ACIDIFICATION OF INTRACELLULAR PH RESULTS IN NEUTRAL PH-SENSITIVE GROWTH OF AN ALKALOPHILIC BACILLUS SP

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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.