Type IV Pilin Post-Translational Modifications Modulate Material Properties of Bacterial Colonies

Bacterial type 4 pili (T4P) are extracellular polymers that initiate the formation of microcolonies and biofilms. T4P continuously elongate and retract. These pilus dynamics crucially affect the local order, shape, and fluidity of microcolonies. The major pilin subunit of the T4P bears multiple post-translational modifications. By interfering with different steps of the pilin glycosylation and phosphoform modification pathways, we investigated the effect of pilin post-translational modification on the shape and dynamics of microcolonies formed by Neisseria gonorrhoeae. Deleting the phosphotransferase responsible for phosphoethanolamine modification at residue serine 68 inhibits shape relaxations of microcolonies after perturbation and causes bacteria carrying the phosphoform modification to segregate to the surface of mixed colonies. We relate these mesoscopic phenotypes to increased attractive forces generated by T4P between cells. Moreover, by deleting genes responsible for the pilin glycan structure, we show that the number of saccharides attached at residue serine 63 affects the ratio between surface tension and viscosity and cause sorting between bacteria carrying different pilin glycoforms. We conclude that different pilin post-translational modifications moderately affect the attractive forces between bacteria but have severe effects on the material properties of microcolonies.