Abstract
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, σB. A transposon screening identified 48 mutants unable to form rings at alternating light conditions, with several of them showing a decreased σB activity/level. However, some of the tested mutants displayed a varied σB 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 σB. All in all, this suggests a distinct pathway for Lmo0799 that converge into a common signalling pathway for σB 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 σB.
Highlights
Bacteria are found in almost all places in the environment, where they successfully occupy different niches
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
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 sB
Summary
Bacteria are found in almost all places in the environment, where they successfully occupy different niches. In order to do so, they have to be able to sense both chemical gradients and physical parameters, such as carbon availability, temperature or light. Sensing such signals might trigger multicellular behaviour in some bacterial species. Nutritional depletion initiates a complex pattern of multicellular behaviour in Myxococcus xanthus, including the formation of fruiting bodies, swarming and rippling, the latter allowing the bacteria to ‘hunt’ for a prey (Shimkets and Kaiser, 1982; Berleman et al, 2006; Berleman and Kirby, 2009). The formation of biofilms as well as other multicellular activities is often triggered by a quorum sensing signal that co-ordinate the action of the entire population (Camilli and Bassler, 2006). Listeria spp. are closely related to Bacillus species, but unlike the latter, Listeria spp. are unable to sporulate, which could prove disadvantageous when encountering soil-derived stress
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