Staphylococcus bacteria. Cultured colonies of Staphylococcus aureus bacteria in a petri dish. S. aureus is a Gram-positive, non- motile, spherical bacterium associated with suppurative infections in humans, such as boils and abscesses, and with respiratory infections, such as pneumonia. It produces toxins responsible for food poisoning. These bacteria have been cultured on Chapman medium, which is selective for Staphylococcus bacteria. Selective media are used to check for the presence of specific bacteria.