Cycles of light and dark co-ordinate reversible colony differentiation in Listeria 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 Listeria 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, sigma B. A transposon screening identified 48 mutants unable to form rings at alternating light conditions, with several of them showing a decreased sigma B activity/level. However, some of the tested mutants displayed a varied sigma B activity depending on which of the two stress conditions tested (light or H2O2 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 sigma B. All in all, this suggests a distinct pathway for Lmo0799 that converge into a common signalling pathway for sigma 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 sigma B.