The orientation of bacteria is similar to a game of Hit the Pot: As in this blindfolded search, the cell only approaches the target by trial or error. It gets there by continuing to move as it approaches the target ("it gets warmer and warmer") - and randomly changing direction in between.

Bacteria decide where to swim depending on whether it is beneficial or harmful for them, i.e. towards food or away from toxins. However, bacteria are too small to be able to assess their immediate environment in multiple dimensions. To do this, they would need two organs that are far enough apart to be able to perceive a signal differently or determine the direction of the signal - like our ears or eyes. Instead, bacteria evaluate a signal while they are moving. If the signal becomes stronger, they swim towards the target; if it becomes weaker, they move away from it. They keep stopping, turning and swimming in a new, randomly selected direction. How long they swim in one direction is also determined by the changing signal. If it becomes stronger, they swim in that direction for a long time. If the strength of the signal does not change or if it becomes weaker, they only follow this direction for a short time. In the long term, this results in a directed movement towards or away from a source without knowing the target position in advance (see illustration).

This directed movement is known as taxis. The increasing or decreasing signal forms a gradient that bacteria follow. Depending on the signal, we speak of phototaxis (light), chemotaxis (dissolved substances, including food), thermotaxis (temperature) or even magnetotaxis (when bacteria follow magnetic fields). Whether bacteria swim straight ahead or change direction is determined by their flagellum, a long cell projection that works like a propeller. If it turns in one direction, the cell swims straight ahead; if it turns in the other direction, it does not swim backwards, but turns in circles. This creates the two movement patterns required for the locomotion described above. In order to determine the direction of rotation of the flagellum, the bacterium must first perceive the external signal via receptors on the cell surface and translate it into an internal signal. This then influences the flagellum directly or indirectly. A decision is often based not only on a single external signal, but on the superposition of several pieces of information.

Bacteria are not the only organisms that use alternating straight-ahead movements and random changes of direction to approach a signal source. Moths and other insects find distant mates by following their scent over several kilometres. And this is how children find the sweets under the pot at a birthday party.

© Text and figure: Tobias Engl / VAAM, tengl[at]uni-mainz.de, use in accordance with CC 4.0