Glucose PTS Modulates Pyruvate Metabolism, Bacterial Fitness, and Microbial Ecology in Oral Streptococci

Spontaneous mutants with defects in the primary glucose phosphotransferase (PTS) permease (manLMNO) of Streptococcus sanguinis SK36 showed enhanced fitness at low pH. Transcriptomics and metabolomics with a manL deletion mutant (SK36/manL) revealed redirection of pyruvate to production of acetate and formate, rather than lactate. The observations were consistent with measurements of decreased lactic acid accumulation and increased excretion of pyruvate and H2O2. Genes showing increased expression in SK36/manL included those encoding carbohydrate transporters, extracellular glycosidases, intracellular polysaccharide (IPS) metabolism, arginine deiminase, and pathways for metabolism of acetoin, ethanolamine, ascorbate and formate; along with genes required for membrane biosynthesis and adhesion. Streptococcus mutans UA159 persisted much better in biofilm co-cultures with SK36/manL than with SK36, an effect that was further enhanced by culturing the biofilms anaerobically but dampened by adding arginine to the medium. We posited that the enhanced persistence of S. mutans with SK36/manL was in part due to excess excretion of pyruvate by the latter, as addition of pyruvate to S. mutans-S. sanguinis co-cultures increased the proportions of UA159 in the biofilms. Reduction of the buffer capacity or increasing the concentration of glucose benefited UA159 when co-cultured with SK36, but not with SK36/manL; likely due to the altered metabolism and enhanced acid tolerance of the mutant. When manL was deleted in S. mutans or Streptococcus gordonii, the mutants presented altered fitness characteristics. Our study demonstrated that PTS-dependent modulation of central metabolism can profoundly affect streptococcal fitness and metabolic interactions, revealing another dimension in commensal-pathogen relationships influencing dental caries development. ImportanceDental caries is underpinned by a dysbiotic microbiome and increased acid production. As beneficial bacteria that can antagonize oral pathobionts, oral streptococci such as S. sanguinis and S. gordonii can ferment many carbohydrates, despite their relative sensitivity to low pH. We characterized the molecular basis for why mutants of glucose transporter ManLMNO of S. sanguinis showed enhanced production of hydrogen peroxide and ammonia, and improved persistence under acidic conditions. Significant metabolic shift involving more than 300 genes required for carbohydrate transport, energy production, and envelope biogenesis was observed. Significantly, manL mutants engineered in three different oral streptococci displayed altered capacities for acid production and interspecies antagonism, highlighting the potential for targeting the glucose-PTS to modulate the pathogenicity of oral biofilms.