An electron microscope survey of the surface structures and hydrophobicity of oral and non-oral species of the bacterial genus Bacteroides

Abstract

Seventeen strains of Bacteroides representing 10 species were examined by negative staining; the majority were from the mouth but a few non-oral strains were included. Seven species had peritrichously-arranged, non-flagellar appendages which could be divided by morphology and ultra-structure into two subgroups, fibrils and fimbriae. Bacteroides asaccharolyticus strains B536 and B537 and Bacteroides gingivalis strains W50, W83, WPH15 and WPH35 had fimbriae with mean width of 4.4 nm and 0.5 to 6.0 μm long depending on the strain. The fimbrial length within each strain also varied. Fibrils were present on two fresh oral isolates of Bacteroides melaninogenicus, Bacteroides intermedius strains T588 and W09, Bacteroides corporis ATCC 33547, Bacteroides oralis ATCC 33269 and Bacteroides buccae ATCC 33574. Fibrils consistently clumped into bundles of variable thickness and formed a fringe around the cell periphery, ranging from 0.27 to 1.2 μm long depending on the strain. Fibril lengths of each strain were uniform. Fibrils had no measurable width and the clumps tapered towards the free ends. Bacteroides loeschii VPI 9085, Bacteroides pentosaceus strains NP333 and Jl and Bacteroides capillosus 29799 had no detectable surface appendages. Fimbriate strains had a layer outside the outer membrane, with a mean thickness of between 17.8 and 28.6 nm. Both fibrillar and fimbriate strains produced many small membranous vesicles budding from the outer membrane. There were two morphological forms of vesicles, ones with either fimbriae or fibrils (species-dependent) and ones with no attached appendages. Of eleven strains tested for cell-surface hydrophobicity by partitioning between hexadecane and buffer, all but one was non-hydrophobic.