Comparison of the effects of galactose and glucose on the pH responses of human dental plaque, salivary sediment and pure cultures of oral bacteria

Abstract

Comparisons made in dental plaque in vivo demonstrated that galactose produces a significantly smaller decrease in pH than does glucose. In vitro studies with plaque, salivary sediment and pure cultures of oral bacteria done in the absence of intraoral factors such as flowing saliva confirmed this lesser acidogenicity of galactose. Pure culture showed that most of the bacteria tested produce a moderate to large decrease with glucose but only a few do so with galactose; most produced a moderate to little or no pH response with this sugar. This suggested that the smaller decreases in pH seen in plaque in vivo with galactose were largely due to bacterial differences, basically that resident micro-organisms individually have less galactolytic than glucolytic capability. Variance in capability was attributed to differences in membrane transport processes and metabolic pathways normally available to bacteria for galactose and glucose catabolism. In the in vitro experiments, because plaque and sediment can produce base as readily as they can produce acid, the nitrogenous substrates identified earlier as major stimulants of base formation, urea and arginine, were concurrently examined for their attenuating effects on the galactose and glucose pH responses. These showed, consistent with its lesser acidogenicity, that galactose could be countered more readily in its ability to reduce the pH by either of these two base-forming substrates than could glucose. The effects were different with urea and with arginine; urea attenuation occurred sooner and arginine attenuation later in both plaque and sediment. The corresponding acid-base pH profiles for pure cultures were different. They showed that most oral bacteria lacked one or more of the galactolytic, glucolytic, ureolytic and arginolytic capabilities clearly evident in both plaque and sediment. A significant fall with glucose was found in the pH profiles of most of the bacteria examined, whereas such was not the case with galactose or with the two base-forming substrates examined at the same time. This suggested that mixing pure cultures would be necessary in constructing models to simulate the acid-base pH profiles seen in plaque and sediment with the above substrates. Also, knowledge of the acid-base pH profiles of individual bacteria should make feasible the selection of the appropriate bacteria for such a purpose, as well as facilitate understanding of how plaque microbial composition and its pH responses are related.