Entry of Shigella flexneri into HeLa cells: evidence for directed phagocytosis involving actin polymerization and myosin accumulation.

The enteroinvasive bacterium Shigella flexneri expresses a plasmid-mediated capacity to penetrate into nonphagocytic cells. By using 7-nitrobenz-2-oxa-1,3-diazole-phallacidin (NBD-phallacidin), a fluorescent dye which specifically stains microfilaments, we observed condensations of filamentous actin underneath the plasma membrane of HeLa cells which interacted with the invasive isolate M90T. As demonstrated by indirect immunofluorescence with the antimyosin monoclonal antibody CC-212, myosin accumulated at the same sites. The entry process could be synchronized by using strain SC301, a pIL22 transformant of M90T. pIL22, a recombinant plasmid encoding the Escherichia coli afimbrial adhesin AFA I, rendered shigellae highly adherent to HeLa cells. Using such a system, we demonstrated that the occurrence of bacterial penetration and the appearance of structures brightly stained by NBD-phallacidin were simultaneous events. Such microfilamentous structures resulted from de novo polymerization of the monomeric actin pool in a DNase I inhibition assay, as shown by measurement of the monomeric versus total actin content of infected HeLa cells. These data provide direct evidence that the penetration of S. flexneri into HeLa cells occurs through a mechanism similar to phagocytosis by professional phagocytes.