Staphylococcus epidermidis is a Gram-positive bacterium. It is part of the normal human flora, typically the skin flora, and less commonly the mucosal flora. Although S. epidermidis is not usually pathogenic, patients with compromised immune systems are at risk of developing infection. These infections are generally nosocomial. S. epidermidis is a particular concern for people with catheters or other surgical implants because it is known to form biofilms that grow on these devices. It also causes endocarditis, most often in patients with defective heart valves.