Why do rod-shaped bacteria form compact microcolonies?
The paper in Nature Communications is here: http://go.nature.com/2G8kVho
When Marie-Cécilia Duvernoy joined our lab, we were completing a study about public goods in bacterial population. We just discovered that in surface attached microcolonies, local trafficking of public goods between neighbouring bacteria save producer cells from exploitation by non-producer cells (Julou et al., PNAS 2013). Soon after she arrived, Marie-Cécilia wondered why rod-shaped bacteria form compact microcolonies whereas one would naively expect them to divide and grow as a chain. The question sounded naive at that time but it turned out to unravel complexities on how individual bacteria adhere to a surface and how it influences the pattern of surface colonization.
We started to focus on the first division and noticed that the two daughter cells slide along one another after cell septation. We tried to find explanations in the literature but only dug out old observations (Shapiro, J. Bacteriol. 1989). So, we decided to take a closer look at isolated cells prior to division. Unexpectedly, we saw that a large fraction of these cells were not elongating around their center of mass. Oh! Our simple assay just unraveled that substrate adhesion was biased towards one pole. Thanks to our collaborators at the Pasteur Institute in Paris, we confirmed with mutants of adhesion that the apparent asymmetric elongation was actually due to substrate adhesion.
But which pole between the old and the new is stickier? To address that, we had to follow at least one division in order to determine the relative age of the poles, while maintaining bacteria isolated. How could we solve this contradictory challenge? Simply by using laser ablation? Yes, but we wanted them to be automatized and we spent quite a long time to figure out which motorized stage is accurate enough to reproducibly position bacteria under the tiny UV diffraction limited spot of our laser. And that's it! We eventually got the explanation to our initial puzzle. Bacteria are pinned at their old pole, so that the only solution for them to keep elongating is to rotate, therefore breaking the line.
And so what? We still had to understand what is the contribution of polar adhesion to the shape of microcolonies. But how could we measure the forces developed by growing microcolonies? We heard about Traction Force Microcopy (TFM) that was initially developed for eukaryotic cells. Yet, we initially thought that for bacteria we could use something simpler. So, we decided to put beads in agar. First experiments rapidly showed that this technique was a mess, but we stayed narrow-minded and kept trying to redo what others have tried and gave up before us. Eventually, and thanks to the tenacity of Marie-Cecilia and our collaborators at the LiPhy in Grenoble, we adapted TFM in the lab to directly measure the forces developed by growing microcolonies on a substrate. And, we confirmed that bacteria adhere in discrete adhesion foci that are transient.
At this point, you might think that the story is finished, but biophysicists always want to confront their understanding to a mathematical model. Just an extra year! But, now with our model you can estimate adhesion forces out of a simple experiment (Duvernoy et al., Nat. Commun. 2018)...