  Microbial colonies cultured on agar Petri dishes have become a model system
to study biological evolution in populations expanding in space. Processes such
as clonal segregation and gene surfing have been shown to be affected by
interactions between microbial cells and their environment. In this work we
investigate the role of mechanical interactions such as cell-surface adhesion.
We compare two strains of the bacterium E. coli: a wild-type strain and a
"shaved" strain that adheres less to agar. We show that the shaved strain has a
selective advantage over the wild type: although both strains grow with the
same rate in liquid media, the shaved strain produces colonies that expand
faster on agar. This allows the shaved strain outgrow the wild type when both
strains compete for space. We hypothesise that, in contrast to a more common
scenario in which selective advantage results from increased growth rate, the
higher fitness of the shaved strain is caused by reduced adhesion and friction
with the agar surface.
