Behavioral responses in bacteria.
As has been stated, bacteria are able to sense a wide range of environmental stimuli through a variety of receptors and to integrate the different signals to produce a balanced response that maintains them or directs them to an optimum environment for growth. In addition, these simple, neuron-less organisms can adapt to the current concentration or strength of stimuli, i.e. they have a memory of the past. Although different species show responses to different chemicals or stimuli, depending on their niche, a consistent pattern is starting to emerge that links environmental sensing and transcriptional control to the chemosensing system, either directly, as in R. sphaeroides and the PTS system, or indirectly, as in the MCP-dependent system. This suggests a common evolutionary pathway from transcriptional activators to dedicated sensory systems. Currently the majority of detailed investigations into bacterial behavior have been carried out on single stimuli under laboratory conditions using well-fed cells. Only limited analysis, using a range of rhizosphere and pathogenic species, has been carried out on the role of behavioral responses in the wild. While laboratory studies are needed to provide the backbone for eventual in vivo investigations, we should remember the responses of whole cells to changes in their environment under laboratory conditions are essentially artificial compared to the natural environment of most species. Once the basic system is understood, it will be possible to investigate the role of these responses in vivo, under competitive, growth-limiting conditions with multiple gradients.