A new form of actin assembly around the Shigella-containing vacuole regulates its intracellular niche

The enteroinvasive bacterium Shigella flexneri forces its uptake into non-phagocytic host cells through the translocation of T3SS effectors that subvert the actin cytoskeleton. Here, we report de novo actin polymerization after cellular entry around the bacterial containing vacuole (BCV) leading to the formation of a dynamic actin cocoon. This cocoon is thicker than any described cellular actin structure and functions as a gatekeeper for the cytosolic access of the pathogen. Host Cdc42, Toca-1, N-WASP, WIP, the Arp2/3 complex, cortactin, coronin, and cofilin are recruited to the actin cocoon. They are subverted by T3SS effectors, such as IpgD, IpgB1, and IcsB. IcsB immobilizes components of the actin polymerization machinery at the BCV. This represents a novel microbial subversion strategy through localized entrapment of host actin regulators causing massive actin assembly. We propose that the cocoon protects the Shigella niche from canonical maturation or host recognition.