Orientation in the Environment

It has long been known that many microorganisms—prokaryote as well eukaryotes—are capable of orientation in the environment and respond to attractants or repellents by motile sensory behaviour. In the case of protozoa, this was first studied by Jennings (1906). Among bacteriologists in particular, the term “taxis” is used for all kind of motile sensory behaviour irrespective of the involved mechanisms, with terms such as “chemotaxis”, “phototaxis”, etc. The tradition among zoologists, on the other hand, is that a taxis means a directed response so that the organisms directly sense the direction of a gradient, whereas “kinesis” refers to other mechanisms, but does not imply that the organisms can at any point detect the direction of a chemical gradient. In this case, taxis is impossible for a microorganism since this would require that the cells could sense the concentration difference between their anterior and their posterior ends, but due to the small number of molecules in the immediate vicinity of the cell and statistical noise, this is impossible. Likewise, microorganisms cannot sense the direction of water flow and thus be able to swim upstream when sensing an attractive compound in the way that many animals move against the direction of wind or water currents when sensing the smell of prey or of a potential mate. Among protozoans, the only examples of a real taxis include phototaxis and geotaxis, both of which occur only in a relatively few species and special organelles are required (see Chap. 1, Fig. 1.2a). Other types of sensory motile behaviour that make microorganisms accumulate in attractive places and avoid deterrents are referred to as “kinesis” (Dunn 1990; Van Houten et al. 1981).