LOW PH ADAPTATION AND THE ACID TOLERANCE RESPONSE OF SALMONELLA-TYPHIMURIUM

Salmonella typhimurium periodically confronts acid environments during its life. These situations arise in chemically compromised ponds, soi l, degradative cellular organelles, host digestive systems, and may ev en result from byproducts of their own metabolism. The levels of acid that are encountered range from mild to extreme. As a neutralophile, S . typhimurium prefers to grow in pH environments above pH 5.5. They ca n survive down to pH 4 for extended periods of time. However, the limi ts of endurance can be stretched if the organisms are first adapted to a moderate acid pH before exposing them to acidity below pH 4.0. This adaptation, called the acid-tolerance response (ATR), includes severa l log phase and stationary phase systems. Some of these systems are de pendent on an alternate sigma factor for RNA polymerase called sigma(s ), whereas other systems are os-independent. A key to the ATR is the s ynthesis of a series of acid shock inducible proteins (ASPs), 51 for l og phase ATR and 15 for stationary phase ATR. Some of these ASPs requi re sigma(s) for their synthesis; others require the participation of t he ferric uptake regulator protein Fur. Effective acid tolerance invol ves RecA-independent DNA repair systems, iron, and facets of fatty aci d metabolism. Aspects of medium composition and carbon metabolism are also known to influence the nature of acid tolerance in this organism. In addition to aiding survival in the natural non-host environment, a spects of acid tolerance are also tied to virulence, as evidenced by t he involvement of the mouse virulence locus mviA and the fact that aci d-sensitive strains of S. typhimurium exhibit reduced virulence. This review summarizes these aspects of acid adaptation and includes a disc ussion of acid-regulated gene expression.