Staphylococcus aureus. Coloured scanning electron micrograph (SEM) of Staphylococcus aureus bacteria. These Gram-positive bacteria cause skin infections and often grow in these 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. Treatment is with antibiotics if the infection is serious. Variants of S. aureus (MRSA) have developed resistance to many antibiotics. Magnification: x6000 when printed at 10 centimetres wide.