Potassium proton antiport system is dispensable for growth of Enterococcushirae at low pH

An energy-dependent K+/H+ antiport system is found in Enterococcus hirae AT CC 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 activit y of this antiport system was totally missing in cells cultured in a define d 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 hig h pH but dispensable at lower pH for ion homeostasis of this bacterium.