Staphylococcus aureus bacteria. Coloured scanning electron micrograph (SEM) of Staphylococcus aureus bacteria (yellow). 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).