Objective   To study the role and possible mechanism of dltD in the acid tolerance of Streptococcus mutans 593 (SM593), and to provide a theoretical basis for the ecological prevention and control of dental caries by constructing the dltD gene deletion strain of SM593 (SM593-ΔdltD).

  Methods  1) SM593-ΔdltD was constructed by homologous recombination. 2) The growth curve of SM593 dltD and SM593-ΔdltD under different pH culture conditions was drawn by the automatic growth curve analyzer to compare their acid tolerance. Colony forming unit (CFU) at different time points was used to calculate the survival rate and to compare the acid tolerance response (ATR) of SM593 and SM593-ΔdltD. 3) Under different pH conditions, glycolysis experiments, proton permeability test and H⁺-ATPase activity test were conducted to make preliminary exploration into the mechanisms of how dltD gene deletion may affect acid tolerance.

  Results  1) PCR and sequencing results showed that the SM593-ΔdltD was constructed successfully. 2) With decreasing pH value of the culture medium, the growth of SM593-ΔdltD slowed down. When the pH value of the culture medium was 5.0, SM593-ΔdltD was not allowed to grow, and its acid tolerance was lower than that of SM593. Compared with SM593, the ATR capability of SM593-ΔdltD was decreased. 3) SM593 dltD and SM593-ΔdltD did not show obvious difference in their glycolysis ability under different pH conditions. Compared with SM593 dltD, the proton permeability of SM593-ΔdltD under different pH conditions was increased significantly (P<0.05), and H⁺-ATPase activity decreased significantly (P<0.05).

  Conclusion  Compared with SM593 dltD, SM593-ΔdltD showed obvious decrease in acid tolerance, which may be caused by the significant increase in proton permeability and significant decrease in the H⁺-ATPase activity induced by the deletion of the dltD gene, hence reducing its ability to maintain intracellular pH homeostasis.