Animation of a colony of Staphylococcus bacteria. These are spherical gram-positive bacteria that form an important part of the microbiome of humans and other animals, particularly on the skin and in the upper respiratory tract. In some cases they can cause opportunistic infections, such as abscesses, sinusitis and food poisoning. The strain methicillin-resistant Staphylococcus aureus (MRSA) is an important drug-resistant pathogen (or "superbug"). It causes infections particularly in hospitals, prisons and other enclosed environments. It is difficult to treat with antibiotics and infections can be fatal.