Ultrastructural visualization of anionic sites on mycoplasma membranes by polycationic ferritin

Anionic sites on mycoplasma membranes were visualized in the electron microscope by a polycationized ferritin derivative. The technique of thin sectioning was used. Staining prior to fixation led to clustering of ferritin granules on the mycoplasma cell surface. On glutaraldehyde-fixed Mycoplasma mycoides subsp. capri, M. gallisepticum, M. pneumoniae, and Acholeplasma laidlawii, the anionic sites were uniformly distributed over the entire membrane surface. M. hominis did not bind the polycationic ferritin label. Chemical and enzymatic treatments of the mycoplasmas indicated that the anionic sites may be lipid phosphate groups. Isolated M. mycoides subsp. capri membranes were labeled exclusively on only one membrane surface, presumably the outer one. Liposomes prepared from diphosphatidylglycerol and phosphatidylcholine were also labeled by the polycationic ferritin.