Coloured scanning electron micrograph (SEM) of a neutrophil phagocytosing (engulfing) Staphylococcus aureus bacteria (red). Neutrophils are the most abundant white blood cell and are part of the body's immune system. Staphylococcus aureus are Gram-positive coccus (round) bacteria found on the skin and mucous membranes of humans and many animals. 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. S. aureus can also cause food poisoning, producing toxins that induce vomiting. Some strains have become resistant to many commonly prescribed antibiotics.