Bacterial strategies to inhabit acidic environments

Bacteria can inhabit a wide range of environmental conditions, including ex tremes in pH ranging from 1 to 11, The primary strategy employed by bacteri a in acidic environments is to maintain a constant cytoplasmic pH value. Ho wever, many data demonstrate that bacteria can grow under conditions in whi ch pH values are out of the range in which cytoplasmic pH is kept constant. Based on these observations, a novel notion was proposed that bacteria hav e strategies to survive even if the cytoplasm is acidified by low external pH, Under these conditions, bacteria are obliged to use acid-resistant syst ems, implying that multiple systems having the same physiological role are operating at different cytoplasmic pH values. If this is true, it is quite likely that bacteria have genes that are induced by environmental stimuli u nder different pH conditions. In fact, acid-inducible genes often respond t o another factor(s) besides pH, Furthermore, distinct genes might be requir ed for growth or survival at acid pH under different environmental conditio ns because functions of many systems are dependent on external conditions. Systems operating at acid pH have been described to date, but numerous gene s remain to be identified that function to protect bacteria from an acid ch allenge. Identification and analysis of these genes is critical, not only t o elucidate bacterial physiology, but also to increase the understanding of bacterial pathogenesis.