Growth association of oral bacteria : lactobacilli and streptococci

Six Lactobacillus spp. and two Streptococcus spp. were isolated from oral cavities of students. These microorganisms were investigated for their capabilities to produce organic acid(s) in Trypticase Soy Broth supplemented with 0.75% (w/v) dextrose or sucrose. Results have demonstrated that lactobacilli grew and fermented sugars more slowly than did streptococci. Mixed cultures of individual lactobacilli with individual streptococci fermented sugars to a greater extent. The reactions of liquid media supporting growth of these mixtures dropped from the initial pH 7.2 to pH 4.6. In mixtures, S. mitis and S. salivarius fermented dextrose more strongly (final pH 4.5) than they did when grown separately (final pH 5.0). Conversely, these two cultures grown in association in Trypticase Soy Broth supplemented with sucrose produced final acidity of PH 5.0, which was not as great as that produced by S. mitis (pH 4.8) and S. salivarius (pH 4.5) grown separately. It appeared that in mixtures, S. mitis and S. salivarius fermented dextrose in a synergistic manner. However, in sucrose the two cultures exhibited a kind of inhibition with regard to acid production.