Mechanisms of acid tolerance development

The mechanisms involved in ATR development in bacteria are not completely understood. However, it is generally recognized that various genes and proteins, including alternative RpoS and shock proteins are involved (Foster, 2000), as well as modifications in membrane fluidity and fatty acid composition (Brown et al., 1997 Alvarez-Ordonez et al., 2009). There are two systems known to provide extra protection against inorganic acid stress (1) RpoS dependent, and (2) PhoPQ dependent. PhoP is 

Increased acid tolerance correlates strongly with a decrease in proton accumulation in the cytoplasm. This altered proton permeability is again associated with changes in the protein composition of the cell membranes. Acid-adapted E. coli changes the lipid composition of its membranes, and elevated levels of cyclopropane fatty acids are often found. This may mean that changes in the protein composition of the cell membrane are a result of changes in the membrane lipid composition. Both lipid and protein alterations may be necessary to protect a bacterial cell in acidic environments (Jordan, Oxford, and O Byrne, 1999). 

Sources From Foster and Hall Journal of Bacteriology 173 5129-5135, 

Bearson, B.L. et. al., Journal of Bacteriology, 178 2572-2579,1998 Audia et al., International Journal of Medical Microbiology, 291 97-106,2001. 

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