Modeling the effect of acid and osmotic shifts above and across the growth boundaries on the adaptation and growth of Listeria monocytogenes

摘要

The effect of acid and osmotic shifts on the adaptation and growth of Listeria monocytogenes was evaluatedat 10°C. Two types of shifts were considered: i) shifts within the growth region ii) shifts from no growth togrowth conditions. A L. monocytogenes isolate was grown to late-exponential phase in TSBYE at 7 pH (5.1-7.2) at a_w 0.995 or 5 NaCl concentrations (0.5-12.5%) at pH 7.2 and then was shifted to all the remaining pHand NaCl levels at 10°C. No-growth/growth shifts were carried out by transferring L. monocytogenes,habituated at pH 4.9 or salt 12.5% for 1, 5 and 10 days to growth permitting conditions. Reducing a_w in therange of 0.995 to 0.930 resulted in linear reduction of μ_(max) and exponential increase of lag time. Shifts in therange of pH 5.5-7.0 reduced μ_(max) but did not markedly affect the lag time. Conversely, downshifts to pH 5.1induced lag times of 88-180 h. The longer the cells were incubated at no growth a_w, the faster they initiatedgrowth subsequently, suggesting adaptation to osmotic stress. Conversely, extended habituation at pH 4.9had the opposite effect on subsequent growth of L. monocytogenes, suggesting potential injury of cells. Wetherefore assumed the existence of an adaptation/injury rate (ν) at no growth conditions. A global fitting ofdata with shifts from growth-ceasing to growth-permitting conditions enabled determination of such anadaptation rate as well as of the dependency of h_0 to a_w and pH shifts. These parameters describing the effectof magnitude and direction of osmotic and acid shifts were introduced in a dynamic growth model, whichsuccessfully predicted the growth of L. monocytogenes in milk under dynamic pH (6.4-4.9) and a_w (0.98 to<0.90) conditions. Our results suggest that quantifying adaptation phenomena under growth-limitingenvironments is essential for reliable growth simulations.