Abstract: Objective To investigate the types of sepM gene mutations and their distribution in clinical isolates of Streptococcus mutans (S. mutans) and explore the association of sepM gene mutation with the capacity of mutacin IV production by S. mutans. Methods We assessed the capacity of mutacin IV production in 80 clinical isolates of S. mutans using an inhibition zone assay. The minimum spanning tree and phylogenetic tree of these isolates were constructed using core genome multilocus sequence typing and maximum likelihood method, respectively. GeneMarkS software was used to predict the coding genes of these isolates, and the predicted genes were blasted against the sepM gene sequence of the reference genome UA159 to determine sepM gene mutations and their distribution characteristics in the clinical isolates. The mutation types affecting mutacin IV production were identified by analyzing the differentially distributed mutations between mutacin IV-producing isolates and mutacin IV-free isolates and by comparing the inhibition zones between isolates with sepM gene mutations and those without mutations. Results Among the 80 clinical isolates of S. mutans, 25 isolates were capable of mutacin IV production and 55 did not produce mutacin IV. The minimum spanning tree showed that the allelic differences were less among the mutacin IV producing isolates than among the mutacin IV-free isolates, and the origins of the mutacin IV-producing isolates were similar. We identified a total of 34 single base mutations in the 80 clinical isolates, and among them, C31T (P=0.001), G533A (P<0.001), C756T (P=0.025), and C1036T (P=0.003) showed significant differential distributions between the mutacin IV- producing and mutacin IV-free isolates. These differentially distributed mutations were positively correlated with the capacity of mutacin IV production of the bacteria. Conclusion sepM gene mutations that affect the capacity of mutacin IV production are present in the clinical strains of S. mutans.

Key words: sepM gene; gene mutation; mutacin IV; Streptococcus mutans