Cysteine toxicity for oral streptococci and effect of branched-chain amino acids.

Cysteine was bactericidal to strains of Streptococcus mutans and S. salivarius in concentrations that were nontoxic to S. sanguis, S. milleri, or S. mitior when these microorganisms were incubated in a saliva protein-based synthetic medium. Cysteine toxicity for S. mutans also occurred after incubation in synthetic base medium supplemented with amino acids as the nitrogen source for growth. The bactericidal effect of cysteine for S. mutans or S. salivarius in the saliva protein medium was influenced by the cysteine oxidative activity associated with the saliva protein fraction. Valine alone or in combination with leucine or isoleucine was effective in overcoming cysteine toxicity for susceptible strains of S. mutans or S. salivarius. Cysteine toxicity for these oral streptococci may be due to cysteine inhibition of an enzymatic step in the valine-leucine biosynthetic pathway.