Cycles of light and dark co‐ordinate reversible colony differentiation in L isteria monocytogenes

Recently, several light receptors have been identified in non‐phototrophic bacteria, but their physiological roles still remain rather elusive. Here we show that colonies of the saprophytic bacterium L isteria monocytogenes undergo synchronized multicellular behaviour on agar plates, in response to oscillating light/dark conditions, giving rise to alternating ring formation (opaque and translucent rings). On agar plates, bacteria from opaque rings survive increased levels of reactive oxygen species ( ROS ), as well as repeated cycles of light and dark, better than bacteria from translucent rings. The ring formation is strictly dependent on a blue‐light receptor, Lmo0799 , acting through the stress‐sigma factor, σ B . A transposon screening identified 48 mutants unable to form rings at alternating light conditions, with several of them showing a decreased σ B activity/level. However, some of the tested mutants displayed a varied σ B activity depending on which of the two stress conditions tested (light or H 2 O 2 exposure). Intriguingly, the transcriptional regulator PrfA and the virulence factor ActA were shown to be required for ring formation by a mechanism involving activation of σ B . All in all, this suggests a distinct pathway for Lmo0799 that converge into a common signalling pathway for σ B activation. Our results show that night and day cycles co‐ordinate a reversible differentiation of a L . monocytogenes colony at room temperature, by a process synchronized by a blue‐light receptor and σ B .