When we think of a bacterium, we imagine an isolated cell, without interaction with its congeners, most of the time dangerous. It is rarely associated with words like “collaboration” or “altruism”. Yet these exciting microorganisms live often within large colonies fixed to a surface, highly organized and places of close collaboration between individuals. These clusters of bacteria, called “biofilms”, generally harmless, even beneficial, are omnipresent in our environment: on cheese crusts, on our teeth, in our digestive tract… This way of life allows the exchange of nutrients between bacteria and guarantees greater resistance to external aggressions.
A team of researchers from the University of Montreal looked into the case of the bacterial strain Caulobacter crescentus. These crescent-shaped bacteria live in biofilms in lakes and streams. They have the particularity 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 parent cell and sports a stalk that ends in a holdfast, firmly attached to the surface on which the colony rests. The motile cell has a flagellum – a long locomotor filament emerging from the bacterium – which allows him to move. This second cell will swim freely until it swaps the flagellum for the spiked rod, which will then attach itself to the nearest surface. It in turn takes the form of a fixed cell and can begin a new round of cell division. Once the clamp is fixed, the metamorphosis is final, the bacteria can no longer detach or return to the swarming form.
In biofilms, the mobile cell generally remains within the group, where it quickly adopts its final form. However, as these biofilms mature, they can become overcrowded, and nutrients, especially oxygen, run out, causing the colony to die. In this case, or during a change in environmental conditions, it becomes necessary for the bacteria to be able to go into exile and colonize new, more hospitable environments.
A counter-intuitive mechanism
But how do they know it’s time to jump ship? Quebec researchers have noticed that when there is a lack of oxygen in the colonies, large amounts of extracellular DNA are found there. It comes from dead cells that have released their DNA, which then finds itself free in the biofilm. Finding this molecule in large quantities is therefore an indicator of a high rate of bacterial death. The DNA prevents the fixing of the crampons; the swarming cells will then continue to wander until they emerge from the colony and find a blank spot to cling to. It has been shown that the more the concentration of free DNA increases in the biofilm, the greater the dispersion of the bacteria will be. Thus, the less oxygen there is, the more the bacteria die and the more the young cells leak.
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