Pathogen-induced remodelling of the host cell actin cytoskeleton drives internalization of invasive Salmon ella by non-phagocytic intestinal epithelial cells. Two Salmonella actin-binding proteins are involved in internalization: SipC is essential for the process, while SipA enhances its efficiency. Using purified SipC and SipA proteins in in vitro assays of actin dynamics and F-actin bundling, we demonstrate that SipA stimulates substantially SipC-mediated nucleation of actin polymerization. SipA additionally enhances SipC- mediated F-actin bundling, and SipC–SipA collaboration generates stable networks of F-actin bundles. The data show that bacterial SipC and SipA cooperate to direct efficient modulation of actin dynamics, independently of host cell proteins. The ability of SipA to enhance SipC-induced reorganization of the actin cytoskeleton in vivo was confirmed using semi- permeabilized cultured mammalian cells.