The study was developed to determine the effectiveness of LISTEX, a commercially available anti-Listeria phage product for topical application, in reducing Listeria monocytogenes (Listeria) 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). It was conducted at the Food Research Center, University of Guelph, Guelph, Canada.

Sliced RTE meat cores at 4° and 10°C were inoculated with cold-adapted Listeria to result in a surface contamination level of 103 CFU/cm2. LISTEX was applied at 107 PFU/cm2 and samples taken at regular time intervals during the RTE product's shelf life to enumerate viable Listeria cells.

LISTEX was effective during incubation at 4°C with initial reductions of Listeria 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 Listeria reductions in the presence of LISTEX for cooked turkey containing PL and roast beef containing SD-PL.

In the samples containing no chemical antimicrobials, the presence of LISTEX resulted in lower Listeria numbers, relative to the untreated control, of about 2 log CFU/cm2 over a 28-day storage period at 4°C. An initial Listeria 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 bacteriophage at an abusive temperature of 10°C.

At this temperature, the Listeria cell numbers of samples treated with LISTEX 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 Listeria numbers on samples containing chemical antimicrobials and treated with LISTEX 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 Listeria when compared to the untreated controls throughout the 28-day storage period.

For roast beef and cooked turkey containing chemical antimicrobials treated with LISTEX and stored at 4°C, no more than a 2 log CFU/cm2 increase of Listeria was observed throughout the stated shelf life of the product. This study shows that LISTEX causes an initial reduction of

Listeria numbers and can serve as an effective hurdle to enhance the safety of RTE meats when used in combination with chemical antimicrobials.

A copy of the complete study is available by contacting Dirk de Meester (d.demeester@micreos.com)

Participants in the study, published in the International Journal of Food Microbiology:

• Andrew Chibeu, Guelph Food Research Centre - Agriculture and Agri-Food Canada and Laboratory for Foodborne Zoonoses, Public Health Agency of Canada;
• Louise Agius, Guelph Food Research Centre - Agriculture and Agri-Food Canada and Department of Food Science, University of Guelph;
• Anli Gao Laboratory Services Division, University of Guelph;
• Parviz M. Sabour, Guelph Food Research Centre - Agriculture and Agri-Food Canada;
• Andrew M. Kropinski Laboratory for Foodborne Zoonoses, Public Health Agency of Canada and Department of Molecular and Cellular Biology, University of Guelph, and
• S. Balamurugan, Guelph Food Research Centre - Agriculture and Agri-Food Canada.