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

Listeria monocytogenes can survive and grow in a variety of environments, including refrigeration, making it difficult to control and highlighting the importance of optimizing control strategies against this pathogen. Listeria phages are attractive biocontrol agents because phages bind to specific wall teichoic acids (WTA) on the bacterial cell wall, inhibiting pathogens without disrupting the normal microbiota or structure of the food. Common stresses found on dairy products can affect cell wall composition and structure and subsequently affect the efficiency of control strategies that target the cell wall. The goal of this study was to determine the effect of a range of pH and temperatures on the effectiveness of a commercial phage cocktail treatment against several strains of L. monocytogenes in a cheese matrix. We developed a laboratory-scale cheese model that was made at different pH, treated with phage, and then inoculated with L. monocytogenes. Cheeses were incubated at 6, 14, or 22°C for 14 d, and bacterial counts were determined on d 1, 7, and 14. Our data show that phage treatment has a limited ability to reduce L. monocytogenes counts at each temperature tested; however, it was more effective on specific strains of L. monocytogenes when cheese was stored at higher temperatures. More specifically, the average counts of L. monocytogenes on phage-treated cheese stored at 22°C were significantly lower than those on phage-treated cheese stored at 6 or 14°C. Similarly, phage treatment was significantly more effective at inhibiting L. monocytogenes on cheese made at higher pH (6 and 6.5) compared with counts on cheese made at pH 5.5, where L. monocytogenes did not grow. Furthermore, serotype was found to affect the susceptibility of L. monocytogenes to phage treatment; serotype 1/2 strains showed significantly higher susceptibility to phage treatment than serotype 4b strains. Overall, our results suggest the importance of considering the efficacy of phage under conditions (i.e., temperature and pH) specific to a given food matrix when applying interventions against this important foodborne pathogen.