and Sth 2 ) that are regulated by the Com two-component regulatory system, which also regulates genetic competence. Previously we found that the thiol-disulfide oxidoreductase SdbA was required for bacteriocin activity; however, the role of SdbA in Com signaling was not clear. Here we demonstrate that ⌬sdbA mutants lacked bacteriocin activity because the bacteriocin gene sthA was strongly repressed and the peptides were not secreted. Addition of synthetic competence-stimulating peptide to the medium reversed the phenotype, indicating that the Com pathway was functional but was not activated in the ⌬sdbA mutant. Repression of bacteriocin production was mediated by the CiaRH two-component system, which was strongly upregulated in the ⌬sdbA mutant, and inactivation of CiaRH restored bacteriocin production. The CiaRH-induced protease DegP was also upregulated in the ⌬sdbA mutant, although it was not required for inhibition of bacteriocin production. This establishes CiaRH as a regulator of Sth bacteriocin activity and links the CiaRH and Com systems in S. gordonii. It also suggests that either SdbA or one of its substrates is an important factor in regulating activation of the CiaRH system. IMPORTANCEStreptococcus gordonii is a noncariogenic colonizer of the human oral cavity. To be competitive in the oral biofilm, S. gordonii secretes antimicrobial peptides called bacteriocins, which inhibit closely related species. Our previous data showed that mutation of the disulfide oxidoreductase SdbA abolished bacteriocin production. In this study, we show that mutation of SdbA generates a signal that upregulates the CiaRH two-component system, which in turn downregulates a second two-component system, Com, which regulates bacteriocin expression. Our data show that these systems are also linked in S. gordonii, and the data reveal that the cell's ability to form disulfide bonds is sensed by the CiaRH system. O ral biofilms are highly competitive, constantly fluctuating environments. The bacteria that colonize this niche contend with a high density of competing bacteria of an estimated 2,000 different taxa and dramatic environmental changes that are dependent on host behavior (1, 2). Streptococcus gordonii is a pioneer colonizer of this environment, where it initiates biofilm formation by binding directly to the acquired salivary pellicle on the tooth surface (2, 3). Once established, S. gordonii persists within the host as part of the oral microbiota. The presence of S. gordonii is associated with oral health (4, 5), and it has been shown to inhibit biofilm formation by cariogenic species (6, 7).S. gordonii uses several strategies to gain advantage over its competitors and to colonize the oral cavity successfully...

Mutation of the Streptococcus gordonii Thiol-Disulfide Oxidoreductase SdbA Leads to Enhanced Biofilm Formation Mediated by the CiaRH Two-Component Signaling System

“…Previously, we found that expression of the two-component signalling system CiaRH is upregulated in the ΔsdbA mutant [19]. Although the role of CiaRH in biofilm formation by S .…”

“…CiaRH represses the ComDE quorum sensing system in the ΔsdbA mutant by inhibiting production of the CSP autoinducer [19]. Accordingly, de-repression of ComDE can be achieved in two ways: (1) inactivate the CiaRH system or (2) expose cells to exogenous CSP [19].…”

“…Accordingly, de-repression of ComDE can be achieved in two ways: (1) inactivate the CiaRH system or (2) expose cells to exogenous CSP [19]. Given that inactivation of CiaRH reduced biofilm formation in the ΔsdbA mutant, we asked whether exogenous CSP would produce a similar effect.…”

“…These bonds are important for the folding and stability of certain proteins, and ΔsdbA mutants are unable to form disulfide bonds. This generates a stress signal that triggers activation of the two-component signalling system CiaRH, presumably in response to an accumulation of misfolded proteins [19]. CiaH is a histidine kinase located at the membrane that activates the response regulator CiaR, which then drives the expression of multiple proteins including DegP (HtrA), a quality control protease that degrades aberrant proteins at the cell surface [19,20].…”

“…This ultimately leads to expression of the bacteriocin genes, as well as genetic competence. However, activation of CiaRH in the ΔsdbA mutant eliminates CSP production, effectively shutting down the ComDE pathway and bacteriocin production [19]. Thus the pleiotropic phenotype of the ΔsdbA mutant involves multiple mechanisms, some of which are not directly related to disulfide bond formation.…”