cProteus mirabilis is an opportunistic pathogen that is frequently associated with urinary tract infections. In the lab, P. mirabilis cells become long and multinucleate and increase their number of flagella as they colonize agar surfaces during swarming. Swarming has been implicated in pathogenesis; however, it is unclear how energetically costly changes in P. mirabilis cell morphology translate into an advantage for adapting to environmental changes. We investigated two morphological changes that occur during swarming-increases in cell length and flagellum density-and discovered that an increase in the surface density of flagella enabled cells to translate rapidly through fluids of increasing viscosity; in contrast, cell length had a small effect on motility. We found that swarm cells had a surface density of flagella that was ϳ5 times larger than that of vegetative cells and were motile in fluids with a viscosity that inhibits vegetative cell motility. To test the relationship between flagellum density and velocity, we overexpressed FlhD 4 C 2 , the master regulator of the flagellar operon, in vegetative cells of P. mirabilis and found that increased flagellum density produced an increase in cell velocity. Our results establish a relationship between P. mirabilis flagellum density and cell motility in viscous environments that may be relevant to its adaptation during the infection of mammalian urinary tracts and movement in contact with indwelling catheters.
Proteus mirabilis is a Gram-negative rod-shaped gammaproteobacterium that is commonly associated with urinary tract infections (1) and the biofouling of catheters (2-4). P. mirabilis may also be present in the human gut microflora (5) and is correlated with the incidence of colitis (6, 7). Broth-grown vegetative cells of P. mirabilis are characteristically ϳ2 m long and have a peritrichous distribution of ϳ4 to 10 flagella. The flagella form a bundle that performs work on the surrounding fluid and propels cells forward via a mechanism that is similar to the motility system of Escherichia coli (8, 9).Broth-grown vegetative cells of P. mirabilis in contact with the surface of agar gels infused with nutrients change their morphology, become "swarmers," and colonize the surface by coordinating the movement of large groups of cells (i.e., "swarming") (see Fig. 1A). P. mirabilis swarm colonies exhibit a terraced pattern of concentric rings (see Fig. S1 in the supplemental material) (10). These rings are produced by alternating phases of consolidation, during which the colony does not expand and cells are dedifferentiated into a vegetative cell-like morphology, and swarming, during which cells are motile and differentiated (11). Motility occurs predominantly at the swarm front and decreases with increasing distance from the front; cells near the center of the swarm are nonmotile. Swarming has several characteristics, including the following: (i) the inhibition of cell division to produce long (10-to 70-m) multinucleate cells, (ii) an increase in the surface dens...