AUTHOR=Dorey Amber L. , Lee Bo-Hyung , Rotter Bjorn , O’Byrne Conor P. 

TITLE=Blue Light Sensing in Listeria monocytogenes Is Temperature-Dependent and the Transcriptional Response to It Is Predominantly SigB-Dependent

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.02497

DOI=10.3389/fmicb.2019.02497

ISSN=1664-302X

ABSTRACT=<p><italic>Listeria monocytogenes</italic> is an important food-borne pathogen that is tolerant to many of the stresses commonly used during food preservation. Outside the host, the bacterium has a saprophytic lifestyle that includes periodic exposure to solar irradiance. The blue component of this light is known to influence the activity of the stress-inducible sigma factor Sigma B (σ<sup>B</sup>). In this study, the influence of temperature and growth phase on the response of <italic>L. monocytogenes</italic> to blue light was investigated and the global transcriptional response to blue light was elucidated using an RNAseq-based approach. Stationary phase cells were found to be significantly more resistant to killing by blue light (470 nm) than exponential phase cells. Temperature also had a marked effect on blue light resistance with cells cultured at 37°C being much more sensitive than cells grown at 30°C. The role of σ<sup>B</sup> in light tolerance was confirmed but this effect was observed only at 30°C. σ<sup>B</sup> activation by blue light was assessed by measuring the transcriptional response of known σ<sup>B</sup>-dependent genes (<italic>sigB</italic>, <italic>lmo2230</italic>, and <italic>opuCA</italic>) to light. The transcripts were induced by blue light only at 30°C suggesting that blue light fails to activate σ<sup>B</sup> at 37°C. The light-induced transcription at 30°C was dependent on a functional blue light sensor, Lmo0799 (which we rename herein as RsbL). A transcriptomic analysis of the response to sub-lethal levels of blue light found that the changes in transcription were almost entirely σ<sup>B</sup>-dependent. A mutant where the light sensing mechanism of RsbL was inactivated through an amino acid substitution (Cys56Ala) was found to have an attenuated response to blue light, but residual activation of σ<sup>B</sup>-dependent genes suggested that alternative routes for activation of σ<sup>B</sup> by light are likely to exist. Overall, the study highlights the central role of σ<sup>B</sup> in the response of this pathogen to visible light and further shows that light sensing is absent at temperatures that exist within the mammalian host.</p>