SCIENCE PHOTO LIBRARY/EAST NEWS Staphylococcus aureus. Coloured Scanning Electron Micrograph (SEM) of a cluster of Staphylococcus aureus bacteria. They cause skin infections. These Gram-positive bacteria often grow in grape-like clusters of small spheres (cocci). S. aureus is extremely common in humans, often living harm- lessly on the skin and inside the nose, throat and large intestine. If S. aureus multiplies inside blocked pores in the skin, it causes pus to build up forming an acne spot or a boil. Inside the body this bacteria may also cause abscesses and most types of suppurative infection. Treatment is with antibiotics if the infection is serious.