  Flocks of birds, schools of fish, mixtures of motors and cytoskeletal
filaments, swimming bacteria and driven granular media are systems of
interacting motile units that exhibit collective behaviour. These can all be
described as active matter systems, since each individual unit takes energy
from an internal energy depot and transduces it into work performed on the
environment. We review a model for cyanobacterial phototaxis, emphasising the
differences from other models for collective behaviour in active matter
systems. The interactions between individual cells during phototaxis are
dominated by mechanical forces mediated by their physical attachments through
type IV pili (T4P) and through the production of "slime", a complex mixture of
non-diffusible polysaccharides deposited by cells that acts to decrease
friction locally. The slime, in particular, adds a component to the interaction
that is local in space but non-local in time, perhaps most comparable to the
pheromones laid down in ant trails. Our results suggest that the time-delayed
component of the interactions between bacteria qualify their description as a
novel active system, which we refer to as "damp" active matter.
