Staphylococcus bacteria, coloured scanning electron micrograph (SEM). Staphylococcus is a genus of Gram-positive, non-motile, spherical bacteria. They are part of the normal flora of human skin and mucous membranes. The bacteria are usually harmless, but infections can occur on broken skin or within a blocked sweat or sebaceous gland, resulting in boils, pustules and abscesses. Some species, such as S. aureus, can cause food poisoning, producing toxins that induce vomiting. Some strains have become resistant to many commonly prescribed antibiotics. Magnification: x4000 when printed at 10 centimetres wide.