This study examined the ultrastructure of two Cytophaga species—C. johnsonae and C. aquatilis—using freeze-etching and frozen-surface replica electron microscopy techniques. Both organisms displayed typical Gram-negative cell envelopes with undulated surfaces and produced copious extracellular slime. A key difference emerged in their internal membrane architecture: C. aquatilis contained large, complex mesosomes associated with septum formation during cell division, while C. johnsonae lacked these structures. Freeze-etching revealed important distinctions in membrane fracture patterns—C. aquatilis showed two major fracture planes through the lipophilic regions of its membranes, whereas C. johnsonae displayed three fracture planes, including an unusual intermediate fracture between the cytoplasmic and outer membranes. C. johnsonae possessed double-stranded longitudinal fibrils (10-12 nm wide) on the intramembrane fracture surface, differing morphologically from previously described peripheral fibrils in related bacteria. Both species exhibited 6-8 nm protein protrusions on fractured cytoplasmic membranes and 4-6 nm beads on outer membranes. The fibrillar extracellular slime of C. aquatilis resembled muopolysaccharide materials found in other gliding bacteria.

Key findings

• C. aquatilis possesses complex mesosomes associated with septum formation during cell division, while C. johnsonae lacks mesosome-like structures
• C. johnsonae has an unusual third fracture plane in freeze-etch preparations between the cytoplasmic and outer membranes, distinct from typical Gram-negative bacteria
• C. johnsonae contains double-stranded longitudinal fibrils (10-12 nm) on the intramembrane fracture surface that differ from peripheral fibrils found in related species
• Both species produce fibrillar extracellular slime and display membrane-associated protein structures (6-8 nm protrusions on cytoplasmic membranes and 4-6 nm beads on outer membranes)