Ultrastructure of the Bacteroides nodosus cell envelope layers and surface

Abstract

The surface structure and cell envelope layers of various virulent B. nodosus strains were examined by light microscopy and by EM by using negative staining, thin-section and freeze-fracture-etch techniques. Three surface structures were described: pili and a diffuse material, both of which emerged from one or both poles of the bacteria (depending on the stage of growth and division), and large rodlike structures (usually 30-40 nm in diameter) associated with a small proportion of the bacterial population. No capsule was detected. The cell envelope consisted of 4 layers: a plasma membrane, a peptidoglycan layer, an outer membrane and an outermost additional layer. The additional layer was composed of subunits, generally hexagonally packed with center-to-center spacing of 6-7 nm. The outer membrane and plasma membrane freeze-fractured through their hydrophobic regions revealing 4 fracture faces with features similar to those of other gram-negative bacteria. Some unusual features were seen on the fracture faces of the outer membrane: large raised ring structure (11-12 nm in diameter) on .**GRAPHIC**. 3 [convex fracture face of the outer membrane] at the poles of the bacteria; complementary pits or ring-shaped depressions on .**GRAPHIC**. 2 [concave fracture face of the outer membrane]; and small raised ring structures (7-8 nm in diameter) all over .**GRAPHIC**. 2.