FIG. 3.

Illustration of interspecies interactions between Streptococcus mutans and selected oral streptococci that have been studied on a molecular level. (a) Interactions of S. mutans with S. gordonii, S. oligofermentans, and S. sanguinis. Depicted is the production of lactic acid, mutacin, and CSP, which also enhances mutacin production by S. mutans. Lactic acid has been found to inhibit proliferation of a variety of acid-sensitive anaerobe species, but it was recently discovered to serve as a substrate for S. oligofermentans for H₂O₂ production. This compound is also generated by S. sanguinis and was previously shown to inhibit growth of S. mutans. S. sanguinis and S. gordonii are sensitive to the mutacins produced by S. mutans. In contrast to S. sanguinis, which can inhibit S. mutans by its ability to produce H₂O₂, S. gordonii utilizes challisin to reduce mutacin production by reducing the levels of the stimulating factor CSP. The dashed arrow in combination with + indicates stimulation, solid arrows symbolize production, and ⊣ in combination with − designates inhibition. The shaded area indicates the timing-sensitive interactions between S. mutans and S. sanguinis illustrated in panel b. (b) Importance of timing for reciprocal inhibition of S. mutans (Sm) and S. sanguinis (Ss). Given a 1-day growth advantage, S. mutans inhibits S. sanguinis via its mutacin production (upper panel), whereas S. sanguinis controls establishment of S. mutans by its ability to generate H₂O₂ when given the same 1-day advantage (middle panel). Without the benefit to either of establishing itself earlier, both species coexist without any apparent mutual inhibition (lower panel). For further details, see the text.