Staphylococcus bacteria, coloured scanning electron micrograph (SEM). Staphylococcus sp. bacteria are Gram positive cocci (spherical) bacteria that often form clusters. A number of Staphylococcus species are found on the skin and mucous membranes without causing disease. However, some species, such as S. aureus, can cause disease in immunocompromised individuals, especially if they enter open wounds. Some strains of S. aureus, which can cause blood poisoning, pneumonia and boils, have become resistant to the antibiotics commonly used to treat them. Magnification: x4000 when printed at 10 centimetres wide.