Adaptive growth responses of Listeria monocytogenes to acid and osmotic shifts above and across the growth boundaries.

The effect of acid and osmotic shifts on the growth of Listeria monocytogenes was evaluated at 10°C. Two types of shifts were tested: (i) within the range of pH and water activity (a(w)) levels that allow growth of L. monocytogenes and (ii) after habituation at no-growth conditions back to growth-permitting conditions. A L. monocytogenes cheese isolate, with high survival capacity during cheesemaking, was inoculated (10(2) CFU/ml) in tryptic soy broth supplemented with 0.6% yeast extract at six pH levels (5.1 to 7.2; adjusted with lactic acid) and 0.5% NaCl (a(w) 0.995), or four a(w) levels (0.995 to 0.93, adjusted with 0.5 to 10.5% NaCl) at pH 7.2 and grown to early stationary phase. L. monocytogenes was then shifted (at 10(2) CFU/ml) to each of the aforementioned growth-permitting pH and a(w) levels and incubated at 10°C. Shifts from no-growth to growth-permitting conditions were carried out by transferring L. monocytogenes habituated at pH 4.9 or a(w) 0.90 (12.5% NaCl) for 1, 5, and 10 days to all pH and a(w) levels permitting growth. Reducing a(w) or pH at different levels in the range of 0.995 to 0.93 and 7.2 to 5.1, respectively, decreased the maximum specific growth rate of L. monocytogenes. The lag time of the organism increased with all osmotic downshifts, as well as by the reduction of pH to 5.1. Conversely, any type of shift within pH 5.5 to 7.2 did not markedly affect the lag times of L. monocytogenes. The longer the cells were incubated at no-growth a(w) (0.90), the faster they initiated growth subsequently, suggesting adaptation to osmotic stress. Conversely, extended habituation at pH 4.9 had the opposite effect on subsequent growth of L. monocytogenes, possibly due to cell injury. These results suggest that there is an adaptation or injury rate induced at conditions inhibiting the growth of the pathogen. Thus, quantifying adaptation phenomena under growth-limiting environments, such as in fermented dairy and meat products or products preserved in brine, is essential for reliable growth simulations of L monocytogenes during transportation and storage of foods.