Effects of diadenylate cyclase deficiency on synthesis of extracellular polysaccharide matrix of Streptococcus mutans revisit

An emerging secondary messenger c-di-AMP plays an important role in bacterial physiology. It was reported by Cheng et al. that inactivation of a gene coding for diadenylate cyclase (DAC), a c-di-AMP producing enzyme, resulted in enhanced synthesis of extracellular polysaccharides (EPS) by a cariogenic bacterium, Streptococcus mutans (Cheng et al., 2016). We constructed a similar mutant and observed a completely different effect, the DAC deficiency resulted in a decrease in the production of EPS. Our studies provided the following compelling evidence, (1) the DAC mutant we constructed can be readily complemented for the production of EPS, while the mutant from the Cheng group cannot; (2) Our mutant exhibits the regular pattern of key enzymes that produce EPS, glucosyltransferases (Gtfs), while Cheng et al. reported an irregular pattern, which was inconsistent with their earlier studies. (3) We demonstrated that the response of the DAC mutant to oxidative stress is independent of GtfB, the key enzyme producing EPS, while the Cheng report suggests that overproduction of EPS is a responsive mechanism for the DAC mutant to adapt to the oxidative stress. Therefore, the validity of the relationship between DAC and EPS reported by Cheng et al. warrants further investigation and clarification.