The membrane-induced proton motive force influences the metal binding ability of Bacillus subtilis cell walls.

Bacillus subtilis 168 is a gram-positive bacterium whose cell wall contains the highly electronegative polymers peptidoglycan (chemotype A1 gamma) and glycerol-based teichoic acid to produce a surface with a net negative charge with high metal binding capacity. During metabolism, a membrane-induced proton motive force continuously pumps protons into the wall fabric. As a result, a competition between protons and metal ions for anionic wall sites occurs, and less metal is bound in living cells than in nonliving cells or those in which the plasma membrane has been uncoupled. This was shown by using two metallic ions, UO2(2+) and Sc3+, on control cells, cells uncoupled with either carbonyl cyanide m-chlorophenylhydrazone or NaN3, or cells killed by gamma radiation. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma atomic-emission spectroscopy showed that more metal was retained in the walls of nonliving cells and those with deenergized membranes than in their living counterparts.