Butyric Acid Inhibits Biofilm Formation but Promotes Initial Attachment and Aggregation by <i>Streptococcus gordonii</i>

Streptococcus gordonii, an initial attachment bacterium, forms a scaffold for oral biofilm formation and has been detected in the human mouths. S. gordonii is a nonpathogenic species associated with infective endocarditis. Extracellular DNA（eDNA） released by the bacteria contributes to biofilm formation. Short - chain fatty acids （SCFAs）increase the initial attachment and aggregation, and biofilm formation of Actinomyces spp. However, their relationship with other initially adhering bacteria remains unclear. Therefore, this study aimed to clarify the effects of butyric acid（a SCFA）on the initial attachment and aggregation, and biofilm formation of S. gordonii. Butyric acid inhibited biofilm formation by S. gordonii but promoted initial attachment and aggregation. However, under conditions that mimicked the pH during butyric acid addition using HCl, initial attachment and aggregation were not promoted. In addition, initial attachment and aggregation promotion of S. gordnoii, facilitated by butyric acid addition, was inhibited by adding DNA degradation enzymes（DNase I）. The initial attachment and aggregation of bacteria were live/dead stained and observed using a confocal laser microscope. Butyric acid increased the number of live and dead bacteria. In contrast, DNase I decreased the numbers of live and dead bacteria. Thus, butyric acid, which is present in the oral cavity at concentrations that can be reached, promotes the initial attachment and aggregation of S. gordonii.