Chemostat Studies of the Effect of Environmental Control on Streptococcus sanguis Adherence to Hydroxyapatite

Abstract

Streptococcus sanguis is a major component of early dental plaque. The ability of S. sanguis to adhere to salivary pellicle appears to involve specific bacterial surface receptors. The nature of these receptors is still not known; however, the component molecules may be subject to environmental control as has been shown for teichoic acids and certain proteins. To study these environmental effects, a chemostat was employed to vary the growth conditions of Streptococcus sanguis strain G9B. This strain has been used extensively to study the adhesion of [³H]thymidine-labeled batch-grown cells to saliva-coated hydroxyapatite beads. The effects of dilution rate, pH, and carbon source on adhesion were studied with a competition assay in which the labeled batch cells were used as a reference standard. In this assay, cells from the chemostat were harvested and compared for their ability to inhibit adhesion of labeled cells relative to unlabeled control batch-grown cells. Subsequent studies used chemostat grown cells labeled with [³H]thymidine as a reference standard so that results were internally controlled and reflected only the particular alteration in environment which was studied. These results indicated that when glucose was used as a growth-limiting substrate, cells grown at relatively high dilution rates (D = 0.5 h⁻¹; mean generation time = 1.4 h) behaved similarly to batch-grown cells and appeared to compete for the same binding sites. Cells grown at D = 0.1 h⁻¹ (mean generation time = 7 h) no longer competed with either batch-grown cells or chemostat cells grown at D = 0.5 h⁻¹. Moreover, adsorption isotherms of such slow-growing cells (D = 0.1 h⁻¹) suggested that binding was no longer specific. When fructose was used as the growth-limiting carbohydrate, cells grown at D = 0.1 h⁻¹ did not show this loss of specificity and competed nearly as well as control batch-grown glucose cells. However, the effect of pH appeared to be independent of carbohydrate source, because cells grown in either glucose or fructose at pH 5.5 at D = 0.1 h⁻¹ lost the ability to compete with reference batch or chemostat cells grown at D = 0.5 h⁻¹. This effect was very sharp, since cells grown in the pH range from 6 to 7.5 at D = 0.5 h⁻¹ competed nearly as well as control cells. A similar effect of pH was found for batch cultures grown with excess glucose. These studies reinforce the idea that the environment can profoundly affect the bacterial surface and consequently the ability of the organism to adhere, a property which appears to be a primary event in some infectious diseases and in dental plaque formation.

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