Staphylococcus epidermidis

Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus. It is part of human skin flora(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 are often at risk for developing an infection. These infections can be both nosocomial or community acquired, but they pose a greater threat to hospital patients. This phenomenon may be the result of continuous use of antibiotics and disinfectants within hospitals, leading to evolutionary pressure toward more virulent strains of the organism.

S. epidermidis is also a major concern for people with catheters or other surgical implants because it is known to cause biofilms that grow on these devices.

Discovery
Friedrich Julius Rosenbach distinguished S. epidermidis from S. aureus in 1884, initially naming S. epidermidis as S. albus. He chose aureus and albus since the bacteria formed yellow and white colonies, respectively.

Cellular morphology and biochemistry
S. epidermidis is a very hardy microorganism, consisting of non-motile Gram-positive cocci, arranged in grape-like clusters. It forms white raised colonies approximately 1–2 millimeter in diameter after overnight incubation and is non-hemolytic on blood agar. It is a catalase-positive, coagulase-negative, facultative anaerobe that can grow by aerobic respiration or by fermentation. Some strains may not ferment.

Biochemical tests indicate this microorganism also carries out a weakly positive reaction to the nitrate reductase test. It is positive for urease production, is oxidase negative, and can utilize glucose, sucrose, and lactose to form acid products. In the presence of lactose it will also produce gas. S. epidermidis does not possess the gelatinase enzyme; therefore, not allowing it to hydrolyze gelatin. It is sensitive to novobiocin, providing an important test to distinguish it from Staphylococcus saprophyticus, which is coagulase-negative as well but novobiocin-resistant.

Similar to those of Staphylococcus aureus, the cell walls of S. epidermidis have a transferrin binding protein that helps the organism obtain iron from transferrin. The tetramers of a surface exposed protein, GAPDH or glyceraldehyde-3-phosphate dehydrogenase, are believed to bind to transferrin and remove its iron. Subsequent steps include iron being transferred to surface lipoproteins, then to transport proteins which carry the iron into the cell.

Virulence and antibiotic resistance
The ability to form biofilms on plastic devices is a major virulence factor for S. epidermidis. One probable cause is surface proteins that bind blood and extracellular matrix proteins. The organism's capsule, known as polysaccharide intercellular adhesion (PIA), is made up of sulfated polysaccharide. It allows other bacteria to bind to the already existing biofilm, creating a multilayer biofilm. Such biofilms decrease the metabolic activity of bacteria within them. This decreased metabolism, in combination with impaired diffusion of antibiotics, makes it difficult for antibiotics to effectively clear this type of infection. S. epidermidis strains are often resistant to antibiotics including penicillin, amoxicillin, and methicillin. Resistant organisms are most commonly found in the intestine, but organisms living freely on the skin can also become resistant due to routine exposure to antibiotics secreted in sweat.

Disease
As mentioned above S. epidermidis causes biofilms to grow on plastic devices placed within the body. This occurs most commonly on intravenous catheters and on medical prostheses. Infection can also occur in dialysis patients or anyone with an implanted plastic device that may have been contaminated. Another disease it causes is Endocarditis. This occurs most often in patients with defective heart valves. In some other cases sepsis can occur in hospital patients.

Antibiotics are largely ineffective in clearing biofilms. The most common treatment for these infections is to remove or replace the infected implant, though in all cases prevention is ideal. The drug of choice is often vancomycin, to which rifampin or aminoglycoside can be added. Hand washing has been shown to reduce the spread of infection.

Preliminary research also indicates that S. epidermidis is universally found inside affected acne vulgaris pores, where Propionibacterium acnes is normally the sole resident.

Identification
The normal practice of detecting S. epidermidis is by using the Baird-Parker Agar with egg yolk supplement. Colonies appear small and black. They can be confirmed using the coagulase test. Increasingly, techniques such as real-time PCR and quantitative PCR are being employed for the rapid detection and identification of Staphylococcus strains. Normally sensitivity to desferrioxamine can also be used to distinguish it from most other staphylococci, except in the case of Staphylococcus hominis, which is also sensitive. In this case the production of acid from trehalose, by Staphylococcus hominis, can be used to tell the two species apart.