This animation shows how pathogenic Salmonella bacteria invade intestinal cells and evade the immune system.

Pathogenic strains of Salmonella can cause diseases, such as typhoid fever and diarrhea, in humans. The animation illustrates how a pathogenic Salmonella bacterium invades and survives inside a human intestinal epithelial cell.

As shown, the bacterium uses a syringe-like mechanism, called the Type III secretion system, to inject bacterial proteins into the host cell. These proteins cause the host cell to take in the bacterium in a vacuole. The bacterium releases more proteins to alter the structure of the vacuole, which protects it against lysosomes. Now safe from the immune system, the bacterium can multiply and infect other cells.

This animation is a clip from a 1999 Holiday Lecture Series, 2000 and Beyond: Confronting the Microbe Menace. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different features.