Staphylococcus: A group of bacteria that cause a multitude of diseases. Under a microscope, staphylococcus bacteria are round and bunched together. They can cause illness directly by infection or indirectly through products they make, such as the toxins responsible for food poisoning and toxic shock syndrome. The best-known member of the staphylococcus family is Staphylococcus aureus. A drug resistant form of Staphylococcus aureus is called MRSA (methicillin resistant Staph. aureus). Staphylococci are the main culprits in hospital-acquired infections, and they cause thousands of deaths every year. Also known as staph.