Detection of capsule and slime polysaccharide layers in two strains of Rhodopseudomonas capsulata

A capsule and slime were visualized electronmicroscopically in Rhodopseudomonas capsulata strain St. Louis (=ATCC 23782) and strain Sp 11 after pre-incubation of the cells in the homologous O/K antisera. The slime consists of loosely associated material surrounding the cell in irregular distribution. The capsule is directly adjacent to the cell wall and has a constant thickness of 75–85 nm in strain St. Louis and 30–40 nm in strain Sp 11. The capsule has a fibrillar fine-structure with radial orientation to the cell surface. In contrast to the slime, it is not removed from the cells by washing with saline.An acidic polysaccharide fraction was obtained from both strains by cetavlon fractionation of hot phenol-water extracts. The composition is strain-specific: the relative amounts of the common sugars found, i.e. rhamnose, galactose, glucose, glucosamine and galacturonic acid are different, the fraction from strain Sp 11 contains additionally fucose, 3-amino-3,6-dideoxygalactose, an unknown amino sugar and an unknown acidic component. Whether the polysaccharides of these fractions are in fact the slime or capsular substances remains to be established.