Listeria monocytogenes strain in response to stress conditions: preliminary results

Listeria monocytogenes is a Gram positive, facultative intracellular, and cold tolerant pathogen that is of concern in ready-to-eat (RTE) foods. It causes listeriosis, a severe invasive disease that can be fatal in individuals with a compromised immune system, including the elderly, pregnant women, and their newborns. L. monocytogenes exhibits resilience under diverse environmental conditions. A research framework that evaluates the resilience of this pathogen could help prevention measures. L. monocytogenes has a crucial impact on the safety of RTE foods because it can grow under refrigeration and resist technological stresses, such as at low/high pH, high salt concentrations and modified atmosphere. According to the Commission Regulation N0 2073/2005, the levels of L. monocytogenes in foods not intended for infants and for special medical purposes and not representing an optimal substrate for its growth shall not exceed 100 CFU/g. In this study, a genomic approach, combined with immunoproteomics and bioinformatics, was carried out to assess the expression levels of genes responsible for the virulence biomarkers expressed by a L. monocytogenes 1/2 strain involved in a listeriosis outbreak in the Marche region (Italy), grown under adverse environmental conditions in terms of temperature (12°C and 37°C), pH (7.0), and salt concentrations (0.5% and 7%). In addition, a strain grown at optimal condition served as baseline. Quantitative reverse transcription PCR (qPCR) assays were developed to quantify the relative expression of a panel of 6 target genes (imo0132, imo0699, imo0723, imo1053, imo2679, trxB) involved in the stress response. 16SrRNA was included as housekeeping gene target. The gene expression levels of lmo0699 gene, which encodes the Flagellar Motor switch protein (FliM), known for regulating flagella formation and motility of the strain, increased at 12°C, whereas at the same temperature but under acidic and osmotic stress conditions the gene expression was inhibited. Lmo0723, which encodes the methyl-accepting chemotaxis protein, identified in the motility and morphology of the microorganism, exhibited high similar expression patterns across all experimental conditions. Other txrB, lmo2679 and lmo0132 genes, associated with detecting environmental hurdles and initiating appropriate cellular responses, exhibited low expression, comparable in all conditions. The application of genomic studies could help classify and characterize L. monocytogenes strains to understand and investigate their physiological behaviour, especially in the perspective of outbreak investigations and surveillance strategies.