The Streptococcus mutans Rhamnose-glucose Polysaccharide Plays an Important Role in Oxidative Stress Resistance and Iron Homeostasis

The rhamnose-glucose polysaccharide (RGP) of Streptococcus mutans is known to play an essential role in cell division and biofilm formation. It has become increasingly apparent that the structure plays a role in protection against external stress, including an undefined role in oxidative stress. In this study, we demonstrate that the loss of RGP impairs the uptake of glucose and the susceptibility of S. mutans to lethal concentrations of iron, while at the same time, generates elevated content of intracellular iron, a prominent producer of reactive oxygen species. Intracellular iron levels were significantly higher in a strain lacking rhamnose production ({Delta}rmlD), compared to a strain deficient in components of the RGP production pathway ({Delta}rgpG). The seemingly paradoxical discrepancy between iron uptake and iron susceptibility may be partially explained by increased expression of dpr and sod, genes that protect against oxidative stress and prime S. mutans against oxidative stressors such as H2O2 and superoxide. Analysis of the promoter of the rhamnose biosynthesis gene rmlD indicated the presence of regulatory motifs, including Rex and PerR, which we have shown control expression for the genes responsible for rhamnose biosynthesis and RGP production. These results provide initial insights into the role that RGP plays in toleration of oxidative stress and suggest a complexity in regulation of the genes responsible for production of the RGP. ImportanceDental caries, a disease caused by Streptococcus mutans, imposes significant healthcare burdens and productivity losses in the US. The outer structure of S. mutans, which is primarily composed of the rhamnose-glucose polysaccharide (RGP), is understudied, though it plays a crucial role in survival of the organism in response to environmental stress. Elucidating the protective role of RGP and the regulatory mechanisms that control the synthesis of RGP in S. mutans will lead to insights into the structure and function of cell wall polysaccharides in other streptococci. This study reveals a role for L-rhamnose, the building block of RGP, in mediating homeostasis of iron uptake, while the loss of rhamnose biosynthesis causes alterations in transcription of sod and dpr.