Regulation of Streptococcus mutans PTS Bio by the transcriptional repressor NigR

XRE family transcriptional regulator XtrSs modulates Streptococcus suis fitness under hydrogen peroxide stress

Streptococcus suis is an important emerging zoonosis that causes economic losses in the pig industry and severe threats to public health. Transcriptional regulators play essential roles in bacterial adaptation to host environments. In this study, we identified a novel XRE family transcriptional regulator in S. suis CZ130302, XtrSs, involved in the bacterial fitness to hydrogen peroxide stress. Based on electrophoretic mobility shift and β-galactosidase activity assays, we found that XtrSs auto-regulated its own transcription and repressed the expression of its downstream gene psePs, a surface protein with unknown function in S. suis, by binding to a palindromic sequence from the promoter region. Furthermore, we proved that the deletion of the psePs gene attenuated bacterial antioxidant response. Phylogenetic analysis revealed that XtrSs and PsePs naturally co-existed as a combination in most S. suis genomes. Collectively, we demonstrated the binding characteristics of XtrSs in S. suis and provided a new insight that XtrSs played a critical role in modulating psePs to the hydrogen peroxide resistance of S. suis.

Spontaneous Mutants of Streptococcus sanguinis with Defects in the Glucose-Phosphotransferase System Show Enhanced Post-Exponential-Phase Fitness

Genetic truncations in a gene encoding a putative glucose-phosphotransferase system (PTS) protein (manL, EIIAB^Man) were identified in subpopulations of two separate laboratory stocks of Streptococcus sanguinis SK36; the mutants had reduced PTS activities on glucose and other monosaccharides. To understand the emergence of these mutants, we engineered deletion mutants of manL and showed that the ManL-deficient strain had improved bacterial viability in the stationary phase and was better able to inhibit the growth of the dental caries pathogen Streptococcus mutans. Transcriptional analysis and biochemical assays suggested that the manL mutant underwent reprograming of central carbon metabolism that directed pyruvate away from production of lactate, increasing production of hydrogen peroxide (H₂O₂) and excretion of pyruvate. Addition of pyruvate to the medium enhanced the survival of SK36 in overnight cultures. Meanwhile, elevated pyruvate levels were detected in the cultures of a small but significant percentage (∼10%) of clinical isolates of oral commensal bacteria. Furthermore, the manL mutant showed higher expression of the arginine deiminase system than the wild type, which enhanced the ability of the mutant to raise environmental pH when arginine was present. To our surprise, significant discrepancies in genome sequence were identified between strain SK36 obtained from ATCC and the sequence deposited in GenBank. As the conditions that are likely associated with the emergence of spontaneous manL mutations, i.e., excess carbohydrates and low pH, are those associated with caries development, we propose that glucose-PTS strongly influences commensal-pathogen interactions by altering the production of ammonia, pyruvate, and H₂O₂. IMPORTANCE A health-associated dental microbiome provides a potent defense against pathogens and diseases. Streptococcus sanguinis is an abundant member of a health-associated oral flora that antagonizes pathogens by producing hydrogen peroxide. There is a need for a better understanding of the mechanisms that allow bacteria to survive carbohydrate-rich and acidic environments associated with the development of dental caries. We report the isolation and characterization of spontaneous mutants of S. sanguinis with impairment in glucose transport. The resultant reprograming of the central metabolism in these mutants reduced the production of lactic acid and increased pyruvate accumulation; the latter enables these bacteria to better cope with hydrogen peroxide and low pH. The implications of these discoveries in the development of dental caries are discussed.

Transcriptional Profiling Reveals the Importance of RcrR in the Regulation of Multiple Sugar Transportation and Biofilm Formation in Streptococcus mutans

The ability of Streptococcus mutans to survive and cause dental caries is dependent on its ability to metabolize various carbohydrates, accompanied by extracellular polysaccharide synthesis and biofilm formation. Here, the role of an rel competence-related regulator (RcrR) in the regulation of multiple sugar transportation and biofilm formation is reported. The deletion of the rcrR gene in S. mutans caused delayed growth, decreased biofilm formation ability, and affected the expression level of its multiple sugar transportation-related genes. Transcriptional profiling revealed 17 differentially expressed genes in the rcrR mutant. Five were downregulated and clustered with the sugar phosphotransferase (PTS) systems (mannitol- and trehalose-specific PTS systems). The conserved sites bound by the rcrR promoter were then determined by electrophoretic mobility shift assays (EMSAs) and DNase I footprinting assays. Furthermore, a potential binding motif in the promoters of the two PTS operons was predicted using MEME Suite 5.1.1. RcrR could bind to the promoter regions of the two operons in vitro, and the sugar transporter-related genes of the two operons were upregulated in an rcrR-overexpressing strain. In addition, when RcrR-binding sites were deleted, the growth rates and final yield of S. mutans were significantly decreased in tryptone-vitamin (TV) medium supplemented with different sugars, but not in absolute TV medium. These results revealed that RcrR acted as a transcription activator to regulate the two PTS systems, accompanied by multiple sugar transportation and biofilm formation. Collectively, these results indicate that RcrR is a critical transcription factor in S. mutans that regulates bacterial growth, biofilm formation, and multiple sugar transportation.

IMPORTANCE The human oral cavity is a constantly changing environment. Tooth decay is a commonly prevalent chronic disease mainly caused by the cariogenic bacterium Streptococcus mutans. S. mutans is an oral pathogen that metabolizes various carbohydrates into extracellular polysaccharides (EPSs), biofilm, and tooth-destroying lactic acid. The host diet strongly influences the availability of multiple carbohydrates. Here, we showed that the RcrR transcription regulator plays a significant role in the regulation of biofilm formation and multiple sugar transportation. Further systematic evaluation of how RcrR regulates the transportation of various sugars and biofilm formation was also conducted. Notably, this study decrypts the physiological functions of RcrR as a potential target for the better prevention of dental caries.

The Route of Sucrose Utilization by Streptococcus mutans Affects Intracellular Polysaccharide Metabolism

Streptococcus mutans converts extracellular sucrose (Suc) into exopolysaccharides (EPS) by glucosyl-transferase and fructosyl-transferase enzymes and internalizes Suc for fermentation through the phosphotransferase system (PTS). Here, we examined how altering the routes for sucrose utilization impacts intracellular polysaccharide [IPS; glycogen, (glg)] metabolism during carbohydrate starvation. Strain UA159 (WT), a mutant lacking all exo-enzymes for sucrose utilization (MMZ952), and a CcpA-deficient mutant (∆ccpA) were cultured with sucrose or a combination of glucose and fructose, followed by carbohydrate starvation. At baseline (0h), and after 4 and 24h of starvation, cells were evaluated for mRNA levels of the glg operon, IPS storage, glucose-1-phosphate (G1P) concentrations, viability, and PTS activities. A pH drop assay was performed in the absence of carbohydrates at the baseline to measure acid production. We observed glg operon activation in response to starvation (p<0.05) in all strains, however, such activation was significantly delayed and reduced in magnitude when EPS synthesis was involved (p<0.05). Enhanced acidification and greater G1P concentrations were observed in the sucrose-treated group, but mostly in strains capable of producing EPS (p<0.05). Importantly, only the WT exposed to sucrose was able to synthesize IPS during starvation. Contrary to CcpA-proficient strains, IPS was progressively degraded during starvation in ∆ccpA, which also showed increased glg operon expression and greater PTS activities at baseline. Therefore, sucrose metabolism by secreted enzymes affects the capacity of S. mutans in synthesizing IPS and converting it into organic acids, without necessarily inducing greater expression of the glg operon.