Digestion of proteoglycan by Bacteroides thetaiotaomicron

It has been shown previously that Bacteroides thetaiotaomicron, a human colonic anaerobe, can utilize the tissue mucopolysaccharide chondroitin sulfate as a source of carbon and energy and that the enzymes involved in this utilization are all cell associated (A. A. Salyers and M. B. O'Brien, J. Bacteriol. 143:772-780, 1980). Since chondroitin sulfate does not generally occur in isolated form in tissue, but rather is bound covalently in proteoglycan, we investigated the extent to which chondroitin sulfate which is bound in such a sterically hindered complex can be utilized by intact bacteria. Intact cells of B. thetaiotaomicron were able to digest chondroitin sulfate in proteoglycan, although at a slightly slower rate than free chondroitin sulfate. Prior digestion of proteoglycan with trypsin to produce small fragments of protein with several chondroitin sulfate chains attached did not increase the rate at which the bound chondroitin sulfate was digested. Accordingly, the slower rate of digestion was probably due to attachment of chondroitin sulfate chains to the protein backbone rather than to steric hindrance by other components of the proteoglycan. When proteoglycan which had been incubated with intact bacteria was treated with sodium borohydride to release the undigested fragments of chondroitin sulfate from the protein backbone, the size and composition of the fragments indicated that intact bacteria were able to digest all but three monosaccharides of the chondroitin sulfate chains. Thus, despite steric hindrance due to attachment of the chondroitin sulfate chains to the protein backbone, digestion of bound chondroitin sulfate by intact bacteria was nearly complete.