For a long time, the ability to communicate has been considered as an exclusive property of multicellular "higher" organisms. However, research over the past few decades has raised serious doubts about this point of view. Complex behaviours such as chemotaxis, quorum sensing and biofilm formation show that bacteria can communicate with the environment, within the species and with other species (O'Toole et al 2000; Miller and Bassler 2001 for reviews). Apparently bacteria use physical signals as part of sophisticated signalling systems that function over distances that are substantially larger than cellular dimensions (which are of the order of one to a few \mu m).