Staphylococcus (in Greek staphyle means bunch of grapes and coccos means granule) is a genus of gram-positive bacteria. Under the microscope they appear round (cocci), and form in grape-like clusters .

There are many species of staphylococci, most are completely harmless, and reside normally on the skin and mucous membranes of humans and other organisms. They are a small component of soil microbial flora. This genus is found world wide.

Staphylococci can cause a wide variety of diseases in humans either through toxin production or invasion. For example the most common cause of food poisoning is staphylococci toxins. The bacteria grow in improperly stored food, the cooking process kills them but the toxins they produce are heat resistant. Staphylococci can grow in foods with relatively low water activity (such as cheese and salami).

One harmful species is Staphylococcus aureus, which can infect wounds. Of this type, Methicillin-resistant Staphylococcus aureus (MRSA) has recently become a major cause of hospital-acquired infections. These bacteria can survive on dry surfaces increasing the chance of transmission. Staphylococcus aureus is also implicated in Toxic Shock Syndrome, during the 1980s some tampons allowed the rapid growth of S. aureus releasing toxins that were absorbed into the bloodstream.

S. epidermidis, a coagulase-negative staphylococcus species, is a commensal of the skin, but can cause severe infections in immune suppressed patients and those with central venous catheters.