A Defined Artificial Saliva Medium Allows Characterization of the Hypothiocyanite Response of  <i>Streptococcus sanguinis</i> and <i>Streptococcus mutans</i>

Hypothiocyanite (OSCN^-/HOSCN) is an antimicrobial molecule found at high concentrations in saliva. HOSCN is thought to differentially affect oral streptococci, since noncariogenic streptococci (e.g. Streptococcus sanguinis) possess HOSCN reductase activity that cariogenic streptococci (e.g. Streptococcus mutans) lack. However, the enzyme responsible for this activity and the effects of HOSCN and HOSCN reductase activity on biofilm formation by oral streptococci have not been previously established. In this work, we developed an artificial saliva medium for growth of oral streptococci with minimal redox-active components, called Defined Recipe Optimized Oral Liquid (DROOL), and used it to characterize the HOSCN responses of S. sanguinis and S. mutans. We identified a homolog of the Streptococcus pneumoniae Har protein in S. sanguinis as HOSCN reductase. S. mutans wild-type and S. sanguinis ∆har mutants were more sensitive to inhibition by physiological concentrations of HOSCN in DROOL than wild-type S. sanguinis when grown planktonically. S. mutans biofilm formation and glucan production were strongly decreased by HOSCN treatment, suggesting HOSCN inhibits S. mutans exopolysaccharide production. Collectively, our data demonstrate the specific ability of HOSCN to inhibit functions of cariogenic but not noncariogenic oral streptococci and show that Har is responsible for mediating this difference.