Ultrastructural and cytochemical studies on anionic surface sites of Mycoplasma membranes.

Negatively charged, chemical groups on the surfaces of mycoplasma membranes were studied in the electron microscope by staining with positively charged ferric oxide hydrosols in the acetic acid (AI-reagent) or propanoic acid (PI-reagent), respectively. The technique of thin sectioning was used. With the AI-reagent, no significant binding of the ferric oxide particles was observed on the mycoplasma membranes. With the lipophilic PI-reagent, the membrane surfaces of M. mycoides subsp. capri, M. mycoides subsp. mycoides, M. gallisepticum, and Acholeplasma laidlawii were heavily stained with focal aggregates of iron granules. M. hominis did not bind the label. Prior chemical and enzymatic treatments indicate that the cationic ferric oxide particles in propanoic acid interact with negatively charged, presumably lipid phosphate groups exposed on the surfaces of mycoplasma membranes.