Abstract
The growth of Listeria monocytogenes on refrigerated, ready-to-eat food products is a major health and economic concern. The natural antimicrobial nisin targets the bacterial cell wall and can be used to inhibit L. monocytogenes growth on cheese. Cell wall composition and structure, and therefore the efficacy of cell wall acting control strategies, can be severely affected by environmental and stress conditions. The goal of this study was to determine the effect of a range of pH and temperatures on the efficacy of nisin against several strains of L. monocytogenes in a lab-scale, cheese model. Cheese was made with or without the addition of nisin at different pH and then inoculated with L. monocytogenes; L. monocytogenes numbers were quantified after 1, 7, and 14 days of incubation at 6, 14, or 22°C. While our data show that nisin treatment is able to reduce L. monocytogenes numbers, at least initially, growth of this pathogen can occur even in the presence of nisin, especially when cheese is stored at higher temperatures. Several environmental factors were found to affect nisin efficacy against L. monocytogenes. For example, nisin is more effective when cheese is stored at lower temperatures. Nisin is also more effective when cheese is made at higher pH (6 and 6.5), compared to cheese made at pH 5.5, and this effect is at least partially due to the activity of cell envelope modification genes dltA and mprF. Serotype was also found to affect nisin efficacy against L. monocytogenes; serotype 4b strains showed lower susceptibility to nisin treatment compared to serotype 1/2 strains. Overall, our results highlight the importance of considering environmental conditions specific to a food matrix when developing and applying nisin-based intervention strategies against L. monocytogenes.
Highlights
Controlling Listeria monocytogenes in food is essential to food safety due to the high mortality rate associated with listeriosis, especially among susceptible populations, such as pregnant women, newborns, the elderly, and those with compromised immune systems (Farber and Peterkin, 1991; Jackson et al, 2018)
Nisin-treated cheese showed lower numbers of L. monocytogenes compared to untreated cheese at each temperature tested; higher L. monocytogenes numbers were observed for nisin-treated cheese stored at 14 and 22◦C compared to nisin-treated cheese stored at 6◦C
A oneway analysis of variance (ANOVA) was used to determine differences in zones of inhibition between the nisin-extracted samples; the results show there was no significant difference among diameters of the zones of inhibition (P = 0.31; Supplementary Table 2), indicating that nisin loss in the whey is similar regardless of the pH (5.5, 6.0, or 6.5) at which the cheese was made, therein nisin loss does not explain the decreased ability of nisin to kill L. monocytogenes in cheese made at pH 5.5
Summary
Controlling Listeria monocytogenes in food is essential to food safety due to the high mortality rate associated with listeriosis, especially among susceptible populations, such as pregnant women, newborns, the elderly, and those with compromised immune systems (Farber and Peterkin, 1991; Jackson et al, 2018). L. monocytogenes can be classified into different serotypes, which are associated with specific environments and differ in their ability to cause disease (Orsi et al, 2011). Low salt content, and near neutral pH make Hispanicstyle fresh cheese an ideal environment for survival and growth of a number of foodborne pathogens (Leggett et al, 2012), including L. monocytogenes (Uhlich et al, 2006; Ibarra-Sánchez et al, 2017). L. monocytogenes can tolerate a number of environmental stressors associated with foods, including a wide range of temperatures (0–45◦C), pH (4.4–9.4), and high salt concentrations (13–14 w/v%) (Swaminathan et al, 2007). The ability of L. monocytogenes to survive adverse environmental conditions increases the likelihood of transmission from the environment to humans via contaminated food products.
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