The current trend of minimization in food formulation and processing – such as the reduction of salt levels in products like meat and cheese and milder processing techniques designed to preserve fresh characteristics of products – should trigger a renewed look at food safety measures in the food industry, and might attract the attention of government regulators. Reducing salt concentrations will change the water activity of products, which affects growth and survival or death of microorganisms. It is likely that foodborne pathogens in general will grow faster in reduced salt conditions. Modeling studies by the Institute of Food Research suggest that in various hypothetical food products like typical bacon or ham samples, Listeria monocytogenes growth would be enhanced in reduced salt conditions. In a study concerning the preservation of natural casings, low water activity (hence, a relatively high salt concentration) prevents the growth of Listeria monocytogenes. Furthermore, the bacterium survived for up to 30 days at a water activity level of 0.85 percent. Even a small increase in the growth rate of Listeria monocytogenes in food products can significantly increase the risk of listeriosis in a sensitive population. Listeria monocytogenes Is a Continuing Threat to Human Health  With a mortality rate of 20 percent to 30 percent, Listeria is considered one of the most dangerous food pathogens. Outbreaks of listeriosis, the deadly foodborne infection caused by Listeria, are in the news frequently these days. Two single outbreaks in the past 2 years – in Canada on RTE meat and in the US on cantaloupes – together killed more than 50 people. The elderly, pregnant women, newborn babies and immunocompromised people are especially at risk. In the first half of 2012, U.S. food recalls caused by suspected Listeria monocytogenes contamination were reported involving a variety of products like soybean and alfalfa sprouts, pizza, sandwiches, beef sausages, cooked eggs, herring fillet, smoked salmon and cheeses. Researchers at the University of Florida’s Emerging Pathogens Institute ranked various pathogen-food combinations based on number of illnesses, the cost of each and the overall public health burden. Listeria monocytogenes in deli meat was ranked #3 and in dairy products #5 out of the top 10 pathogen-food combinations. Elimination of Listeria monocytogenes in food materials and from equipment and surfaces in food processing areas is crucial because it survives adverse environmental conditions well compared to many other bacteria. It even grows at refrigeration temperatures and in low oxygen environments, such as in vacuum-packaged products. Changes in processing methods can also influence the microbiological stability of a product.  Mildly processed, ready-to-use foods are becoming increasingly popular because of good retention of sensory and nutritional characteristics of the ingredients. However, many of the food safety issues related to these processing methods have not been fully investigated. We know that these treatments do not always result in complete inactivation of the microbes present, so unwanted effects cannot be excluded. Mild thermal treatment of cut cabbage, for instance, promoted Listeria monocytogenes growth, indicative of the destruction of an endogenous growth-inhibiting compound within the cabbage by the applied thermal treatment. Even more important, various microbial survival strategies contribute to the persistence of pathogenic bacteria in food-processing environments. With respect to Listeria monocytogenes, mechanisms like biofilm formation stress adaptive response and the occurrence of resistant subpopulations. An example is a phenomenon referred to as ‘tailing’ where bacteria are able to adapt to some degree to an intervention. High hydrostatic pressure-resistant variants have been found that were also more resistant to heat, and differences among variants were observed in acid resistance, growth rate, motility, and biofilm-forming capacity. Adequate control When salt levels in food products are reduced, or when processing conditions are changed, it might be necessary to decrease the shelf life of a product or to include other preservative factors to ensure an adequate safety margin. This should be carefully evaluated for each reformulated product. Adequate control of Listeria monocytogenes can be obtained cost effectively by the use of phage-based products. Bacteriophages (Greek for ‘bacteria-eaters”), also known as ‘phages’ were discovered nearly a century ago. They exist ubiquitously in our environment, including the human body and many foodstuffs. The unique property of phages lies in the fact that they attack and kill targeted bacteria. Hence, specific phages can be used against specific dangerous bacteria in the production of food. Phages can be used to address post-lethality contamination of Listeria monocytogenes under ‘alternative 2’ or ‘alternative 1′ anti-Listeria protocols as defined by the U.S. Department of Agriculture and its Food Safety Inspection Service branch. Phages have been confirmed as GRAS [Generally Recognized as Safe] by the FDA and do not require labeling when used as a processing aid. Furthermore, phages are suitable for natural and organic products (OMRI-listed) and can be integrated easily within the daily routine of the normal production process. The LISTEX™ phage, which is active against thousands of strains of Listeria monocytogenes, withstands a wide range of food processing conditions; does not affect organoleptic properties of the treated products such as taste, texture, or color; leaves starter cultures unaffected and is non-corrosive. This phage, selected from Micreos’ proprietary collection of food-grade phages, show bacteriocidal effects that can be measured within hours. A dose-dependent control of Listeria monocytogenes is typically observed during shelf life.

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    Uaing phages can be an effective tool in food portection as phage typing results in host cell lysis, however, not all strains of pathogenic bacteria are typable. In a study done by Carlton et al., 2005, L.monocytogensis in soft cheese was controlled by using a Listeria phage that resulted in a 3.5 log reduction.
    The following journal articles are an interesting read:

  • The nucleic acid (DNA or RNA) present within a given bacteriophage must necessarily have come from its previous host. Phage packaging into a new bacteriophage particle is not usually very precise. Some nucleic acid from the host may be incorporated as well. Now this hybrid bacteriophage/bacterial nucleic acid gets injected into a new host. The result is transduction. Since we don’t know the function of the nucleic acid picked up from the previous host, we have no way to tell what effect such bacteriophage-mediated nucleic acid transfer will have on the newly-infected cell. This sort of thing is generally called bacteriophage transduction and from the bacterium’s point of view this sort of thing is just another way to experience sex (in either lytic or lysogenic cycles). Yes, transduction events are rare, but bacteria are so small and their populations so large that a one in a million chance may actually be a near-certainty.
    Myself, I would be much happier with bacteriophage therapy of anything from humans to ham if it could be shown that the bacteriophage used caused all (i.e, 100%) of the susceptible cells to die. That way, any transduced genetic elements have no way to propagate and cause problems later on. Papers concerned with bacteriophage therapy tend to report “cures.” They don’t go into details of how effective a kill the bacteriophage produced.
    petifun Colombo,