The relationship of acid adaptation to tolerance of other environmental stresses was examined in Salmonella typhimurium. S. typhimurium was adapted to acid by exposing the cells to mildly acidic conditions (pH 5.8) for one to two cell doublings. Acid-adapted cells were found to have increased tolerance towards various stresses including heat, salt, an activated lactoperoxidase system, and the surface-active agents crystal violet and polymyxin B. Acid adaptation increased cell surface hydrophobicity. Specific outer membrane proteins were induced by acid adaptation, but the lipopolysaccharide component appeared to be unaltered. These results show that acid adaptation alters cellular resistance to a variety of environmental stresses. The mechanism of acid-induced cross-protection involved changes in cell surface properties in addition to the known enhancement of intracellular pH homeostasis.
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