Research

A detailed view of Listeria monocytogenes’ adaptation and survival under cold temperature stress

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Abstract

The human pathogen Listeria monocytogenes (Lm) continues to be a challenge for the food industry where it is known to contaminate ready-to-eat foods and grow during refrigerated storage. In order to gain increased control of Lm in the food-supply-chain, an improved understanding of low temperature stress adaptation methods is needed. In this study, RNA-seq (strand specific Illumina libraries;22-39 million 2x100bp reads) and cell membrane fatty acid profiling were used to analyze adaptation mechanisms used by a fast growing, serotype 1/2a, Lm food plant isolate at 4°C. Brain heart infusion (BHI) broth pre-tempered to 20 or 4°C, was inoculated with 107 CFU/ml of stationary phase (SP) cells grownat 20°C in BHI, and incubated at 20 and 4°C. RNA and lipids were extracted at T1– early lag phase (LP),T2 – end of LP, T3 – mid-exponential, T4 – transition to SP, and T5 – late SP (T5+4 h or 2 days for 20 and4°C respectively). The number of coding transcripts upregulated (>2 log2, p<0.05) at 4°C relative to 20°C was 142, 96, 91, 45, and 388 from T1-T5 respectively, while the number of down regulated genes at T1-T5was 91, 38, 56, 125, and 256 respectively. Notably, the greatest differential gene expression occured in Lm cells during late SP at 4°C, the most relevant physiological state to Lm’s survival in chilled food products. Common among all time points was the upregulation of nine genes required for branched-chain fatty acid (BCFA) synthesis, which was supported by an increase in membrane BCFAs from 77% at T1-4°C to 93%at T5-4°C. Putative cold stress regulatory mechanisms could be observed through negatively correlated expression levels of sense and antisense RNA. This research highlights Lm’s response to cold stress and provides deeper insight into how refrigerated storage conditions influence microbial gene expression and physiology.