Demonstration of mother-to-child transmission of Streptococcus mutans using multilocus sequence typing

A new reliable genotyping method, multilocus sequence typing (MLST), was used to evaluate vertical transmission of the cariogenic pathogen Streptococcus mutans. A total of 136 S. mutans strains were isolated from saliva samples of 20 Japanese mother-child pairs, including 5 girls and 5 boys with primary dentition, and 5 girls and 5 boys with mixed dentition. The nucleotide sequences of 8 partial housekeeping genes, aroE, murI, gltA, glnA, glk, tkt, lepC, and gyrA, were analyzed and a similarity for all of those sequences between strains from a mother-child pair was regarded as indicating transmission, which was shown in 70% of the pairs. Interestingly, the rate of transmitted strains from mothers was significantly higher in the girls (90%) than in the boys (p = 0.001). Furthermore, the S. mutans sequence type (ST) with the highest distribution percentage in each maternal saliva sample was found to be transferred to their children. In addition, variations in two large conjugative-transfer associated regions, TnSmu1 and TnSmu2, were determined and compared with the STs defined by MLST. No variations in those two regions shown by PCR patterns were present in any of the strains isolated from the same families with the same STs, though isolates of some STs from different families showed distinct patterns for TnSmu2. Our results indicate that mothers are the main source for transmission of S. mutans to their children, while the present MLST method was also shown to be useful for investigating bacterial transmission.