Type IV pilus retraction force shapes physiology in early biofilms
Physical forces have been recently implicated in many aspects of biology from metazoan to bacteria. Many bacteria use mechanical forces to form cellular aggregates, crucial for their physiology, called microcolonies. We have used the causative agent of the human disease gonorrhea, Neisseria gonorrhoeae; as a model system to study bacterial microcolonies. The advantage of this system is that cell-cell interactions are controlled by extracellular filaments, called type IV pilus. Those ubiquitous bacterial polymers can undergo cycles of elongations and retractions and thus exert forces on their surroundings. Through experiments and modeling, I was able to demonstrate that the various pilus interactions produce motility gradients in microcolonies potentially establishing a force gradient across the microcolonies. I am interested in testing the biological implications of those motility and force gradients. In order to do so I have measured the level of gene expression of eight pilus-related genes in two backgrounds: WT and a pilus retraction-deficient mutant, ∆pilT. The ∆pilT mutant allowed us to measure physiological response in cells that do not produce retractive force from its pilus. I found that WT microcolonies express pilus-related genes in a heterogeneous fashion, which produce spatiotemporal patterns in the microcolony that are modified in a ∆pilT background. The presence or absence of retraction forces between bacteria also has a profound impact on bacterial physiology: the WT and ∆pilT background do not survive in a classical static biofilm assay at the same rate. Altogether those results point towards a fundamental role for intercellular forces in shaping bacteria physiology. This work aims at laying the grounds for the mechanistic understanding of bacterial multicellular development.