Quantifying the Formation and Dissolution of Multilayer Regions in the Expansion of Twitching Bacterial Colonies

Type IV pili (T4P) are very thin (5-8 nm in diameter) protein filaments that can be extended and retracted by certain classes of Gram-negative bacteria including P. aeruginosa [1]. These bacteria use T4P to move across viscous interfaces, referred to as twitching motility. Twitching can occur for isolated cells or in a collective manner [2]. In collective motion, the advancing front of an expanding colony consists of finger-like protrusions consisting of many aligned bacteria, with 5 to 30 cells across the fingers, followed by cells that twitch within a lattice-like pattern. During the outward radial motion of the fingers along an agar-glass interface, cells can vertically displace the agar to form multilayer regions. Using our custom-built, temperature- and humidity-controlled environmental chamber, we have studied the formation and dissolution of multilayer regions within fingers for a range of agar concentrations. We find that there is a minimum finger width required for the stability of multilayer regions.

[1] Burrows, L.L. (2012) Annu. Rev. Microbiol. 66: 493–520.
[2] Semmler, A.B., Whitchurch, C.B., Mattick, J.S. (1999) Microbiology 145: 2863-2873.