Potassium/proton antiport system is dispensable for growth of Enterococcus hirae at low pH

An energy-dependent K+/H+ antiport system is found in Enterococcus hirae ATCC 9790 cultured in a standard complex medium (Y. Kakinuma, and K. Igarashi, J. Biol. Chem. 263:14166-14170, 1988). We have now found that the activity of this antiport system was totally missing in cells cultured in a defined medium. In this defined medium, E. hirae did not grow well at pH near 9, but grew normally at pH below 7.5. This antiport system is important at high pH but dispensable at lower pH for ion homeostasis of this bacterium.