Stress fiber-based movement of Shigella flexneri within cells

icsA (virG), a gene located on pWR100, the virulence plasmid of Shigella flexneri serotype 5 (M90T), encodes a 120-kDa outer membrane protein. This protein promotes a random intracellular movement of the bacteria and leads to the infection of adjacent cells by the formation of protrusions. This movement, which involves the nucleation, polymerization, and subsequent polarization of actin, is referred to as the Ics phenotype (intra/intercellular spread). Here we present evidence that a second, distinct form of locomotion is also elaborated by S. flexneri in chicken embryo fibroblasts in which the Ics phenotype is not expressed. Using a combination of phase-contrast microcinematography and confocal microscopy, we have demonstrated that bacteria adopt parallel orientations by interacting with stress fibers. This interaction subsequently results in bacterial movement along the stress fibers themselves. This phenomenon occurs independently of the presence of a phagocytic vacuole which is lysed shortly after entry of the bacteria into the cell. It is expressed by M90T and SC560, its icsA mutant. This movement has been termed organelle-like movement (Olm phenotype) and is thought to account for the early accumulation of bacteria seen near the nucleus.