Effectiveness of Bacteriophage Listex™ P100 in Reducing Listeria monocytogenes on Roast Beef and Cooked Turkey

Introduction: The use of phages as biocontrol agents in food and food animals has been successfully applied to poultry, pigs, cattle, cheese, sprouts, melons and other foods. Previous studies have shown that successful phage-based pathogen intervention in food greatly depends on the chemical composition of the food and its specific matrix. It has thus been suggested that there is need to individually optimize protocols for the application of phages with respect to the phages and the target bacteria as well as putting into consideration specifications for the food matrix.

Purpose: To verify the effectiveness of commercially available anti-Listeria phage LISTEX™ P100 in reducing Listeria monocytogenes on ready-to-eat (RTE) roast beef and cooked turkey.

Methods: A proof-of-concept study examined the activity of commercial anti-Listeria phage preparation, LISTEX™ P100, on a four-strain L. monocytogenes cocktail in two RTE food products; roast beef and cooked turkey. Foods at 4°C and 10°C were inoculated with cold-adapted L. monocytogenes to result in surface a contamination level of 10³ CFU/cm².  LISTEX™ P100 was applied at manufacturer recommended dose (10⁷ PFU/cm²) and samples taken at regular time intervals during the RTE product’s shelf life (t=30 mins, 24 hours, 48 hours, 72 hours, 7 days, 10 days, 14 days, 20 days and 28 days) were treated with a virucide (tea infusion containing 4.3 mmol FeSO₄) to neutralize free phages and plated on Oxford media to recover viable L. monocytogenes. Controls comprised of RTE meat samples un-inoculated with L. monocytogenes with no phage treatment and RTE meat samples inoculated with four strain L. monocytogenes cocktail with no phage treatment.

Results: LISTEX™P100 was effective during incubation at 4°C with initial reductions of L. monocytogenes of 2.2 log CFU/cm² and 1.7 log CFU/cm², respectively for cooked turkey and roast beef.  For the phage treated samples, the L. monocytogenes cell numbers remained below those of the untreated control with a reduction of between 0.9-2.8 log CFU/cm² and 1.9-4.1 CFU/cm², respectively for cooked turkey and roast beef.  An initial L. monocytogenes cell reduction of 1.6 log CFU/cm² and 1.7 log CFU/cm², for cooked turkey and roast beef was achieved by the phage at 10°C (abusive temperature), respectively. At this temperature, the L. monocytogenes cell numbers remained below those of the untreated control only during the first 14 days of the experiment for roast beef samples after which there was no difference between the treated and untreated controls.

Significance: An overall reduction of at least 2 logs/cm² has an impact on the growth of L. monocytogenes on RTE roast beef and cooked turkey. This represents a hurdle which can be used to enhance safety in foods contaminated with L. monocytogenes.