Allelic Variation in gtfB–gtfC Region of Natural Variant of Streptococcus mutans Without Biofilm Formation

Streptococcus mutans is primarily found in biofilms on tooth surfaces and is associated with the development of dental caries. S. mutans synthesizes water-insoluble glucan (WIG) using sucrose as a substrate, inducing the formation of three-dimensional biofilms. WIG is produced by glucosyltransferases (GTFs) encoded by the tandem and highly homologous gtfB and gtfC genes. Conversely, the homologous recombination of gtfB and gtfC readily happens, producing natural variants without WIG. These WIG− variants are thought to have ecologically pleiotropic functions; however, the molecular basis for their appearance is unclear. This study aimed to determine the sequence of the gtfB–gtfC regions of WIG− variants. We sequenced the gtfB–gtfC regions of 23 WIG− variants derived from S. mutans UA159 and revealed the presence of five alleles and four types of single homologous recombination patterns. Regardless of the allele type, the WIG− variants showed low biofilm formation and GTF activity. To the best of our knowledge, this is the first study to report the presence of alleles in WIG− variants. These findings provide important information for explaining the appearance of mechanisms in WIG− variants.