Role of ABC transporter proteins in stress responses of Streptococcus mutans

Streptococcus mutans possesses a number of ATP-binding cassette (ABC) transporters, which function as multiple sugar metabolism transporters and are promising targets for antimicrobial strategies. In the present study, we performed functional analyses of SMu0836 and SMu0837 products, which are possible ABC transporters. Isogenic mutant strains Δ0836 and Δ0837 were generated by insertional inactivation of SMu0836, and SMu0837, respectively, of strain MT8148 and found to be more sensitive to antibiotics as compared to MT8148. In addition, assays of membrane transport functions using a fluorescent probe showed that export pumps did not function properly in strain Δ0836. Expression of those genes was elevated when strain MT8148 was cultured with a sublethal concentration of tetracycline, as well as when exposed to heat shock, osmotic stress, and oxidative stress conditions. Furthermore, the expressions of other genes including SMu0374, SMu0215, SMu0475, SMu0986, and SMu1051, which encode possible ABC transporters, were also elevated when strain MT8148 was cultured with a sublethal concentration of tetracycline. Together, these results suggest that the products of SMu0836 and SMu0837 are ABC transporters, which may function in stress response.