Saliva from subjects harboring Actinobacillus actinomycetemcomitans kills Streptococcus mutans in vitro

Previous research indicated that patients with localized aggressive periodontitis (LAgP) had minimal proximal decay. We speculated that differences in these two proximal dental diseases (LAgP and proximal decay) in LAgP could be due to the effect of saliva on the growth of key microorganisms related to these two infections. Carbon dioxide (CO(2)) is required for growth of Actinobacillus actinomycetemcomitans (Aa), the reputed cause of LAgP. Bicarbonate, a source of CO(2), buffers acid production by Streptococcus mutans (Sm), a key organism associated with caries. The purpose of this study was to determine whether the saliva of LAgP patients and subjects with Aa had higher levels of bicarbonate, or an elevated pH, and/or reduced survival of Sm.

Eleven Aa-positive subjects (seven with LAgP) were matched with 11 Aa-negative controls. A total of 5 ml saliva obtained from each subject was tested for CO(2) levels, pH, and effects on survival of Aa and Sm. Saliva from 22 additional subjects was used for confirmatory data.

CO(2) levels in the test group (Aa-positive subjects) and controls (Aa-negatives) were similar. No clinically relevant differences were found in salivary pH. However, saliva from the test group killed Sm by more than two logs (P <0.05). No effect was seen on Aa. The saliva from the Aa-negative group killed Aa by two logs (P <0.05). No effect was seen on Sm.

Aa-positive subjects had a salivary factor that significantly reduced survival of Sm, which may help to explain the fact that this group typically has minimal proximal decay.