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RESEARCH PRODUCT

Frequent emergence of sigB mutant alleles in Listeria monocytogenes is likely to be linked to a growth advantage at elevated temperatures

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International audience; Background: The food-borne pathogen Listeria monocytogenes is a Gram-positive bacteria and highly resistant to a variety of stresses. Such resistance is determinate by the activation of the alternative sigma factor B (σB). Genes within sigB operon encode for the signal transduction that controls σB activity. Mutations within this operon generally result on reduced σB activation. Objectives: Characterize the impact of secondary mutations within L. monocytogenes sigB operon, and identify laboratory conditions that promote the selection of such mutations among wild type (WT) populations. Methods: Transposon mutants (Tms) that lost the ability to form rings in response to cycles of light and dark were analyzed through whole genome sequencing. Challenged for acid tolerance (pH 2.5). σB activity was measured using the EGFP reporter fusion system. Growth kinetics measured at different temperatures. Competition experiments were performed using mixed cultures incubated at 42°C. Results: Frameshift mutations within the sigB operon were identified in Tms, which showed decreased acid tolerance and less fluorescence in the EGFP reporter strains, demonstrating a reduced σB activation. Increased growth rates of Tms and ΔsigB when compared to WT during the transition to stationary phase at elevated temperatures (40-42°C). Tms and ΔsigB were able to overtake the WT population overtime, but not vice versa, in competition experiments. Elevated growth temperature can result in the selection for mutations that result in deceased σB activity in L. monocytogenes. Researchers use temperature based selection strategies should be cognizant of this finding during routine culture and genetic work on this organism.