Comparative ultrastructure of Bacteroides melaninogenicus subspecies.

A total of 10 strains of Bacteroides melaninogenicus subspecies were examined by transmission electron microscopy after conventional processing, and staining with uranyl acetate and lead citrate. All strains examined had a cell periphery with a basic ultrastructure typical of that reported for other Gram-negative bacteria. However, only s.s. asaccharolyticus strains exhibited a moderately electron-dense covering layer, 16-19 nm thick, in contact with the outer membrane of the cell wall, and numerous intercellular membranous vesicles. S.s. melaninogenicus strains, in contrast to those of other subspecies, had no extramural structures, and exhibited a distinct, intercellular, fibrillar matrix. S.s. intermedius cells featured a 9-14 urn thick, electron-dense extramural layer which was separated by an electron-lucent layer of similar thickness from the outer membrane of the wall. These morphological differences may be useful in differentiating these subspecies from one another. [ABSTRACT FROM AUTHOR]