Listeria monocytogenes cells may induce alkali stress adaptation when exposed to sublethal concentrations of alkaline cleaners and sanitizers that may be frequently used in the food processing environment. In the present study, the effect of temperature on the induction and the stability of such alkali stress adaptation in L. monocytogenes were investigated. Our results demonstrate that temperature plays a critical role in the induction of alkali stress adaptation in L. monocytogenes under sublethal alkaline conditions. Pre-exposure to pH 9.0 tryptic soy broth supplemented with 0.6% yeast extract (TSB-YE) at 37 °C induced pronounced alkali stress adaptation whereas sublethal alkaline pre-exposure at 4 °C failed to induce any alkali stress adaptation. Furthermore, this pattern of alkali stress adaptation in L. monocytogenes was not dependent on the length of pre-exposure time, the concentration of sublethal alkali, the types of alkaline agents and the growth phases of cells. In addition, alkali stress adaptation induced at 37 °C was completely reversed in pH 7.2 TSB-YE within 2 h at 37 °C or within 4 h at 22 °C. However, once it was induced at a higher temperature, alkali stress adaptation in L. monocytogenes remained stable at 4 °C for at least 4 h. Our findings suggest that even though cold temperatures do not induce alkali stress adapted phenotypes, but it can maintain the previously acquired alkali stress adaptation much longer in L. monocytogenes.
The safe preservation of new generation foods (high moisture, low salt, high pH and shelf stable under ambient conditions) is microbiologically challenging. The growth of Clostridium botulinum in low acid foods is a hazard for consumers. In this study the combined effect of salt (sodium chloride) (0–4% w/v), potassium sorbate (0–4% w/v) and nisin (0–300 ppm) at two different pHs (5.5 and 7) on the probability of growth of Clostridium sporogenes spores, as a non-toxigenic surrogate of C. botulinum, was evaluated in nutrient broth. Nutrient broth was used as it can easily and accurately be adjusted and controlled in terms of composition, and allows more rapid growth than is observed in food. The aims of this study were to develop probability of growth models in which the logit is expressed as a function of the concentrations of the selected preservatives. The developed models fit the data adequately. The c-values models were close to 1, indicating good predictive power. This indicates the performance of models would be satisfactory. The results of this study indicated that salt, potassium sorbate and nisin had significant inhibitory effects on the growth of C. sporogenes in high moisture (>95%) and high pH conditions (pH > 4.5). Combinations of the selected preservatives were more effective than any of them individually. The inhibitory effects of all three preservatives in the current study were pH dependent. Less stringent combinations of preservative concentrations were required experimentally to stop growth at pH 5.5 compared with pH 7.
FoodWorld 