A non-alkalophilic mutant of Bacillus alcalophilus lacks the Na⁺ H⁺ antiporter

A non-alkalophilic mutant strain of Bacillus alcalophilus grows on L-malate over a pH range from 5.0 to 9.0. The mutant does not exhibit the energy-dependent efflux of Na⁺ that has been used to assay a Na⁺ H⁺ antiporter in the wild type organism. The mutant also fails to transport α-aminoisobutyric acid, at pH 9.0, either in the presence or absence of Na⁺; at pH 5.5, the amino acid analogue is taken up by a Na⁺-independent mechanism. The properties of the mutant constitute strong evidence that the Na⁺ H⁺ antiporter is involved in maintaining an acidified cytoplasm in B. alcalophilus.