Form and structure of a bacterial colony - a question of interaction

When bacteria operate successive cell divisions, they form mini-colonies with a very variable macroscopic structure, that can be more or less elongated. In a collaboration between the ENS, the University of California, the Grenoble Alpes University and the Institut Pasteur, LPS researchers have shown that this complex structure is determined by the anisotropy of the interaction between bacteria and their environmental substrate.

During cell division, a rod-shaped bacterium elongates along its main axis and ends up dividing perpendicularly to this axis. Surprisingly, these bacteria form round colonies when they grow on solid substrates while their aspect ratio and stiffness should cause them to align and thus generate elongated colonies reflecting the anisotropy of the elements constituting it. This question of the nematic order in bacterial communities is particularly critical according to the ecological contexts in which they evolve. Indeed, if the elongated colonies have a large surface of interaction with the environment, the round colonies will rather minimize it. In a rich environment, it is more advantageous for individuals to be in direct contact with the environment, whereas they will tend to protect themself by being surrounded with neighbors in a hostile environment.

LPS researchers conducted a series of experiments to measure how bacteria interact mechanically with their substrate. They have shown that substrate adhesion is a determining parameter in the shape of colonies. Moreover, these adhesion forces are polar because they are not uniformly distributed on the bacterium: after a division, the end of the bacterium which did not appear with the division (or pole A of FIG. 1a) adheres more to the substrate than the new end. This asymmetry is accentuated, and the forces of adhesion of the colony are concentrated in certain discrete points.
This asymmetry is at the origin of the structure of the colony: the stronger it is, the less the colony is elongated, undergoing buckling instabilities all the more frequent (see Figure 1b).
This also explains the transition from a single layer of cells to a 3D distribution. A second layer of bacteria is formed as soon as the energy that a bacterium has to spend to move all its neighbors in the plane exceeds the energy needed to deform the gel above it. This occurs preferentially in the center of the colony, since it is the place where the bacteria have to grow the largest number of cells and the cost to pay is maximum. This transition has been modeled theoretically and numerically.

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This study, published in the journal Nature Communications, will help to better understand how this asymmetry is generated at the cellular level, and try to disrupt it in order to be able to more effectively expose bacteria to antibiotic treatments.

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