Identification of Bacitracin Resistance Mechanisms in Streptococcus sobrinus

Streptococcus sobrinus is a member of the Streptococcus mutans group and is a cariogenic bacterium similar to S. mutans. One of the characteristics of these bacteria is bacitracin resistance, and BceAB (MbrAB), an ABC transporter of S. mutans, has been reported to be involved in bacitracin resistance. However, the factors responsible for bacitracin resistance in S. sobrinus are unknown. In this study, 11 S. sobrinus strains were isolated from the oral cavity, and their bacitracin susceptibility was assessed. The results showed that 9 strains were low-susceptible to bacitracin (MIC = 128-512 µg/mL), whereas 2 strains were highly susceptible (MIC = 32 µg/mL). Whole-genome analysis of all strains identified genes encoding an ABC transporter (designated as BceAB) and its adjacent two-component system (BceRS), which showed high homology with the BceAB-BceRS locus in S. mutans. We successfully generated bceAB-deletion strains from bacitracin-low-susceptibility strains (HSB4 and HSB10) and a bacitracin-high-susceptibility strain (HSB8). Upon verifying their bacitracin susceptibility, the deletion strains derived from the bacitracin-low-susceptibility strains exhibited increased susceptibility compared to the wild-type strains, whereas the deletion strain of HSB8 showed no change in susceptibility. Quantitative RT-PCR revealed that, in the low-susceptibility HSB5 strain, bacitracin induced bceAB expression in a dose-dependent manner, whereas no such induction was observed in the high-susceptibility HSB7 strain. Comparison of the amino acid sequences of the BceAB-BceRS region among representative clinical isolates revealed that only BceS contained a mutation at the 179th amino acid residue (E179K) specific to the bacitracin-high-susceptibility strain. Based on these results, BceAB was concluded to be an intrinsic factor for bacitracin resistance, and BceRS may be associated with bacitracin-induced expression of BceAB in S. sobrinus.