Listeria monocytogenes senses blue light via the flavin mononucleotide-containing sensory protein Lmo0799, leading to the activation of the general stress response sigma factor SigB (¿(B)). In this study we investigate the physiological response of this food-borne pathogen to blue light. We show that blue light (460-470nm) doses of 1.5 -2 mW cm(-2) cause inhibition of growth on agar-based and liquid culture medium. The inhibitory effects are dependent on cell density, with reduced effects evident when high cell numbers are present. The addition of 20 mM dimethylthiourea, a scavenger of reactive oxygen species, or catalase to the medium reverses the inhibitory effects of blue light, suggesting that growth inhibition is mediated by the formation of reactive oxygen species. A mutant strain lacking ¿(B) (¿sigB) was found to be less inhibited by blue light than the wild-type, likely indicating the energetic cost of deploying the general stress response. When a lethal dose of light (8 mW cm(-2)) was applied to cells the ¿sigB mutant displayed a marked increase in sensitivity to light compared to the wild-type. To investigate the role of blue light sensor Lmo0799 mutants were constructed that either deleted the gene (¿lmo0799) or altered a conserved cysteine residue at position 56, predicted to play a pivotal role in the photocycle of the protein (lmo0799 C56A). Both mutants displayed phenotypes similar to the ¿sigB mutant in the presence of blue light providing the first genetic evidence that residue 56 is critical for light sensing in L. monocytogenes Taken together these results demonstrate that L. monocytogenes is inhibited by blue light in a manner that depends on reactive oxygen species and demonstrate clear light-dependent phenotypes associated with ¿(B) and the blue light sensor Lmo0799.
Listeria monocytogenes is a bacterial food-borne pathogen that can cause life-threatening infections in humans. It is known to be able to sense and respond to visible light. In this study we examine the effects of blue light on the growth and survival of this pathogen. We show that growth can be inhibited at comparatively low doses of blue light and that at higher doses L. monocytogenes cells are killed. We present evidence suggesting that blue light inhibits this organism by causing the production of reactive oxygen species such as hydrogen peroxide. We help to clarify the mechanism of light sensing by constructing a "blind" version of the blue light sensor protein. Finally we show that the activation of the general stress response by light has a negative effect on growth, probably because cellular resources are diverted into protective mechanisms rather than growth.