Staphylococcus aureus bacteria. Coloured transmission electron micrograph (TEM) of a Staphylococcus aureus bacterium (round) in skin tissue. S. aureus is a Gram-positive coccus (round) bacteria that is 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. Certain strains cause the potentially fatal toxic shock syndrome (TSS).