Interaction between wild-type, mutant and revertant forms of the bacterium Streptococcus sanguis and the bacterium Actinobacillus actinomycetemcomitans in vitro and in the gnotobiotic rat.

In vitro, Streptococcus sanguis inhibits the growth of Actinobacillus actinomycetemcomitans, a presumed aetiological agent of localized juvenile periodontitis. When provided with glucose and good aeration, a growing culture of Strep. sanguis was found to produce hydrogen peroxide at concentrations in excess of the maximum LD50 reported for strains of A. actinomycetemcomitans. This concentration of hydrogen peroxide also inhibits the growth of the producer organism. A mutant of Strep. sanguis was isolated that lacked the ability to produce alpha-haemolysis on blood agar. This mutant had less than 3 per cent of its parent's level of pyruvate-oxidase activity, and made no detectable hydrogen peroxide. In vitro, the mutant had also lost the ability to inhibit the growth of A. actinomycetemcomitans. A spontaneous revertant, isolated by its ability to produce alpha-haemolysis, was found to have regained parental levels of pyruvate-oxidase activity and hydrogen-peroxide production and could inhibit the growth of A. actinomycetemcomitans in vitro. A gnotobiotic rat model was used to demonstrate that Strep. sanguis and A. actinomycetemcomitans interact in vivo and that this interaction depends on hydrogen-peroxide production by Strep. sanguis.