Most pathogenic microorganisms have evolved methods of resisting phagocytosis. For example, grup A streptococci have cell surface structures called M proteins of which there are now more than 120 antigenically distinct molecules that inhibit direct phagocytosis, mainly by preventing deposition of complement on the organism. Another example is the pneumococcal polysaccharide capsule of which there are thirty or forty distinct polysaccharides. Another approach (taken by both group A streptococci and staphylococci) is the release of potent extracellular toxins, which kill phagocytes with the formation of pus. An intriguing bacterium, mycobacterium tuberculosis, can be ingested by phagocytes but resists intracellular killing, often persisting for years in the macrophage.