Circulation of H+ and K+ across the plasma membrane is not obligatory for bacterial growth.

Streptococcus faecalis grows normally in the presence of gramicidin and other ionophores under conditions such that there is no gradient of pH or of electrical potential across the plasma membrane and that currents of H+, K+, and Na+ are short-circuited. Growth requires a rich medium, a slightly alkaline pH, and a high concentration of external K+. The proton circulation maintains the cytoplasmic pH and pools of ions and other metabolites but is not obligatory for biosynthetic functions including DNA replication, cell division, or assembly of the structural framework of the cell.