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There are more than 20 species of Staphylococci bacteria, although only three are responsible for most cases of human infection. The most common is Staphylococcus aureus (S. aureus). S. aureus are typically more harmful and more likely to cause disease than other types of staph bacteria. S. aureus are a common cause of serious hospital-acquired staph infections such as pneumonia and bloodstream infections (bacteremia). Methicillin-resistant Staphylococcus aureus (MRSA) is a well-known variation of this species.

Staphylococcus epidermidis (S. epidermis) is the type of staph bacteria that commonly colonize the skin of people without causing infection. Infection by these bacteria is most likely to occur after skin injury or among immunosuppressed people.

Staphylococcus saprophyticus (S. saprophyticus) is believed to cause 10 percent of urinary tract infections in otherwise healthy young women.

Additional staph bacteria that less commonly affect humans include S. haemolyticus, S. hominis, S. warnerii, S. sacharolyticus and S. cohnii.