F. Ross Hallett Memorial Graduate Scholarship Presentation and Seminar - 2019

Date and Time


MACN 222


Refreshments: MacNaughton Building Room 217 at 10:00 a.m.
Citation: Dr. John Dutcher
Award Presentation: Mrs. Barbara Hallett

Seminar Presentation:

Quantifying 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]. We have used optical microscopy, together with a custom-built, temperature- and humiditycontrolled environmental chamber, to study the expansion of twitching colonies across an agar-glass interface for a range of agar concentrations 1.0 % w/v < C < 1.9% w/v. The advancing front consists of finger-like protrusions (fingers) consisting of many bacteria, with the cells within the expanding colony arranged in a lattice-like pattern. The fingers consist of aligned bacteria 5 to 30 cells across, which move radially outward across the agar-glass interface. We observed a significant decrease in edge speed and a corresponding transition from monolayer to multilayer coverage within the fingers at an agar concentration of 1.6% w/v. We have studied this transition by characterizing multilayer formation and dissolution, and transient and stable multilayer regions within fingers. We observed that a minimum finger width is required for multilayer stability, and we have developed a simple nucleation model that describes the dependence of multilayer lifetime on finger width.

[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.

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