SUMMARY: The ultrastructure of strain B15LD was examined in detail using thin section and freeze-etch techniques. The cell wall complex consisted of five discrete layers external to the cytoplasmic membrane. The surface layer was the most distinct, containing linearly arranged longitudinal fibrils, 10-12 nm in diameter. Sulphur inclusions observed in thin sections had a 12-14 nm thick pentalaminar envelope and were external to the cytoplasmic membrane. Similar pentalaminar envelope structures were observed in cells grown without hydrogen sulphide, but they were small, folded and probably lacked sulphur. After a few hours of exposure to hydrogen sulphide, large sulphur inclusions were present in the cells, presumably due to the expansion of the folded ‘rudimentary’ vesicles to accommodate the deposited sulphur. The location and morphology of the cytoplasmic membrane invaginations which surrounded the sulphur inclusions were elucidated by thin sectioning and freezeetching. A three-dimensional model of the cell envelope structure is presented.


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