Actin-based motility. 3D model made from optical sections of Shigella sp. bacteria (pink) infecting cells. Filaments of the protein actin, which is part of the cell's cystoskeleton, have been stained green. The bacteria use the actin to help them infect other cells. They initiate the growth of actin filaments that push them against the cell membrane. This causes the cell membrane to grow outwards and allows the bacteria to infect other cells without passing into the extracellular space. Shigella sp. are Gram-negative rod-shaped bacteria. They infect the large intestine, causing dysentery, which can vary in severity from a mild attack of diarrhoea to an acute infection.