Exploring the global transcriptomic response of L. monocytogenes to desiccation on stainless steel

The ability of L. monocytogenes to survive desiccation for extended periods on food contact surfaces remains a challenge for the food industry. The purpose of this study was to further our understanding of the bacterium’s survival by investigating the global transcriptomic response of L. monocytogenes to desiccation (43% RH, 15°C) on food grade stainless steel surfaces. Two strains (a food and an outbreak strain) of L. monocytogenes were desiccated (43% RH, 15°C) on stainless steel under conditions simulating a food processing plant. Survivor counts and RNA extracts were obtained after 0 (control), 6, 12, 24 and 48 hours for subsequent rRNA-depleted Illumina TrueSeq RNA library preparations and strand specific Illumina Hiseq 2000 paired end RNA-sequencing. Differentially expressed genes were reported as significant (padjust < 0.05) if log2 fold change were >1 (fold change > 2). Both strains were reduced by 1.8 – 2.0 log CFU/cm2 over 48 hours (from 7.7 log CFU/cm2), with the first log reduction occurring after 6 hours. The number of differentially expressed genes varied among the food (336±20) and outbreak strains (646±32). After commencement of the desiccation, gene expression remained stable over the 48 hours for both strains. A core set of 154 genes were differentially (padjust < 0.05) expressed in both strains throughout the desiccation and included the downregulated cheY and cheA (two component system involved in chemotaxis), the upregulated qoxABCD operon (sigB dependent quinol oxidase), and the upregulated phdA (general metabolism related to osmotic stress). In contrast, genes such as inlH (internalin H) and lmo0781-0784 (PTS mannose system) were differentially up- or down- regulated in the strains. The present study detected novel desiccation associated stress genes in L. monocytogenes and revealed strain differences. Taken together this will increase our knowledge of the bacterium’s desiccation-stress response and lead to improved control in food processing plants.