Investigating Acid Tolerance Mechanisms of Streptococcus sobrinus via Interspecies Microarrays

The dental pathogen Streptococcus sobrinus is capable of generating an adaptive acid tolerance response (ATR), a key virulence trait of cariogenic bacteria. S. sobrinus thrives in an acidic environment within the oral cavity where it is capable of undergoing glycolysis at low pH values, allowing for a competitive advantage over other bacteria.   However, exploration of the molecular basis for acid tolerance of this organism has been inadequate in part because of the overwhelming focus on its relative, Streptococcus mutans.  Previously, we showed critical differences in the molecular mechanisms of acid adaptation by S. sobrinus and S. mutans.  Objectives: To further characterize the ATR of S. sobrinus, we took advantage of the availability of S. mutans microarrays, and of the close genetic relationship between these species to conduct interspecies microarray hybridizations. Methods: RNA isolated from S. sobrinus 6715 grown in a chemostat to steady-state pH 7 or pH 5 was used to generate cDNAs that were hybridized to the S. mutans arrays.  Results: The microarrays revealed 76 genes (P ≤ 0.05) with altered levels of expression at pH 5.  Among the down-regulated genes were several genes encoding hypothetical proteins (n =24) and genes involved in amino acid biosynthesis and transport (n = 11).  A small number of genes found up-regulated are known to participate in the S. mutans ATR, including atpA, atpD, dpr, ldh, and smx.  The mleS gene encoding the malolactic enzyme displayed the greatest change in expression (17.9-fold increase). This enzyme is involved in malolactic fermentation (MLF) and has been found to contribute to the ATR of S. mutans.  MLF assays of S. sobrinus revealed a significant increase in MLF activity at pH 5.5 compared to pH 7.  Conclusion: This study further reveals that cariogenic streptococci have developed both common and unique traits for acid tolerance. Funding: NIDCR DE017101