The greatest level of resistance to acid-killing was found after preadaption at pH 4.2 for 15 min(Fig 3 A) and at pH low pH 5.0 for 60 min(Fig 3B). Above and below these values the level of acid resistance declined gradually. The survival rate was not increased by a longer incubation of the cells in the rang of pH 3.8-4.6, whereas at pH 5.0 or higher the cultures only displayed acid tolerance after 60 min preadaptation . These results would suggest that different mechanisms are induced as pH was lowered from 5.0 to 4.2. In fact, the rapid increase in acid resistance at pH 4.2 may indicate that the response was occurring through the increased activity of enzymes already present in the cells. On the contrary, the more gradual acquisition of acid tolerance at pH 5.0 would indicate that major changes in cellular composition are required, representing a total cellular response at genetic levels (Hill et al., 1995).