title: The role of mechanical forces in the planar-to-bulk transition in growing Escherichia coli microcolonies.
abstract: Mechanical forces are obviously important in the assembly of three-dimensional multicellular structures, but their detailed role is often unclear. We have used growing microcolonies of the bacterium Escherichia coli to investigate the role of mechanical forces in the transition from two-dimensional growth (on the interface between a hard surface and a soft agarose pad) to three-dimensional growth (invasion of the agarose). We measure the position within the colony where the invasion transition happens, the cell density within the colony and the colony size at the transition as functions of the concentration of the agarose. We use a phenomenological theory, combined with individual-based computer simulations, to show how mechanical forces acting between the bacterial cells, and between the bacteria and the surrounding matrix, lead to the complex phenomena observed in our experiments-in particular the observation that agarose concentration non-trivially affects the colony size at transition. Matching these approaches leads to a prediction for how the friction between the bacteria and the agarose should vary with agarose concentration. Our experimental conditions mimic numerous clinical and environmental scenarios in which bacteria invade soft matrices, as well as shedding more general light on the transition between two- and three-dimensional growth in multicellular assemblies.
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK matthewaagrant@gmail.com.
authors.forename: Matthew A A
authors.initials: MA
authors.lastname: Grant
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, James Clerk Maxwell Building, King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, UK bwaclaw@googlemail.com.
authors.forename: Bartłomiej
authors.initials: B
authors.lastname: Wacław
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, James Clerk Maxwell Building, King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, UK.
authors.forename: Rosalind J
authors.initials: RJ
authors.lastname: Allen
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Pietro
authors.initials: P
authors.lastname: Cicuta
mesh_terms: D058491:Bacterial Load / Q000379:methods*; D002450:Cell Communication / Q000502:physiology*; D002452:Cell Count; D049109:Cell Proliferation / Q000502:physiology; D003198:Computer Simulation; D004926:Escherichia coli / Q000166:cytology* / Q000254:growth & development* / Q000502:physiology*; D017276:Friction; D040542:Mechanotransduction, Cellular / Q000502:physiology*; D008954:Models, Biological; D033081:Shear Strength / Q000502:physiology; D013314:Stress, Mechanical; D014783:Viscosity
keywords: bacterial biofilm; bacterial microcolony; cell growth; mechanics