Bacterial chemotaxis: Rhodobacter sphaeroide and Sinorhizobium meliloti - variations on a theme?

Abstract

Overview More than half the known bacterial species swim, with swimming being used to direct their overall movement towards an optimum environment for growth, which could include sites of invasion for pathogens and symbionts. A great deal is now known about swimming and its environmental control in enteric species (for recent reviews see Amsler & Matsumura, 1995; Blair, 1995), but only recently has there been a significant increase in the study of non-enteric species. From these studies it has become apparent that the chemosensory system must have a very ancient origin as it is present in some form not only in all the genera examined, but also in both bacteria and archaea (Alex & Simon, 1994). The central theme, an ion-driven motor rotating a semi-rigid helix, controlled by a phosphorelay system to bias the overall movement of the bacterium to its optimum environment for growth, is common to all species. However, as might be expected of a system with such a long history, it has been adapted by different bacterial subgroups in a way which may relate to the different environmental niches into which they evolved. The chemosensory system in enteric species is relatively straightforward, but that of other species is turning out to be much more complex, and more varied. This variation on a central theme is illustrated well if the behaviour of different members of the a-subgroup of bacteria is examined. This group, characterized by DNA with a high G+ C content (over 60 mol%), includes species found in a very wide range of natural environ- ments, for example Agrobacterium, Axospirillum, Caulo- bacter, Sinorhizobium and Rhodobacter, with most of