Regulation of internal pH in acidophilic and alkalophilic bacteria

The ability to regulate pH_inis a crucial physiological function that is made all the more important—even when the vicissitudes in external pH are not that great—by the likelihood that at least some critical cell processes is tightly controlled by pH_in.In neutralophiles, the relative roles of proton pumping adenosine triphosphatses (ATPases), respiration, and other possible means of raising pH_induring acid stress are still incompletely determined. Extreme acidophiles and alkalophiles are of intrinsic interest because of their broader bioenergetic problems and characteristics, the properties of their surfaces, and those processes that are exposed to the external milieu, their industrial potential, and their relationships to the ecosystem outline the protocols used to characterize features and mechanisms of pH homeostasis in the obligately acidophilic and alkalophilic bacilli. Information derived from studies by other investigators of gram-negative acidophiles and of alkaline-tolerant organisms can be introduced only peripherally to indicate where important differences in experimental detail may obtain. In both the acidophile Bacillus acidocaldarius and in alkalophilic Bacillus alcalophilus and Bacillus firmus RAB, work with right-side-out membrane vesicles has provided unequivocal evidence of outward, respiration-dependent proton translocation, and has been part of the battery of approaches to the characterization of antiport activities relevant to pH regulation.