FIG 1.

A nonmotile strain inhibits A-motility swarm expansion of a related strain and depletes motile cells from the swarm edge by a Tra-dependent mechanism. (A) Nonmotile strain DK8601 was mixed 1:1 with the indicated isogenic A-motile strains DK8615 (ΔpilQ) and DW1415 (ΔpilQ traA::km) and incubated for 72 h. Bar, 1 mm. (B) Phase-contrast micrographs of the same tra⁺ strain mixture 24 h after mixing. Top panel, cells (arrow) that have migrated out from the inoculum edge (red dashed line); bottom panel, the identical field 24 h later revealing that most motile cells at the swarm fringe have disappeared (arrow), although slime trails remain. Bar, 100 μm. The dashed arrow at the top of panel B shows the direction of swarm expansion.