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Wetting Dynamics and Mass Swarming of Bacillus Subtilis over Soft Gel Substrates
Bacillus subtilis forms dendric colonies on gel surfaces through the collective motion of groups of 10^4 to 10^5 cells which form the compact tips of elongating dendrites. We investigate some physical aspects of this phenomenon, for which humidity and the surfactant production by the bacteria play as much of a role as does the motility of the hyperflagellated swarmer cells. I will present observations at different spatial and temporal scales which yield a wealth of information that cannot be simply inferred from the final colony shape. In particular we can learn which physical interactions between bacteria and the gelose substrate regulate the dynamics of mass swarming. Finally I will discuss a novel mode of locomotion, tentatively termed "mass sliding" or "colony sliding", which we discovered with a modified experimental protocol.