Pathways of glutamate catabolism among Fusobacterium species

Glutamate is a major source of energy for Fusobacterium species but its mode of catabolism has not hitherto been elucidated. Cell suspensions of F. nucleatum and F. varium, as representative species from the oral cavity and gastrointestinal tract, respectively, both decarboxylated position-labelled glutamate but by different pathways. 14CO2 was released only from C-5 by F. nucleatum whereas F. varium decarboxylated glutamate at either C-1 or C-5. In both species, 2 mols of glutamate fermented yielded 2 mols of acetate and 1 mol of butyrate, suggesting the possibility of three metabolic pathways: the 2-oxoglutarate, mesaconate and 4-aminobutyrate pathways. Enzymes representative of the three pathways were assayed for in cell-free extracts of fusobacteria. All species tested possessed high levels of both glutamate dehydrogenase and 2-oxoglutarate reductase, indicating the presence of the 2-oxoglutarate pathway. Enzymes representative of the mesaconate pathway were detected in F. sulci, F. ulcerans, F. mortiferum and F. varium, while the latter two species also possessed the 4-aminobutyrate pathway. The pathways of glutamate catabolism therefore bore no relationship to the site of isolation of the fusobacteria tested but instead correlated with their chemotaxonomic properties. Thus, F. varium, F. mortiferum, F. ulcerans and F. sulci, which possess a peptidoglycan structure based on diaminopimelic acid, have either two or three pathways for glutamate catabolism whereas F. nucleatum and other species that have a lanthionine-based murein metabolized glutamate solely by the 2-oxoglutarate pathway.