Taxing questions in development.

Bacteria use taxis-controlled movement to reach their optimum environment. Chemotaxis is probably the best understood behavioural system in biology, biasing the normal random movement of bacteria using a phospho-relay pathway from receptors to the motility organelles. The pathways are typified by signal recognition and receptor adaptation, enabling bacteria to sense and respond to changing environments. Models have been derived from the single chemosensory pathway of Escherichia coli but the sequencing of an increasing number of bacterial genomes is revealing genes that apparently encode multiple chemosensory pathways. Recently, data have accumulated suggesting that some of these pathways might not control motility, although the mechanisms by which this might happen remain unclear. Information from the soil bacterium Myxococcus xanthus could lead the way to an understanding of such mechanisms.