Staphylococcus aureus. Coloured Scanning Electron Micrograph (SEM) of Staphylococcus aureus bacteria. These Gram-positive bacteria cause skin infections and often grow in grape-like clusters of small spheres (cocci). S. aureus is extremely common in humans, living harmlessly 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. Magnification: x11,200 at 6x4.5cm size. Magnification; x12,000 at 5x7cm size.