Acid adaptation of Listeria monocytogenes strains does not offer cross-protection against an activated lactoperoxidase system

Listeria monocytogenes has been implicated in foodborne illness outbreaks i nvolving several types of cheeses made from acidified milk. Acid shock resp onse (ASR) and acid tolerance response (ATR) could be possible reasons for its survival. The ASR and ATR of three strains of L. monocytogenes (V7, V37 , and CA) in skim milk acidified to pH 4.0 and 3.5 with lactic acid and hel d at 32 degrees C were studied. Studies were also done to determine if acid adaptation of the organism enhanced survival in the presence of an activat ed lactoperoxidase system. The cells were directly shocked at pH 4.0 and 3. 5 in skim milk to study the ASR. To study the ATR, cells were initially ada pted in skim milk at a mild pH of 5.5 for the equivalent of one generation before being shocked at pH 4.0 and 3.5 in skim milk. Cells adapted at pH 5. 5 in tryptic soy broth without dextrose and nonadapted cells were challenge d at pH 4.5 in skim milk with or without an activated lactoperoxidase syste m. In all cases, viability and pH were measured 24 or 48 h after challenge. In pH 4.0 skim milk, for all three strains, the adapted cell population su rvived better (0.5 to 1.0 log higher) than that of nonadapted cells for 24 h. In pH 3.5 skim milk, the acid-adapted populations of all three strains w ere 3 to 4 logs greater than those of nonadapted cells at 6 h. The acid ada pted cells of all three strains had survival rates similar to those of the nonadapted cells at pH 4.5 both in the presence and absence of an activated lactoperoxidase system. It was also evident that these strains do not exhi bit an adaptive ATR at pH 4.5, although they do at lower pH levels (pH 4.0 and 3.5). Survival due to the ATR was better seen at pH 3.5 than at pH 4.0.