1990 Volume 36 Issue 1 Pages 7-17
A mechanism that represses the growth of facultative alkalophilic Bacillus sp. ASSC-2 at neutral pH has been examined. When the pH of K+-limited culture medium was shifted from 9 to 7, acidification of the intracellular pH (pHin) occurred causing cessation of growth and a decrease in cell viability. When the pHin was lowered below 7 by treatment with nigericin, the viability decreased with decreasing pHin. Apparently the impairment of the pHin regulation at neutral pH caused the striking loss of viability. Compared with neutrophilic Bacillus subtilis, strain B. ASSC-2 involved low K+-transport activity: the K+ transporter allows more H+ to be extruded from the cells by the respiratory pump and alkalizes the pHin. Further, the Na+/H+ antiporter, which acidified the pHin in the external alkaline pH range was active even at neutral pH. These results indicate that inability of the growth at pH 7 in K+-limited medium may be due to a decrease in the activity in the K+-transport system, causing excessive acidification of the pHin under the active Na+/H+ antiport system.