Staphylococcus aureus

Staphylococcus aureus is a round bacterium, that is, a coccus, and on the Gram stain it holds onto the purple dye, which is why it is called gram-positive. Under the microscope it usually appears in clusters, like grapes, though it also shows up singly, in pairs or in short chains. It does not move, it does not form spores, and it is coagulase positive, meaning it makes an enzyme that can clot plasma.

Much of the time it does nothing at all. It can live as a harmless colonizer on the skin, on the mucous membranes and in the digestive tract, and one of the sites it colonizes most often is the anterior nares, the entrance of the nose. It lives as a tenant in the nose without anyone noticing.

The story changes when the skin breaks. Then it invades the tissue, and it does so with tools of its own: adhesins are proteins on its surface that stick tightly to fibrinogen, fibrin and collagen, the proteins of the wound and of the tissue's supporting framework. It also makes toxins and enzymes that let it hide from the body's defenses.