For a long time, bacteria were considered insensitive to light, with the exception of phototrophs that use sunlight as an energy source. The photosensory proteins in these phototrophs were believed to help them find optimally illuminated environments for solar energy harvesting. Six classes of photoreceptors exist, and representatives of five of these classes were first identified in phototrophic bacteria. In these organisms, photoreceptors control production of the photosynthetic apparatus, coordinate composition of light-absorbing antennas to better capture available light, and trigger phototaxis responses.
Recent studies have established that pathogenic bacteria rely on light as one of the means to assess their location and increase virulence in preparation for entering a new host. Given that a number of animal and plant pathogens contain photoactivated proteins, it is intriguing to imagine that the transition between environmental and host-associated lifestyles is commonly regulated by light. Several experimental examples and bioinformatic analysis support the view that light helps bacteria make another important lifestyle decision, i.e. between a single-cellular planktonic state and a surface-attached multicellular community (biofilm). Many outstanding questions regarding these light-regulated lifestyle choices remain. If we want to better control bacterial virulence and biofilms, we need to literally shed light on bacteria more often.
Light helps bacteria make important lifestyle decisions. Trends Microbiol. Jun 9 2011
Until recently, bacterial responses to changes in light environments were regarded as specialized adaptations in a small number of phototrophs. However, the genomes of many photosynthetic and chemotrophic bacteria not known to have photophysiological responses also encode photoreceptor proteins. What new trends in the biological responses triggered by these photoreceptors are emerging? Here, we review several instances where members of different blue-light receptor classes (LOV, BLUF and PYP) photoregulate a lifestyle choice between the motile single-cellular state and the multicellular surface-attached community state (biofilm) by a range of mechanisms including bacterial two-component systems, the second messenger cyclic di-GMP and direct interactions of photoreceptors with transcription factors. We also discuss how ‘seeing’ helps some pathogenic bacteria make another important choice, i.e. between environmental and host-associated lifestyles.