*** Appendix: The mechanism of directed movement in bacteria

Back to: Does directed movement qualify as agency? *** SUMMARY of Conclusions reached References

Staph bacteria. Picture courtesy of Janice Carr/CDC via BBC.

Bacteria possess specialised "receptors" or information-encoding devices, which are sensitive to light, chemicals, magnetic fields and so on. These receptors may or may not be activated, depending on the local environment. Bacterial receptors possess a short-term (three-second) "memory": they are activated by changes in their environment, rather than by the mere presence of their object. These receptors can alter the motion of bacteria.

A bacterium has two kinds of motion: directed movement (a "run", which occurs when a bacterium's rotary motors, or flagella, rotate in a counter-clockwise direction) and random tumbling (which occurs when a bacterium's flagella suddenly change direction and rotate clockwise). When the external section of a bacterial receptor recognizes and binds its target, a signal passes through the rest of the receptor and causes sequential changes in two proteins inside the bacterium. (This two-protein sensing system is found in all bacteria and in many other life-forms, but not in animals.) The first protein is called a kinase and sits next to the receptor. Normally, when there is no signal, this protein activates a second protein, the regulator, which interacts with the gear shift of a bacterium's flagella, causing them to turn clockwise and the bacterium to tumble randomly, about once every second.

However, when there is a signal from the receptor, the kinase cannot activate the regulator protein. Thus, the flagella continue to turn counterclockwise, and the bacterium, instead of tumbling, swims smoothly towards the target (Aegerter, 1997).