Verticalization Of Bacterial Biofilms

Biofilms are communities of bacteria adhered to surfaces. Recently, biofilms
of rod-shaped bacteria were observed at single-cell resolution and shown to
develop from a disordered, two-dimensional layer of founder cells into a
three-dimensional structure with a vertically aligned core. Here, we
elucidate the physical mechanism underpinning this transition using a
combination of agent-based and continuum modelling. We find that
verticalization proceeds through a series of localized mechanical
instabilities on the cellular scale. For short cells, these instabilities
are primarily triggered by cell division, whereas long cells are more likely
to be peeled off the surface by nearby vertical cells, creating an 'inverse
domino effect'. The interplay between cell growth and cell verticalization
gives rise to an exotic mechanical state in which the effective surface
pressure becomes constant throughout the growing core of the biofilm surface
layer. This dynamical isobaricity determines the expansion speed of a
biofilm cluster and thereby governs how cells access the third dimension. In
particular, theory predicts that a longer average cell length yields more
rapidly expanding, flatter biofilms. We experimentally show that such
changes in biofilm development occur by exploiting chemicals that modulate
cell length.
 

Event resources