Staphylococcus epidermidis
Overview
 
Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus
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.
Encyclopedia
Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus
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. 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
Evolutionary pressure
Any cause that reduces reproductive success in a proportion of a population, potentially exerts evolutionary pressure or selection pressure. With sufficient pressure, inherited traits that mitigate its effects - even if they would be deleterious in other circumstances - can become widely spread...

 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
Friedrich Julius Rosenbach
Friedrich Julius Rosenbach, also known as Anton Julius Friedrich Rosenbach, was a German physician and microbiologist. He is credited for differentiating Staphylococcus aureus and Staphylococcus albus, which is now called Staphylococcus epidermidis, in 1884. He also described and named...

 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
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

 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
Catalase
Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...

-positive, coagulase
Coagulase
Coagulase is a protein produced by several microorganisms that enables the conversion of fibrinogen to fibrin. In the laboratory, it is used to distinguish between different types of Staphylococcus isolates. Coagulase negativity excludes S. aureus. That is to say, S...

-negative, facultative anaerobe that can grow by aerobic respiration or by fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

. Some strains may not ferment.

Biochemical tests indicate this microorganism also carries out a weakly positive reaction to the nitrate reductase test
Nitrate reductase test
The nitrate reductase test is a test to differentiate between bacteria based on their ability or inability to reduce nitrate to nitrite using anaerobic respiration.-Procedure:Various assays for detecting nitrate reduction have been described...

. It is positive for urease
Urease
Urease is an enzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia. The reaction occurs as follows:In 1926, James Sumner showed that urease is a protein. Urease is found in bacteria, yeast, and several higher plants. The structure of urease was first solved by P.A...

 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
Gelatinase
In biology and chemistry, gelatinase is a proteolytic enzyme that allows a living organism to hydrolyse gelatin into its sub-compounds that can cross the cell membrane and be used by the organism...

 enzyme; therefore, not allowing it to hydrolyze gelatin. It is sensitive to novobiocin
Novobiocin
Novobiocin, also known as albamycin or cathomycin, is an aminocoumarin antibiotic that is produced by the actinomycete Streptomyces niveus, which has recently been identified as a subjective synonym for S. spheroides a member of the order Actinobacteria . Other aminocoumarin antibiotics include...

, providing an important test to distinguish it from Staphylococcus saprophyticus
Staphylococcus saprophyticus
Staphylococcus saprophyticus is a coagulase-negative species of Staphylococcus bacteria. S. saprophyticus is often implicated in urinary tract infections. S. saprophyticus is resistant to the antibiotic novobiocin, a characteristic that is used in laboratory identification to distinguish it from S...

, 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
Transferrin
Transferrins are iron-binding blood plasma glycoproteins that control the level of free iron in biological fluids. In humans, it is encoded by the TF gene.Transferrin is a glycoprotein that binds iron very tightly but reversibly...

. 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
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

. 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
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

, amoxicillin
Amoxicillin
Amoxicillin , formerly amoxycillin , and abbreviated amox, is a moderate-spectrum, bacteriolytic, β-lactam antibiotic used to treat bacterial infections caused by susceptible microorganisms. It is usually the drug of choice within the class because it is better absorbed, following oral...

, and methicillin
Methicillin
Meticillin or methicillin is a narrow-spectrum beta-lactam antibiotic of the penicillin class. It should not be confused with the antibiotic metacycline.-History:Methicillin was developed by Beecham in 1959...

. 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 catheter
Catheter
In medicine, a catheter is a tube that can be inserted into a body cavity, duct, or vessel. Catheters thereby allow drainage, administration of fluids or gases, or access by surgical instruments. The process of inserting a catheter is catheterization...

s 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
Vancomycin
Vancomycin INN is a glycopeptide antibiotic used in the prophylaxis and treatment of infections caused by Gram-positive bacteria. It has traditionally been reserved as a drug of "last resort", used only after treatment with other antibiotics had failed, although the emergence of...

, to which rifampin or aminoglycoside
Aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...

 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
Acne vulgaris
Acne vulgaris is a common human skin disease, characterized by areas of skin with seborrhea , comedones , papules , pustules , Nodules and possibly scarring...

 pores, where Propionibacterium acnes
Propionibacterium acnes
Propionibacterium acnes is a relatively slow growing, typically aerotolerant anaerobic gram positive bacterium that is linked to the skin condition acne; it can also cause chronic blepharitis and endophthalmitis, the latter particularly following intraocular surgery...

 is normally the sole resident.

Identification

The normal practice of detecting S. epidermidis is by using the Baird-Parker Agar
Baird-Parker Agar
Baird–Parker agar is a type of agar used for the selective isolation of gram-positive Staphylococci species. It contains lithium chloride and tellurite to inhibit the growth of alternative microbial flora, while the included pyruvate and glycine promote the growth of Staphylococci. Staphylococcus...

 with egg yolk
Egg yolk
An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...

 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
Staphylococcus hominis
Staphylococcus hominis is a coagulase-negative member of the bacterial genus Staphylococcus, consisting of Gram positive, spherical cells in clusters. It occurs very commonly as a harmless commensal on human and animal skin. However, like many other coagulase-negative staphylococci, S...

, which is also sensitive. In this case the production of acid from trehalose
Trehalose
Trehalose, also known as mycose or tremalose, is a natural alpha-linked disaccharide formed by an α,α-1,1-glucoside bond between two α-glucose units. In 1832, H.A.L. Wiggers discovered trehalose in an ergot of rye, and in 1859 Marcellin Berthelot isolated it from trehala manna, a substance made...

, by Staphylococcus hominis
Staphylococcus hominis
Staphylococcus hominis is a coagulase-negative member of the bacterial genus Staphylococcus, consisting of Gram positive, spherical cells in clusters. It occurs very commonly as a harmless commensal on human and animal skin. However, like many other coagulase-negative staphylococci, S...

, can be used to tell the two species apart.
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