Phylogenetic, Phenotypic And Transcriptomic Characterization Of Ready-To-Eat Food Associated Listeria Monocytogenes

Abstract

Listeria monocytogenes is of particular concern in ready-to-eat (RTE) food product. It is able to survive and grow in RTE foods from low numbers to potentially hazardous levels. With the aim of developing control strategies that prevent or reduce growth of this pathogen in RTE food products, I combined both applied and basic research approaches to perform phylogenetic, phenotypic and transcriptomic characterizations of RTE food associated L. monocytogenes, enabling a better understanding of L. monocytogenes population genetics, biological capacities, as well as the mechanisms that it uses to survive and grow in food matrices. First, we used a 10-gene multilocus sequence typing scheme to investigate the diversity and phylogenetic distribution of 124 L. monocytogenes strains with various genetic backgrounds. Our data show that 10-gene MLST allows for high level of discrimination of L. monocytogenes and demonstrated the distinct ability of growth of L. monocytogenes under nutrient limited conditions between some of the subgroups of L. monocytogenes. Second, we quantify the effects of organic acids, nisin, and their combinations on controlling 18 strains (individually) of L. monocytogenes growing on the RTE seafood, coldsmoked salmon (CSS), and in modified BHI broth (MBHIB) at 7oC. The combination of potassium lactate and sodium diacetate, and the combination of potassium lactate and nisin, were identified as the most effective bacteriostatic treatment and bactericidal treatment against L. monocytogenes, respectively. We also observed that the quantitative prediction of the variability of growth parameters in a food matrix such as CSS by using a laboratory medium could only be achieved for lineage II strains but not lineage I strains. RNA-seq was used to understand the transcriptional landscape of L. monocytogenes strain H7858 grown on cold-smoked salmon (CSS, water-phase salt 4.65%, pH 6.1) relative to in modified brain heart infusion broth (MBHIB, water-phase salt 4.65%, pH 6.1) at 7oC. We found that genes encoding proteins involved in cobalamin biosynthesis as well as ethanolamine and 1,2-propanediol utilization have significantly higher transcript levels in H7858 grown on CSS compared to in MBHIB. Our data identify specific transcriptional profiles of L. monocytogenes growing on vacuum-packaged CSS, which may provide targets for development of novel and improved strategies to control L. monocytogenes growth on this RTE food.