Environmental conditions and serotype affect Listeria monocytogenes susceptibility to phage treatment in a laboratory cheese model

摘要

Listeria monocytogenes can survive and grow in a varietyof environments, including refrigeration, makingit difficult to control and highlighting the importanceof optimizing control strategies against this pathogen.Listeria phages are attractive biocontrol agents becausephages bind to specific wall teichoic acids (WTA) onthe bacterial cell wall, inhibiting pathogens without disruptingthe normal microbiota or structure of the food.Common stresses found on dairy products can affectcell wall composition and structure and subsequentlyaffect the efficiency of control strategies that target thecell wall. The goal of this study was to determine theeffect of a range of pH and temperatures on the effectivenessof a commercial phage cocktail treatmentagainst several strains of L. monocytogenes in a cheesematrix. We developed a laboratory-scale cheese modelthat was made at different pH, treated with phage,and then inoculated with L. monocytogenes. Cheeseswere incubated at 6, 14, or 22°C for 14 d, and bacterialcounts were determined on d 1, 7, and 14. Ourdata show that phage treatment has a limited abilityto reduce L. monocytogenes counts at each temperaturetested; however, it was more effective on specific strainsof L. monocytogenes when cheese was stored at highertemperatures. More specifically, the average counts of L.monocytogenes on phage-treated cheese stored at 22°Cwere significantly lower than those on phage-treatedcheese stored at 6 or 14°C. Similarly, phage treatmentwas significantly more effective at inhibiting L. monocytogeneson cheese made at higher pH (6 and 6.5)compared with counts on cheese made at pH 5.5, whereL. monocytogenes did not grow. Furthermore, serotypewas found to affect the susceptibility of L. monocytogenesto phage treatment; serotype 1/2 strains showedsignificantly higher susceptibility to phage treatmentthan serotype 4b strains. Overall, our results suggestthe importance of considering the efficacy of phage underconditions (i.e., temperature and pH) specific to agiven food matrix when applying interventions againstthis important foodborne pathogen.