Competence-dependent endogenous DNA rearrangement and uptake of extracellular DNA give a natural variant of Streptococcus mutans without biofilm formation.

The production of water-insoluble glucan (WIG) enables Streptococcus mutans to survive and persist in the oral niche. WIG is produced from sucrose by glucosyltransferase encoded tandemly by the highly homologous gtfB and gtfC genes. Conversely, a single hybrid gene from the endogenous recombination of gtfB and gtfC is easily generated using RecA, resulting in S. mutans UA159 WIG- (rate of ∼1.0×10(-3)). The pneumococcus recA gene is regulated as a late competence gene. comX gene mutations did not lead to the appearance of WIG- cells. The biofilm collected from the flow cell had more WIG- cells than among the planktonic cells. Among the planktonic cells, WIG- cells appeared after 16 h and increased ∼10-fold after 32 h of cultivation, suggesting an increase in planktonic WIG- cells after longer culture. The strain may be derived from the biofilm environment. In coculture with donor WIG+ and recipient WIG- cells, the recipient cells reverted to WIG+ and acquired an intact gtfBC region from the environment, indicating that the uptake of extracellular DNA resulted in the phenotypic change. Here we demonstrate that endogenous DNA rearrangement and uptake of extracellular DNA generate WIG- cells and that both are induced by the same signal transducer, the com system. Our findings may help in understanding how S. mutans can adapt to the oral environment and may explain the evolution of S. mutans.