Table 1.

The parameters of the fitted models for the survival curves of Listeria monocytogenes LO28 wild type and two stress-resistant variantsa

ParameterHHP ([biphasic] linear model)Heat ([biphasic] linear with shoulder model)
Wild typectsR variantImmotile variantWild typectsR variantImmotile variant
N0,measured (log CFU/ml)9.49.7
N0,extrapolated (log CFU/ml)10.310.09.69.79.69.6
D1 (min)1.57.7*16.3**0.863.1**4.5**
D2 (min)7.17.6
fres2.1 × 10−56.5 × 10−61.0 × 10−71.6 × 10−46.5 × 10−61.0 × 10−7
Sl (min)3.67.8***5.2
  • a Cells were treated under conditions of 350 MPa, 20°C (HHP) or at 55°C (Heat) in ACES buffer. N0,extrapolated is the model-based-fit amount of cells at t0. N0,measured is the measured amount of cells at t0. D is the decimal reduction time, where D1 and D2 are the D-values of the first and second part, respectively, of the biphasic linear inactivation curve. fres is the fraction of resistant cells in the population based on N0,measured for the wild type, where the fres given for the wild type is the total fraction of resistant cells (both stably and temporarily resistant) and the fres given for each of the two variants is the fraction of that specific variant in the wild-type population, assuming no shoulder for the two variants (see behavior in Fig. 1b) and taking into account the shoulder in the wild type by using N0,measured. Sl is the shoulder length. *, statistical analysis (P < 0.05) showed that the ctsR variant's D-value for HHP was only higher than the D-value of the sensitive fraction of the population of the wild type; **, the D-value for HHP of the immotile variant and both variants' D-values for heat were significantly different from the D-values of the sensitive and resistant fractions of the wild-type population; ***, the shoulder length of the ctsR variant was higher than that of the wild type for heat inactivation.