Membrane surface potential of Spiroplasma floricola.

Anionic charges, cytochemically identified as lipid phosphate groups, cover the outer membrane surface of Spiroplasma floricola. They induce a negative membrane surface potential which affects the distribution of ions, including protons. Accordingly, the pH at the interface differs from the bulk pH. By using the fluorescent lipoid pH indicator 4-heptadecyl-7-hydroxycoumarin, the pH at the membrane surface was determined. From the difference of the bulk and the interfacial pH the membrane surface potential of S. floricola was calculated to be phi = -118 mV.