Ultrastructure of the capsule of Bacillus megaterium ATCC 19213

The ultrastructure of the firmly adherent capsule produced by Bacillus megaterium cultured on fructose mineral salts medium was examined using thin sectioning, freeze-etching, and critical point drying by transmission and scanning electron microscopy. The capsule material was shown to be fibrillar, with most fibrils containing bulbous protrusions. Two types of fibres were resolved. These were termed primary and cross-linking fibres. Primary fibres originated at the cell wall and had a diameter of 34-50 nm. They also contained bulbous protrusions and enlarged areas where branching occurred. Cross-linking fibres connected the primary fibres. The cross-linking fibres were much smaller, usually 15 micro m in diameter, and contained few enlarged areas. The primary fibres originated at sites on the cell wall approximately equidistant and 0.26 micro m apart.