This study examined the ultrastructure of Beggiatoa alba strain B15LD, a filamentous bacterium that deposits sulfur when grown in hydrogen sulfide. Using thin-section electron microscopy and freeze-etch techniques, researchers documented the detailed architecture of the cell envelope and sulfur inclusions. The cell wall complex consists of five distinct layers external to the cytoplasmic membrane, with the outermost layer (E) displaying longitudinal fibrils 10-12 nm in diameter. Sulfur inclusions are enclosed by a unique pentalaminar (five-layered) envelope measuring 12-14 nm thick, distinguishing this strain from other Beggiatoa species. Cells grown without sulfur compounds contained small, folded structures with similar envelope morphology, suggesting these are rudimentary precursors that expand upon sulfur exposure. The study also revealed cytoplasmic membrane invaginations associated with sulfur inclusion formation and provided detailed characterization of each cell wall layer's morphology and dimensions. A three-dimensional model was constructed integrating freeze-etch and thin-section data, demonstrating the complex organization of this organism's cell envelope.

Key findings

• The cell envelope consists of five discrete layers (A-E) external to the cytoplasmic membrane, with total thickness of 70-105 nm, making it among the thickest bacterial cell walls observed
• Sulfur inclusions possess a unique pentalaminar envelope (12-14 nm thick) composed of alternating electron-dense and electron-translucent layers, structurally distinct from sulfur inclusions in other sulfur bacteria
• Cells grown without hydrogen sulfide contain small, convoluted rudimentary envelope structures that expand and become densely filled with sulfur when exposed to sulfide
• The external surface features longitudinal fibrils 10-12 nm wide arranged in regular arrays, revealed through non-cryoprotected freeze-etching
• Cytoplasmic membrane invaginations (48-72 nm diameter) surround sulfur inclusions, representing points where the membrane invokes to form compartments containing these storage granules