The growth of bacterial colonies is a very ubiquitous phenomenon occurring
in nature and observed in the laboratories. However, there is a limited knowledge
on how at the microscopic level these colonies develop and the individual cells
spatially organize.
In this thesis, we experimentally investigate the physics of growing microcolonies
at the level of the individual Escherichia coli (E. coli ) cells, focussing
on the order-disorder evolution and the understanding of it in the context of
active matter. Bacterial cells are indeed constantly transducing energy from the
environment to move and grow, therefore they behave as active microscopic units,
defining an inherently far from equilibrium system.
In Part I, we present a brief summary of passive liquid crystals that provide
us with some basic concepts and tools for investigating the bacterial microcolony
evolution. Then an overview of the biology of E. coli cell is given, both as part
of multicellular structures (biofilm) and as individuals. Active matter is then
discussed along with some examples of active nematics. This first part ends with
the materials and methods used in the experiments and analysis.
In Part II, we provide our experimental results on the study of growing
bacterial microcolonies as active nematics. We describe the way a bacterial
microcolony evolves from the first mother cell into a layer of hundreds of
cells, and we study the global and local orientational order. We find that
a transition from an anisotropic to an isotropic phase occurs as the colony
increases and that instabilities and topological defects develop, in analogy to
the case of an active nematic. We also compare the real system with simulated
ones by investigating (i ) the case of equilibrated configurations with respect to
experimental nonequilibrium ones, and (ii ) long-time behaviour of nonequilibrium
analogues.
In Part III, we discuss the buckling of bacterial microcolonies, that is, the
transition from a 2D layer of cells to a 3D structure. We investigate the link
between the buckling event and the presence of topological defects in the nematic
map of the bacterial microcolony, finding that the buckling sites tend to be closer
to topological defects with a negative charge. Later, we present some results of
buckling in microcolonies composed of mutants lacking some appendages that
play a role in the motion in and attachment to the surrounding environment,
finding that buckling occurs at earlier times in the case of these mutants than
the wild type.
The aim of this work is to show that a growing bacterial microcolony is an
interesting model of active matter to experiment on, and that the investigation
tools developed for the study of liquid crystals can be useful for describing the
evolution of these living systems in mechanistic terms, and for improving the
current understanding of nonequilibrium phenomena
