Alkalophilic Bacillus firmus RAB generates variants which can grow at lower Na+ concentrations than the parental strain

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Abstract
Obligately alkalophilic Bacillus firmus RAB cannot grow well on media containing less than 5 mM Na+. However, variant strains can be isolated on plates containing 2 to 3 mM Na+. These variants are observed only rarely in cultures that are plated before being subjected to repeated transfers in liquid medium. Cultures which have been transferred several times produce variants at an apparent frequency of 2 X 10(-4). Most of these variants are unstable, generating parental types at the high frequency of 10%; however, stable variants can be isolated. These strains grow better than the parental strain at very high pH values in the presence of 5 mM Na+ and have enhanced activity of the Na+ -H+ antiporter that has been implicated in pH homeostasis. By contrast, Na+ -coupled solute uptake is indistinguishable from that of the parental strain, and no obvious changes in the respiratory chain components are apparent in reduced versus oxidized difference spectra. The membranes of the variants show a marked enhancement, on sodium dodecyl sulfate-polyacrylamide gradient electrophoresis, in one polypeptide band with a molecular weight in the range of 90,000. The findings are discussed from the point of view of genetic mechanisms that might confer adaptability to even more extreme environments than usual and in view of earlier models relating the Na+ -translocating activities of the alkalophiles.