Food-borne Listeria monocytogenes is a serious threat to human health, and new strategies to combat this opportunistic pathogen in foods are needed. Bacteriophages are natural enemies of bacteria and are suitable candidates for the environmentally friendly biocontrol of these pathogens. In a comprehensive set of experiments, we have evaluated the virulent, broad-host-range phages A511 and P100 for control of L. monocytogenes strains Scott A (serovar 4b) and WSLC 1001 (serovar 1/2a) in different ready-to-eat (RTE) foods known to frequently carry the pathogen. Food samples were spiked with bacteria (1 ؋ 10 3 CFU/g), phage added thereafter (3 ؋ 10 6 to 3 ؋ 10 8 PFU/g), and samples stored at 6°C for 6 days. In liquid foods, such as chocolate milk and mozzarella cheese brine, bacterial counts rapidly dropped below the level of direct detection. On solid foods (hot dogs, sliced turkey meat, smoked salmon, seafood, sliced cabbage, and lettuce leaves), phages could reduce bacterial counts by up to 5 log units. Variation of the experimental conditions (extended storage over 13 days or storage at 20°C) yielded similar results. In general, the application of more phage particles (3 ؋ 10 8 PFU/g) was more effective than lower doses. The added phages retained most of their infectivity during storage in foods of animal origin, whereas plant material caused inactivation by more than 1 log 10 . In conclusion, our data demonstrate that virulent broad-host-range phages, such as A511 and P100, can be very effective for specific biocontrol of L. monocytogenes in contamination-sensitive RTE foods.Listeria monocytogenes is an opportunistic human pathogen, widely distributed in the environment and transmitted to humans and animals via contaminated foods (50). The organism is well adapted to very different environmental conditions encountered in foods; it tolerates high levels of salt content (10 to 20%), can grow at pH values below 6, with low oxygen, and at temperatures down to 1°C (48). L. monocytogenes causes listeriosis, a severe disease which may result in septicemia, meningitis, encephalitis, or loss of the fetus during pregnancy (52). Although listeriosis is comparatively rare compared to other food-borne infections, the high mortality rate of 15 to 40% is of great concern (49, 52). Cases of sporadic and epidemic listeriosis are increasing (4, 6, 11), and it has been estimated that about 2,000 hospitalizations and 500 deaths occur annually in the United States as a result of the consumption of Listeria in foods (39). Although many foods can serve as vehicles for this pathogen, Listeria was often isolated from ready-to-eat (RTE) foods, such as milk and cheeses, cold-cut meats, smoked fish, seafood, and vegetables (45). RTE foods have been implicated in most of the major listeriosis outbreaks in the last 30 years (13,18,20,21,42,(45)(46)(47). Of particular concern is the fact that they are consumed directly, without a final bactericidal processing step. Since the preservation methods applicable to minimally processed RTE foods of...

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The growth of Listeria monocytogenes WSLC 1364, originating from a cheese-borne outbreak, was examined in the presence and in the absence of a pediocin AcH-producing Lactobacillus plantarum strain on red smear cheese. Nearly complete inhibition was observed at 10 2 CFU of L. monocytogenes per ml of salt brine solution, while contamination with Listeria mutants resistant to pediocin resulted in high cell counts of the pathogen on the cheese surface. The inhibition was due to pediocin AcH added together with the L. plantarum culture to the brine solution but not to bacteriocin production in situ on cheese. Pediocin resistance developed in vitro at different but high frequencies in all 12 L. monocytogenes strains investigated, and a resistant mutant remained stable in a microbial surface ripening consortium over a 4-month production process in the absence of selection pressure. In conclusion, the addition of a L. plantarum culture is a potent measure for combating Listeria in a contaminated production line, but because of the potential development of resistance, it should not be used continuously over a long time in a production line.