Ribosomal Ribonucleic Acid Homology among Species of the Genus Bacteroides

Abstract

We determined the levels of ribosomal ribonucleic acid similarity among 30 Bacteroides species. The ribosomal ribonucleic acid homology values (determined by using the membrane competition method and both 16S and 23S ribosomal ribonucleic acid subunits) ranged from 100 to 4%. The organisms presently placed in the genus Bacteroides represent a major phylogenetic unit, as shown by the sequence diversity of the ribosomal ribonucleic acid cistrons among the species. There were three major clusters of species, which had intercluster homology values of 20 to 30%. The first cluster consisted of Bacteroides fragilis and nine other saccharolytic species which are common fecal organisms. The second cluster contained Bacteroides melanogenicus and 11 other moderately saccharolytic species, many of which are commonly isolated from human oral cavities. The third, rather heterogeneous cluster contained Bacteroides asaccharolyticus and four other mostly asaccharolytic species.