Ultrastructure and location of cytoplasmic fibrils inSpiroplasma floricola OBMG

Abstract

The ultrastructure ofSpiroplasma floricola OBMG was investigated to identify subcellular structures that might be involved in motility and helicity. Optimal preservation for thin sectioning was achieved with either glutaraldehyde or a mixture of glutaraldehyde plus paraformaldehyde followed by OsO₄ and uranyl acetate. In thin sections, a 94-nm-wide band consisting of 4-nm-diameter fibrils was observed apposed to the cytoplasmic side of the plasma membrane. The band of fibrils extended axially the entire length of the cell. The addition of rethenium red to fixative solutions resulted in condensation of the fibrils. Freeze-substitution increased the apparent thickness of membranes but did not improve preservation of the fibrils. Freeze-fracturing revealed a 99-nm-wide zone containing few particles in fractured membrane surfaces. Treatment with deoxycholate or Triton X-100 to dissolve membranes yielded bands of fibrils comparable to those seen in thin sections. Based on these findings, a model indicating the intracellular location of the fibrils is proposed.