Temperature-Dependent Phage Resistance in Listeria monocytogenes Epidemic Clone II Strains

Abstract

Of at least 400 Listeria phages, only a few have been characterized as having wide host range among listeriae and no phages have been isolated from the environment of food processing plants, where contamination of highly processed, ready to eat foods is likely to occur. To understand the ecology of listeriaphage and Listeria monocytogenes in processing plant environments, we pursued the isolation of Listeria-specific phages from environmental samples from four turkey-processing plants in the United States. Nine of twelve isolated phages showed wide host range and the majority of L. monocytogenes of the serotype 4b complex (serotypes 4b, 4d, 4e) could be readily infected by these wide host range phages, but many isolates of other serotypes (1/2a or 3a, and 1/2b or 3b), which represented the majority of L. monocytogenes isolated from the environmental samples, were resistant to infection. L. monocytogenes epidemic clone II (ECII) has been responsible for two multistate outbreaks in the United States in 1998-99 and 2002, in which contaminated ready-to-eat meat products (hot dogs and turkey deli meats, respectively) were implicated. We found that broad host range phages were unable to produce plaques on ECII strain lawns when the bacteria were grown at temperatures lower than 37°C (4, 10, 25, 30°C) , regardless of the temperature during infection and subsequent incubations. To identify genes responsible for this temperature-dependent resistance, use of a transposon led to successful isolation of a mutant (J46C), which was phage susceptible at both 25°C and 37°C. The ORF 2753, where the transposon was localized, and the adjacent ORF 2754 exhibited similarity with several restriction endonucleases and methylases, respectively, suggesting that they may constitute a restriction-modification system. Temperature-dependent phage resistance mechanisms may contribute to the ability of L. monocytogenes ECII to become established in the environment of processing plants, other environments characterized by relatively low temperatures, and foods. Further studies are needed to elucidate the molecular mechanism by which ECII-specific genes confer temperature-dependent resistance to phage, and to assess other possible roles of these genes in the ecology of the bacteria and during infection.