Staphylococcus epidermidis

Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus. It is part of our skin flora and can also be found in the mucous membranes and in animals. It is the most common species found in laboratory test due to contamination.

It is not usually pathogenic; however, patients with a compromised immune system often risk infection. Infections can be both nosocomial and community acquired and are more of a threat to hospital patients. Hospitals carry more virulent strains of the organism due to the constant use of antibiotics and disinfectants. It is also a major concern for people with catheters or other surgical implants due to it causing biofilms that grow on these devices.

Friedrich Julius Rosenbach found the differences between S. epidermidis from S. aureus in 1884. It is a hard microorganism consisting of non-motile Gram-positive cocci, arranged in grape-like clusters.

It forms white raised colonies about 1-2 millimeters in diameter. It is catalase-positive, coagulase negative, facultative anaerobe that can grow by aerobic respiration or by fermentation. Tests indicate this microorganism also carries out a weakly positive reaction to the nitrate reduction test. It is sensitive to novobiocin, providing an important test to distinguish it from Staphylococcus saprophyticus, which is coagulase-negative as well but novobiocin-resistant.

The cell walls have a transferrin binding protein that helps the organism obtain iron from transferrin. The ability to form biofilms on plastic devices is a major virulence factor for S. epidermidis. Surface proteins bind blood and extracellular matrix proteins. These biofilms decrease the metabolic activity of bacteria with them. S. epidermidis strains are often resistant to antibiotics including penicillin, amoxicillin, and methicillin. Resistant organisms are most commonly found in the intestine although organisms on the skin can become resistant to routine exposure to antibiotics secreted in sweat.

The biofilm that grows on plastic devices most often occurs on catheters and on medical prostheses although infection can contaminate any implanted plastic device. Another disease it causes is Endocarditis which occurs most often in patients with defective heart valves.

Antibiotics are mostly ineffective in clearing biofilms thus the most common treatment is to remove or replace the plastic implant. Prevention is ideal in all cases in which vancomycin is the most common drug used to do this. Generally Baird-Parker Agar with egg yolk supplement is used to detect S. epidermidis. Colonies can be confirmed using the coagulase test.