Transmission electron micrograph (TEM) of an infected Tetrahymena pyriformis protozoa reveals in cross section, a chain of intracellular Legionella pneumophila bacteria. The bacteria are seen multiplying while they are contained inside an intracellular vesicle within the protozoan host cell. Experiments have shown that Legionella pneumophila bacteria can reproduce intracellularly acting as a parasite inside of the ciliated protozoan, Tetrahymena pyriformis, forming serpentine chains Legionella pneumophila.