Objective To explore the feasibility of methyl thiazolyl tetrazolium（MTT） colorimetric method and the applied condition for the normal bacteria in the mouth, as Streptococcus mutans（S.mutans）, Streptococcus sanguis（S. sanguis）, Haemophilus actinomycetemcomitans（H.actinomycetemcomitans）. Methods Colony forming units（CFU） which was the standard antitheses was used to count bacteria. This study would gain some parameters by changing wavelength, reactive time, dosage and so on. MTT colorimetric method was applied in the counting of S.mutans, S. sanguis and H.actinomycetemcomitans. Results When counting S.mutans, the best wavelength was 510 nm, the best range was 1.5×105～1.0×107 CFU·mL-1. When counting S.sanguis, the best wavelength was 545 nm, the best range was 1.5×105～2.0 ×107 CFU·mL -1. When counting H.actinomycetemcomitans, the best wavelength was 557 nm, the best range was 1.0×106～5.0×107 CFU·mL-1. MTT colorimetric method can be used for different aged S.mutans, S.sanguis and H.actinomycetemcomitans. Conclusion Oral bacteria could be counted by MTT colorimetric method, which is fast and convenient.
