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 (pH
in) occurred causing cessation of growth and a decrease in cell viability. When the pH
in was lowered below 7 by treatment with nigericin, the viability decreased with decreasing pH
in. Apparently the impairment of the pH
in 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 pH
in. Further, the Na
+/H
+ antiporter, which acidified the pH
in 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 pH
in under the active Na
+/H
+ antiport system.
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