Enumeration of polysaccharide-degrading Bacteroides species in human feces by using species-specific DNA probes

Abstract

DNA probes that are specific for each of five predominant species of human colonic Bacteroides (B. thetaiotaomicron, B. uniformis, B. distasonis, 'Bacteroides group 3452-A', and B. ovatus) were used to detect and enumerate these species in fecal samples from two adult volunteers. These five species are capable of fermenting many of the complex polysaccharides that are thought to be sources of carbon and energy for bacteria in the colon. Estimates for the concentrations of some of these species in feces have not been previously available because of the difficulties in differentiating colonic Bacteroides spp. by conventional biochemical tests. Our results indicate that all the species except B. ovatus were present in high numbers (>109/g [dry weight]) in the feces of both volunteers. However, the concentrations of the more versatile polysaccharide-degrading species within this group of organisms (7.6 x 109 to 12.0 x 109/g [dry weight] for B. thetaiotaomicron; 2.9 x 109 to 6.3 x 109/g [dry weight] for 'Bacteroides group 3452-A') did not differ significantly from the concentrations of less versatile polysaccharide-degrading species (1.2 x 1010 to 2.0 x 1010/g [dry weight] for B. uniformis; 5.8 x 109 to 8.4 x 109/g [dry weight] for B. distasonis). B. ovatus was not detectable by our method. Since our lower limit of detection is approximately 1 x 109 to 2 x 109 g [dry weight] of feces, this is consistent with earlier estimates that indicated that the concentration of B. ovatus in feces is near or below this value. Thus, it appears that the ability to degrade a wide variety of polysaccharides is not the only major factor in determining which bacteria are the most numerous in the colon.