Staphylococcus epidermidis
Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

. It is part of human skin flora
Skin flora
The skin flora are the microorganisms which reside on the skin. Most research has been upon those that reside upon the 2 square metres of human skin. Many of them are bacteria of which there are around 1000 species upon human skin from 19 phyla. The total number of bacteria on an average human has...

(commensal), and consequently part of human flora. It can also be found in the mucous membranes and in animals. Due to contamination, it is probably the most common species found in laboratory tests.

Although S. epidermidis is not usually pathogenic, patients with a compromised immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 are often at risk for developing an infection.