Type III secretion systems translocate proteins directly from the bacteria into the cytoplasm of eukaryotic cells. (A) Schematic representation of translocation of bacterial proteins by the type III secretion apparatus. (B) Isolated Salmonella "needle complexes" visualized by transmission electron microscopy. "Needle complexes" represent the components of the type III secretion apparatus that span the bacterial envelope. (C) Needle complexes (arrows) visualized within the bacterial envelope of osmotically-shocked Salmonella typhimurium. (Figure courtesy of Tyler Kimbrough.)