INTERNAL PH CRISIS, LYSINE DECARBOXYLASE AND THE ACID TOLERANCE RESPONSE OF SALMONELLA-TYPHIMURIUM

Salmonella typhimurium possesses an adaptive response to acid that inc reases survival during exposure to extremely low pH values. The acid t olerance response (ATR) includes both log-phase and stationary-phase s ystems. The log-phase ATR appears to require two components for maximu m acid tolerance, namely an inducible pH homeostasis system, and a ser ies of acid-shock proteins. We have discovered one of what appears to be a series of inducible exigency pH homeostasis systems that contribu te to acid tolerance in extreme acid environments. The low pH-inducibl e lysine decarboxylase was shown to contribute significantly to pH hom eostasis in environments as low as pH 3.0. Under the conditions tested , both lysine decarboxylase and sigma(s)-dependent acid-shock proteins were required for acid tolerance but only lysine decarboxylase contri buted to pH homeostasis. The cadBA operon encoding lysine decarboxylas e and a lysine/cadaverine antiporter were cloned from S. typhimurium a nd were found to be 79% homologous to the cadBA operon from Escherichi a coil. The results suggest that S. typhimurium has a variety of means of fulfilling the pH homeostasis requirement of the ABR in the form o f inducible amino acid decarboxylases.