Bacterial degradation of glycoprotein sugars in human saliva.

Abstract

PREVIOUS reports1,2 have indicated that sialic acid in human, wax-stimulated saliva is rapidly released and metabolized by the oral flora. Heat (100°, 5 min), or antibiotic, prevents both these reactions from taking place. Further investigations have now shown that the other sugars associated with the salivary glycoproteins (3 hexoses, 2 hexosamines and fucose) are all spontaneously lost from saliva by bacterial action, and this loss may also be prevented by either heat or antibiotics. Wax-stimulated saliva contains, with the exception of fucose, all these sugars in two distinct forms: those in the salivary glycoproteins and those in the many and varied bacteria (108/ml.) also present in the saliva. Many of the bacteria in the saliva contain rhamnose, a 6-deoxy-hexose of the same group of sugars as fucose, but it is possible to distinguish between these two sugars by either paper or thin-layer chromatography3. By this method it was shown that the 6-deoxyhexoses in freshly collected, wax-stimulated saliva consisted of rhamnose and fucose. It was also shown that, on incubation, the saliva rapidly lost all its fuoose and sialic acid, while the rhamnose remained. The hexoses and hexosamines, however, occurred in the salivary glycoproteins and the bacteria, and it was not possible to distinguish between these sources by direct analysis or by chromatography. On incubating wax-stimulated saliva, 50–70 per cent of the hexoses and hexosamines were lost, but it was not possible in this instance to distinguish whether the remaining sugar was derived from the salivary glycoproteins or from the bacteria. If the saliva was collected directly from the salivary ducts (parotid, submaxillary and sublingual) and pooled, there were now present so few bacteria that all the sugars from the glycoproteins remained intact on incubation and the saliva remained clear and viscous. The addition of bacteria, in the form of dental plaque or salivary sediment, to the viscous duct saliva caused a rapid loss of sugars, a drop in viscosity and the formation of a precipitate. After incubation no fucose or sialic acid was present and the amount of residual sugars was approximately equal to that from the added bacteria. In none of the experiments described here has it been possible to detect any but traces of free sugars in the saliva when bacteria have been present, and these sugars were also rapidly metabolized when added in a free state to the saliva. It is thus apparent that the carbohydrate components of the glycoproteins in saliva are labile in the oral environment and are rapidly released and metabolized by bacterial action. The loss of these sugars, particularly sialic acid, provides an explanation for the observed drop in viscosity and formation of a precipitate that occurs spontaneously in wax-stimulated saliva1,2. Previous suggestions that the drop in viscosity is due to a mucinolytic depolymerization of the salivary glycoproteins4 are valid so far as the viscosity-drop is concerned, but this mechanism should produce a product of increased solubility, and, in fact, a heavy precipitate always appears concomitantly with the drop in viscosity. It is tempting to postulate that this is the mechanism whereby the material from the saliva forms a matrix holding together the many bacteria adjacent to the teeth, which is commonly referred to as dental plaque. Support for this is found in the observation that dental plaque is practically devoid of both sialic acid and fucose (< 0.002 per cent), the two essential sugar constituents of salivary glycoproteins that can be identified as belonging exclusively to the glycoproteins and not to the oral bacteria.