A 5-year-old boy from an urban slum in Delhi presents with watery diarrhea and mild abdominal discomfort for 2 days. His mother reports he attends a crowded daycare center. On examination, he is afebrile with mild dehydration. Stool culture yields non-motile, non-lactose-fermenting colonies. The isolate ferments glucose with acid and gas production, is catalase-positive, and produces acid from mannitol. Serotyping identifies it as Shigella flexneri. Which virulence factor is MOST important for the pathogenesis of this infection?