  We study the migration of chemotactic wild-type Escherichia coli populations
in semisolid (soft) agar in the concentration range C = 0.15-0.5% (w/v). For C
< 0.35%, expanding bacterial colonies display characteristic chemotactic rings.
At C = 0.35%, however, bacteria migrate as broad circular bands rather than
sharp rings. These are growth/diffusion waves arising because of suppression of
chemotaxis by the agar and have not been previously reported experimentally to
our knowledge. For C = 0.4-0.5%, expanding colonies do not span the depth of
the agar and develop pronounced front instabilities. The migration front speed
is weakly dependent on agar concentration at C < 0.25%, but decreases sharply
above this value. We discuss these observations in terms of an extended
Keller-Segel model for which we derived novel transport parameter expressions
accounting for perturbations of the chemotactic response by collisions with the
agar. The model makes it possible to fit the observed front speed decay in the
range C = 0.15-0.35%, and its solutions qualitatively reproduce the observed
transition from chemotactic to growth/diffusion bands. We discuss the
implications of our results for the study of bacteria in porous media and for
the design of improved bacteriological chemotaxis assays.
