From Microbes to Cell Giants - Animate Earth: Science, Intuition and Gaia - page 134

ganise themselves into a vast, extracellular self-making network of which Gaia is the
final expression.
In order to find new sources of food and avoid danger, some bacteria use an 'organ'
known as the flagellum, a kind of whip which is used to move the cell around ( Figure
32 ) . A bacterial cell may have up to 400 individual flagella which reach out into the
environment from every part of its cell membrane. When a bacterium such as Salmon-
ella finds itself in a situation where a concentrated food source (say a rotting mouse in
the soil) is leaching nutrients, receptors on its cell membrane detect the food molecules
and instruct the flagella to rotate counter-clockwise. The flagella respond by tying them-
selves up into a single super-flagellum that beats in unison, causing the microbe to move
in a straight line towards the food source. There is purposeful behaviour here—as the
bacterium moves it keeps tabs on how many food molecules lock with its receptors, and
movement towards the food continues if the number of 'hits' keeps increasing. Once the
number of hits levels off the flagella rotate clockwise, causing them to separate so that
each one beats to a different rhythm and at a different point along the cell membrane.
The microbe then moves about randomly and with luck stays close to the source of food.
In fact, both kinds of movement take place all the time, but straight-line movement dom-
inates when the microbe is heading for food, with random movement happening most
often when it is near food, or when an adverse situation is being avoided.
Figure 32: Pseudomonas sp., each with two composite flagella.( photo © Dr Linda Stannard,
UCT / Science Photo Library )