Bacteria exposed to transient host environments can elicit adaptive responses by triggering the differential expression of genes via two-component signal transduction systems. This study describes the vicRK signal transduction system in Streptococcus mutans. A vicK (putative histidine kinase) deletion mutant (SmuvicK) was isolated. However, a vicR (putative response regulator) null mutation was apparently lethal, since the only transformants isolated after attempted mutagenesis overexpressed all three genes in the vicRKX operon (Smuvic ؉ ). Compared with the wild-type UA159 strain, both mutants formed aberrant biofilms. Moreover, the vicK mutant biofilm formed in sucrose-supplemented medium was easily detachable relative to that of the parent. The rate of total dextran formation by this mutant was remarkably reduced compared to the wild type, whereas it was increased in Smuvic ؉ . Based on real-time PCR, Smuvic ؉ showed increased gtfBCD, gbpB, and ftf expression, while a recombinant VicR fusion protein was shown to bind the promoter regions of the gtfB, gtfC, and ftf genes. Also, transformation efficiency in the presence or absence of the S. mutans competence-stimulating peptide was altered for the vic mutants. In vivo studies conducted using SmuvicK in a specific-pathogenfree rat model resulted in significantly increased smooth-surface dental plaque (Pearson-Filon statistic [PF], <0.001). While the absence of vicK did not alter the incidence of caries, a significant reduction in SmuvicK CFU counts was observed in plaque samples relative to that of the parent (PF, <0.001). Taken together, these findings support involvement of the vicRK signal transduction system in regulating several important physiological processes in S. mutans.

The gene encoding fimA, a 36 kDa fimbrial adhesion of Streptococcus parasanguis FW213, is highly conserved in all four genetic groups of sanguis streptococci. FimA-like peptides were produced by all strains tested. The nucleotide sequence directly upstream of fimA contains two open reading frames, ORF5 and ORF1, whose deduced protein products are homologous to members of a superfamily of ATP-binding cassette membrane transport proteins, including both prokaryotic and eukaryotic uptake and export systems. The amino acid sequence of FimA contains the consensus prolipoprotein cleavage site (LxxC) common to the 'periplasmic' binding proteins of Gram-positive transport systems. The deduced product of ORF5 is a 28.6 kDa membrane-associated protein that has the consensus binding site for ATP (GxxGxGKS). It shares significant homology with AmiE of Streptococcus pneumoniae as well as with Escherichia coli proteins involved in iron(III) uptake. Allelic-replacement mutagenesis of ORF5 resulted in greatly increased resistance to aminopterin. These data demonstrate functionality with the amiE locus as well. The deduced product of ORF1 is an extremely hydrophobic integral membrane protein of 30.8 kDa with a pattern of six potential membrane-spanning regions, typical of a component of these types of transport system. The nucleotide sequence downstream of fimA, ORF3, encodes a 20 kDa protein having 78% identity with the 20 kDa protein encoded downstream of ssaB, a fimA homologue in S. sanguis 12. It also exhibits significant homology with bacterioferritin co-migratory protein (Bcp) of E. coli K-12. Allelic-replacement mutagenesis in the fimA locus of FW213 showed that (i) expression of fimA was initiated at a site far upstream of the fimA start codon, and (ii) expression of fimA was not linked to expression of ORF3. Northern blots probed with internal fragments of ORF5, ORF1, fimA or ORF3 hybridized to the same transcript of 3.3 kb, which suggested that these loci were transcribed as a polycistronic message. The ORF3 probe also hybridized to a 540 bp transcript consistent with the size of ORF3 alone and supportive of the mutagenesis data of non-linkage. Strains mutated in fimA continued to produce fimbriae, indicating that FimA was not the fimbrial structural subunit. Immunoelectron microscopy revealed FimA was localized at the tips of the fimbriae of FW213. This is the first study that demonstrates that an adhesin which binds a bacterial cell to a substrate is associated with an ATP-binding cassette.

Streptococcus mutans is considered one of the primary etiologic agents of dental caries. Previously, we characterized the VicRK two-component signal transduction system, which regulates multiple virulence factors of S. mutans. In this study, we focused on the vicX gene of the vicRKX tricistronic operon. To characterize vicX, we constructed a nonpolar deletion mutation in the vicX coding region in S. mutans UA159. The growth kinetics of the mutant (designated SmuvicX) showed that the doubling time was longer and that there was considerable sensitivity to paraquat-induced oxidative stress. Supplementing a culture of the wild-type UA159 strain with paraquat significantly increased the expression of vicX (P < 0.05, as determined by analysis of variance [ANOVA]), confirming the role of this gene in oxidative stress tolerance in S. mutans. Examination of mutant biofilms revealed architecturally altered cell clusters that were seemingly denser than the wild-type cell clusters. Interestingly, vicX-deficient cells grown in a glucose-supplemented medium exhibited significantly increased glucosyltransferase B/C (gtfB/C) expression compared with the expression in the wild type (P < 0.05, as determined by ANOVA). Moreover, a sucrose-dependent adhesion assay performed using an S. mutans GS5-derived vicX null mutant demonstrated that the adhesiveness of this mutant was enhanced compared with that of the parent strain and isogenic mutants of the parent strain lacking gtfB and/or gtfC. Also, disruption of vicX reduced the genetic transformability of the mutant approximately 10-fold compared with that of the parent strain (P < 0.05, as determined by ANOVA). Collectively, these findings provide insight into important phenotypes controlled by the vicX gene product that can impact S. mutans pathogenicity.

Early epidemiological studies implicated manganese (Mn) as a possible caries-promoting agent, while laboratory studies have indicated that manganese stimulates the expression of virulence-related factors in the dental pathogen Streptococcus mutans. To better understand the importance of manganese homeostasis to S. mutans pathophysiology, we first used RNA sequencing to obtain the global transcriptional profile of S. mutans UA159 grown under Mn-restricted conditions. Among the most highly expressed genes were those of the entire sloABC operon, encoding a dual iron/manganese transporter, and an uncharacterized gene, here mntH, that codes for a protein bearing strong similarity to Nramp-type transporters. While inactivation of sloC, which encodes the lipoprotein receptor of the SloABC system, or of mntH alone had no major consequence for the overall fitness of S. mutans, simultaneous inactivation of sloC and mntH (ΔsloC ΔmntH) impaired growth and survival under Mn-restricted conditions, including in human saliva or in the presence of calprotectin. Further, disruption of Mn transport resulted in diminished stress tolerance and reduced biofilm formation in the presence of sucrose. These phenotypes were markedly improved when cells were provided with excess Mn. Metal quantifications revealed that the single mutant strains contained intracellular levels of Mn similar to those seen with the parent strain, whereas Mn was nearly undetectable in the ΔsloC ΔmntH strain. Collectively, these results reveal that SloABC and MntH work independently and cooperatively to promote cell growth under Mn-restricted conditions and that maintenance of Mn homeostasis is essential for the expression of major virulence attributes in S. mutans. IMPORTANCE As transition biometals such as manganese (Mn) are essential for all forms of life, the ability to scavenge biometals in the metal-restricted host environment is an important trait of successful cariogenic pathobionts. Here, we showed that the caries pathogen Streptococcus mutans utilizes two Mn transport systems, namely, SloABC and MntH, to acquire Mn from the environment and that the ability to maintain the cellular levels of Mn is important for the manifestation of characteristics that associate S. mutans with dental caries. Our results indicate that the development of strategies to deprive S. mutans of Mn hold promise in the combat against this important bacterial pathogen.