Researchers from the University of Montreal have discovered that within colonies certain individuals commit suicide when the conditions deteriorate. By indicating to the others that it is time to flee, they save the group.
by Eva Desvigne-Hansch
When you think of a bacteria, you imagine an isolated cell, without interaction with its fellows, mostly dangerous. It is rarely associated with words like “collaboration” or “altruism”. However, these fascinating microorganisms often live within large colonies fixed to a highly organized surface and places of close collaboration between individuals. These clusters of bacteria, called “biofilms”, generally harmless, even beneficial, are omnipresent in our environment: on crusts of cheese, on our teeth, in our digestive tract … This way of life allows indeed exchanges of nutrients between bacteria and guarantees greater resistance to external aggressions.
A team of researchers from the University of Montreal Panted in the case of the Caulobacter Crescentus bacterial strain. These crescent -shaped bacteria live in biofilms in lakes and rivers. They have the distinction of having a dimorphic cell cycle, that is to say that they exist in two forms.
Each cell division produces a fixed cell and a swarming mobile cell. The fixed cell is the replica of the mother cell and has a rod which ends with a crampon, solidly fixed to the surface on which the colony rests. The mobile cell has a flagellum – a long locomotor filament emerging from the bacteria – which allows it to move. This second cell will swim freely until the flagellum against the crampon rod, which will then be attached to the nearest surface. It in turn takes the form of a fixed cell and can start a new cell division cycle. Once the crampon is fixed, the metamorphosis is final, the bacteria can no longer detach or find the swarming form.
In biofilms, the mobile cell remains in general within the group, where it quickly adopts its final form. However, by maturing, these biofilms can become overcrowded, and nutrients, especially oxygen, come to fail, causing the death of the colony. In this case, or during a change in environmental conditions, it becomes necessary for bacteria to be able to go into exile and colonize new more hospital environments.
a counter-intuitive mechanism
But how do they know it’s time to leave the ship? Quebec researchers have noticed that when oxygen fails in the colonies, there are large amounts of extracellular DNA. It comes from the dead cells having released their DNA, which is then free in the biofilm. Finding this molecule in large quantities is therefore indicator of a significant rate of bacterial death. DNA prevents fixing of crampons; The swarming cells will then continue to wander to leave the colony and find a virgin location to hang on. It has been shown that the more the concentration of free DNA increases in the biofilm, the greater the dispersion of bacteria. Thus, the less oxygen, the more the bacteria die and the more young cells flee.
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