Bacteria move cooperatively for many reasons: to invade host tissues,
to seek out nutrients, or to erect structures for spore spreading.
Swarming is a type of cooperative surface-associated motility, in
which each cell is self-propelled by its own flagella. Twitching and
social gliding also depend on a motor driven cellular appendage; type
IV pili are extended toward a surface, adhered, then retracted,
pulling the individual cell forward toward the adhesion. Intriguingly,
many types of collective motility do not require the use of known
motor-driven appendages; these include gliding, adventurous gliding,
sliding, and spreading. The physical mechanisms underlying these kinds
of cooperative motility are not yet clear, yet a clue comes from the
physical chemistry of their excretions; all of these motor-independent
types of motility involve amphiphilic molecules. Here we show that
spatial concentration gradients of these amphiphilic molecules give
rise to surface tension gradients that drive biofilm spreading.
Moreover, we show that polymer production also drives bacterial
biofilm spreading, and is triggered by a gradient of nutrient
depletion within the colony. Thus, it is the spatially heterogeneous
excretion of complex fluids by bacteria that gives rise to these two
types of biofilm spreading force.
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