Efficacy of bacteriophage LISTEX™P100 combined with chemical antimicrobials in reducing Listeria monocytogenes in cooked turkey and roast beef

• LISTEX™P100 reduction of L. monocytogenes at real-life contamination concentrations.
• Efficacy of LISTEX™P100 in the presence and absence of chemical antimicrobials.
• LISTEX™P100 efficacy assay designed to avoid overestimation of phage killing effect.
• At 4 °C, no more than a 2 log CFU/cm2 L. monocytogenes increase throughout shelf life.
• Phage and chemical antimicrobials synergistic effect was pronounced in cooked turkey.

The aim of this study was to verify the effectiveness of the commercially available anti-Listeria phage preparation LISTEX™P100 in reducing Listeria monocytogenes on ready-to-eat (RTE) roast beef and cooked turkey in the presence or absence of the chemical antimicrobials potassium lactate (PL) and sodium diacetate (SD). Sliced RTE meat cores at 4 and 10 °C were inoculated with cold-adapted L. monocytogenes to result in a surface contamination level of 103 CFU/cm2. LISTEXTMP100 was applied at 107 PFU/cm2 and samples taken at regular time intervals during the RTE product's shelf life to enumerate viable L. monocytogenes. LISTEX™P100 was effective during incubation at 4 °C with initial reductions of L. monocytogenes of 2.1 log10 CFU/cm2 and 1.7 log10 CFU/cm2, respectively, for cooked turkey and roast beef without chemical antimicrobials (there was no significant difference to the initial L. monocytogenes reductions in the presence of LISTEXTMP100 for cooked turkey containing PL and roast beef containing SD-PL). In the samples containing no chemical antimicrobials, the presence of phage resulted in lower L. monocytogenes numbers, relative to the untreated control, of about 2 log CFU/cm2 over a 28-day storage period at 4 °C. An initial L. monocytogenes cell reduction of 1.5 log10 CFU/cm2 and 1.7 log10 CFU/cm2, respectively, for cooked turkey and roast beef containing no chemical antimicrobials was achieved by the phage at 10 °C (abusive temperature). At this temperature, the L. monocytogenes cell numbers of samples treated with LISTEX™ P100 remained below those of the untreated control only during the first 14 days of the experiment for roast beef samples with and without antimicrobials. On day 28, the L. monocytogenes numbers on samples containing chemical antimicrobials and treated with LISTEXTMP100 stored at 4 and 10 °C were 4.5 log10 CFU/cm2 and 7.5 log10 CFU/cm2, respectively, for cooked turkey, and 1.2 log10 CFU/cm2 and 7.2 log10 CFU/cm2, respectively, for roast beef. In both cooked turkey samples with and without chemical antimicrobials stored at 10 °C, the phage-treated samples had significantly lower numbers of L. monocytogenes when compared to the untreated controls throughout the 28-day storage period (P < 0.0001). For roast beef and cooked turkey containing chemical antimicrobials treated with LISTEXTMP100 and stored at 4 °C, no more than a 2 log CFU/cm2 increase of L. monocytogenes was observed throughout the stated shelf life of the product. This study shows that LISTEX™P100 causes an initial reduction of L. monocytogenes numbers and can serve as an additional hurdle to enhance the safety of RTE meats when used in combination with chemical antimicrobials.