Introduction: Previously we have shown that human lactoferrin decreases significantly the invasiveness of S. flexneri in a HeLa cell model. A unique aspect of Shigella invasion is that the bacteria induces its own uptake by mammalian cells that are not normally phagocytic (bacterium-induced phagocytosis). The earliest step in this process is induction of pseudopods projecting from the HeLa cell surface, a process that can be seen within minutes of exposure of HeLa cells to Shigella.

Objective: We sought to determine whether lactoferrin affected the first steps in the interaction of HeLa cells and S. flexneri.

Methods: S. flexneri 5 M90T, a virulent strain was used. Invasion was evaluated in the classic HeLa cell model and studied by light and transmission electron microscopy, immunofluorescence and deconvolved microscopy. Bacteria not exposed to lactoferrin treatment were used as controls.

Results: HeLa cells were examined 30 minutes after S. flexneri 5 M90T was added to the monolayer to detect early changes. Pseudopods develop on 100% of HeLa cells exposed to S. flexneri 5 M90T; in contrast, Shigella briefly exposed to human recombinant lactoferrin (10 mg/mL, 60 minutes at 37C) were unable to induce the formation of these projections (0% of HeLa cells).

Conclusions: Brief exposure of Shigella to lactoferrin impairs release of the bacterial signal that triggers development of pseudopods on the surface of HeLa cells. Since the formation of these membrane projections is critical to bacterium-induced phagocytosis, failure to induce these cell membrane changes may account in part for impaired invasion of HeLa cells by lactoferrin treated Shigella. The mechanism by which lactoferrin impairs shigella invasiveness may be relevant to other enteropathogens that share similar virulence strategies.