Epithelial cells are polarized, with their apical surface oriented toward the lumen of an organ and their basolateral side facing the blood. Layers of epithelial cells play an important and essential role in keeping the outside out and the inside in. Helping them to achieve this are the so-called tight junctions, which link neighboring cells and resist any intercellular infiltration of macromolecules. Kierbel et al. have studied how the pathogenic bacterium Pseudomonas aeruginosa (a major contributor to nosocomial infections) manages to invade epithelia, despite having a preference for binding to the basolateral surfaces of epithelial cells. Most of the invading bacteria will, in fact, be confronted by an apical surface when they attempt to colonize a new host. The authors find that the bacterium actually induces the remodeling of a small portion of the apical membrane into a basolateral-like protrusion. P. aeruginosa binds to epithelial monolayers and recruits phosphatidylinositol 3-kinase (PI3K). The PI3K generates phosphatidylinositol 3,4,5-trisphosphate (PIP3), which attracts actin filaments and the delivery of basolateral membrane proteins. The invading bacteria then become engulfed by the transplanted patch of basolateral membrane and thus succeed in breaching the barrier defenses of the epithelium. — SMH
J. Cell Biol. 177, 21 (2007).