Sodium ATPase and sodium/proton antiporter are not obligatory for sodium homeostasis of Enterococcus hirae at acid pH

Enterococcus hirae grows in a broad pH range from 5 to 11. An E. hirae muta nt 7683 lacking the activities of two sodium pumps, Na+-ATPase and Na+/H+ a ntiporter, does not grow in high Na+ medium at pH above 7.5. We found that 7683 grew normally in high Na+ medium at pH 5.5. Although an energy-depende nt sodium extrusion at pH 5.5 was missing, the intracellular levels of Naand K+ were normal in this mutant. The Na+ influx rates of 7683 and two oth er strains at pH 5.5 were much slower than those at pH 7.5. These results s uggest that Na+ elimination of this bacterium at acid pH is achieved by a d ecrease in Na+ entry and a normal K+ uptake.