Taxonomy and Ecology of Bacteroidaceae in the Alimentary Tract of Man

The bacterial flora of the alimentary tract of man is very complex and varied and reflects the enormous diversity of this ecosystem . Members of the Bacteroidaceae colonize both the upper and lower regions of this system but several factors contribute to limit specific taxa to certain niches . In Bergey's manual of systematic bacteriology (2) , 3 genera, Bacteroides, Fuso-b acterium and Leptotrichia are recognized on the basis of physiological tests and acid end product analysis. The genus Leptotrichia remains monospecific , with L. buccalis,( which produces lactic acid as a major end product of metabolism) as its only species. Members of the genus Fusobacterium are characterized by the production of butyric acid and their general asaccharolytic nature . Though the genus appears to be homogeneous, considerable intrageneric heterogeneity exists. Whereas most species have DNA base compositions within the range 25-34 M % G+ C, some such as F. sulci (39%) and the type strain of F . naviforme (43%) are clearly atypical . Some species such as F. nucleatum (type species) and F. periodonticum possess a peptidoglycan structure based upon meso-lanthionine (AIo) while others such as F. varium contain a dl-diaminopimelic acid structure (7, 11, 22) . F. mortiferum atypically has a mixture of both diamino acids. The nature of cross-linkage of these peptidoglycan chemotypes is shown in Fig. 1. Recently we have shown that glutamate is both a useful diagnostic marker at the species and generic levels (4, 6, 20) and is utilized as a major source of energy (5, 10) . Three pathways, viz the 2-oxoglutarate , mesaconate and 4-aminobutyrate, were reported for the catabolism of glutamate (3) . The relationship between these pathways of glutamate catabolism and the distribution of peptidoglycan chemotypes is shown in Fig . 2. Thus despite the apparent homogeneity of the genus, it is in need of urgent chemotaxonomic analyses. The type species was recently shown to be heterogeneous by both DNA-DNA hybridization , enzyme electrophoretic patterns and ribosomal RNA gene restriction patterns