Methods: Each bacterial strain was challenged in batch culture, to maize, samp, brown bread, maize+milk+sugar, maize+gravy, samp+beans and bread+margarine+peanut butter. This was compared against BHI+3% sucrose and 3% sucrose. Samples of bacteria were collected after eight hours of fermentation, pelleted, washed, pre-lysed and total RNA prepared by the hot phenol method. A two-step RT-PCR was used to identify gtf gene transcripts. RFLPs of amplified genes were examined after digestion with HaeIII restriction endonuclease.
Results: The gtfBC genes were expressed in BHI+3% sucrose by all test strains but not in clinical strains challenged to 3% sucrose. In comparison, the gtfI gene was transcribed by all test strains growing in both media containing sucrose. The effect of staple foods on the gtfBC gene in clinical isolates was mainly inhibitory, but variable between strains when challenged to food-mixtures. No statistically significant association between the food challenge, test strain and gtf expression was shown. Gtf restriction profiles between S. mutans and S. sobrinus reference and clinical strains differed from each other and within the same strain.
Conclusions: The level of expression of the gtfBC operon and the gtfI gene is increased in the presence of sucrose compared to other carbohydrates, but is enhanced by additional growth factors. The variation in response between test strains exposed to the same food challenge, suggests strain specificity to different carbohydrate sources.