Cross-protection of acid-adapted Escherichia coli O157:H7 against inimical stresses is mediated by the glucose-repressed sigma factor RpoS. However, many food systems in which E. coli O157:H7 occurs are complex and contain glucose. This study was aimed at investigating the contribution of acid and lactoperoxidase(LP)-inducible genes to cross-
protection of E. coli O157:H7 against LP system and lactic acid(LA)in Tryptone Soy Broth(TSB). Acid-adapted and non-adapted E. coli O157:H7 were challenged to activated LP and LA at pH4.0 and 5.0 in TSBf or 6 h at 25 1C followed by expression of acid and LP- inducible genes. Acid-adapted E. coli showed cross-protection against activated LP and LA. All the acid-inducible genes tested were repressed at pH4.0 with or without activated LP system. At pH7.4, gadA, ompC and ompF were induced in acid-adapted cells.
Induction of corA occurred in non-adapted cells but was repressed in acid-adapted cells. Although acid-inducible genes were repressed at pH4.0, high resistance of acid-adapted cells indicates that expression of acid-inducible genes occurred during acid adaptation and not the actual challenge. Repression of rpoS indicates that RpoS-independent systems contribute to cross-protection in acid-adapted E. coli O157:H7.