How to move through a very dense crowd

Dale Kaiser, Stanford University

Abstract

Many different bacteria are able to spread rapidly over the surface on which they are growing, using a movement strategy known as swarming. When cells cover a surface at high density and growth is limited by competition between cells for nutrients, that strategy permits them to continue rapid growth, especially at the colony edge. Bacterial swarming can be propelled by rotating screw-like flagella, by pulling with type IV pili, by pushing with the secretion of slime from the rear, or by using compatible pairs of these engines. Although swarming with flagella has been investigated for many years, and much has been learned of its regulation, the choreography of movement has remained elusive. Myxococcus xanthus, one of the myxobacteria, expands its swarms at one third the speed of individual cell gliding over the same surface. Its cells have pilus engines at their front end that pull by retracting the pili, and slime secreting engines at their rear. Swarming, which is the bacterial equivalent of being able to dance toward an exit in a crowd of moving bodies that usually are in contact with each other, is found to obey rules of highway traffic flow. Computational simulation shows that some of the cell patterns observed in swarming can be explained by the regulation and mechanical properties of the two engines.

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