Methods: Twenty Streptococus mutans clinical strains (serotype c) were isolated from 10 caries-free and 10 caries-active individuals (DMFT°Ý6). These strains had been cultured for 24h in Tryptone Polypeptone Yeast Extract (TPY) broth with 10mmol/L or 50mmol/L glucose, whose initial pH was pH7.0(pH0). Then the pH values of media(pH1) were determined by the pH electrode, and the differences between pH1 and pH0 represented their aidogenic abilities. Meanwhile the mid-exponential-phase cells of Streptococcus mutans clinical trains were collected. TRIzol agent was used to extract the total RNA according to the direction of the manufacturer. Real-time PCR with specific TaqMan probes was used to measure the relative ldh mRNA levels. The differences of acidogenic abilities and ldh mRNA levels of the strains between the caries-free and ¨Cactive groups or between groups with different glucose concentrations were compared with t test.
Results: The strains cultured with 10mmol/L glucose produced less acid than those with 50mmol/L glucose. When the glucose was limited, the acidogenic abilities of the strains from caries-free individuals were significantly lower than those of the strains from caries-active ones(p<0.05). The differences of ldh mRNA levels were not significant between the strains cultured with 10mmol/L glucose and those with 50mmol/L. When they were cultured in the TPY broth with 10mmol/L glucose, the ldh mRNA levels of the strains from caries-free groups were significantly higher than those of the strains from caries-active ones(p<0.05).
Conclusion: When the glucose amount in the growth environment was limited, Streptococcus mutans produced less acid and their ldh expression was induced.