Staphylococcus epidermidis

Definitions & Key takeaways

Staphylococcus epidermidis is a round, gram-positive, catalase-positive, coagulase-negative, and urease-positive bacteria, which is normally part of the normal human flora, especially on the skin and mucosa. While S. epidermidis is generally harmless, it can cause opportunistic infections in people with weakened immune systems.

S. epidermidis is known to cause skin infections such as impetigo or cellulitis, but it's best known to contaminate and make biofilms on indwelling medical devices. From there, it can then get into the blood and cause severe infections, mostly in newborns. Treatment relies on antibiotics like Vancomycin and removing and replacing the infected medical device with a new, clean one.

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Staphylococcus epidermidis or simply Staph epidermidis can be broken down into staph which means grapes, coccus which means round shape, and epidermidis referring to the superficial layer of the skin.
So, Staphylococcus epidermidis are round bacteria that tend to live clustered together as if they were grapes, and they are part of the normal skin flora even though they may also be found living on the mucosa of the gut.
Now, a little bit of microbe anatomy and physiology. Staph epidermidis has a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so this is a gram-positive bacteria.
It’s non-motile and doesn’t form spores, and also, it’s a facultative anaerobe, meaning that it can survive in both aerobic and anaerobic environments.
Staph epidermidis is catalase positive, so it makes an enzyme called catalase. We can use this to differentiate Staph epidermidis from other gram positive cocci, like streptococci and enterococci, which are catalase negative.
To test for this, a few drops of hydrogen peroxide are added to the colony of the suspected bacteria. So, if catalase is present, like in staph epidermidis, it makes the hydrogen peroxide dissociate into water and oxygen, causing the mixture to foam.
Staph epidermidis is also urease positive, meaning it produces an enzyme called urease that dissociates urea into carbon dioxide and ammonia.
This can be tested by transferring a pure sample of bacteria from the culture to a sterile tube containing a mixture of “urea agar” broth and phenol red.
Then, the mixture is incubated. So, with Staph epidermidis, urease does it’s thing, making urea dissociate into carbon dioxide and ammonia.
Ammonia then makes the mixture change color from orange-yellow to bright pink. This doesn’t happen with urease negative Gram-positive cocci, like Streptococcus pneumoniae or Enterococcus faecalis.
Furthermore, unlike many other Staphylococcus species, Staph epidermidis and its close relative, Staph saprophyticus, are both coagulase negative, meaning they don’t produce an enzyme called coagulase.
Testing for coagulase is done by transferring a colony of the suspected bacteria in test tube containing fibrinogen-rich plasma.
Coagulase-positive bacteria, like Staph aureus, convert the soluble fibrinogen into sticky fibrin, which then visibly clumps up.
With coagulase negative species, like Staph epidermidis or Staph saprophyticus, the fibrin doesn’t clump up. Finally, to distinguish Staph epidermidis from Staph saprophyticus, the novobiocin test is done.
This is when a disk imbued with Novobiocin, an antibiotic, is added to the culture. Staph epidermidis is novobiocin sensitive, so the colonies will die off, whereas Staph saprophyticus is novobiocin resistant, so the colonies remain intact.
Alright, now, Staph epidermidis is actually the most dominant bacteria on the human skin. It colonizes mostly the scalp, the face, the nasal cavities and the axillae, where it lives as an innocent commensal without causing any harm.
And since it's the most abundant bacteria on the human skin, it is a common contaminant of blood cultures. Besides blood cultures, Staph epidermidis can also contaminate indwelling medical devices, such as intravenous and urinary catheters; and prosthetic devices, like prosthetic heart valves and prosthetic joints.
Here, Staph epidermidis can form adherent biofilms, which is basically a layer of “slime” made of exopolysaccharides or EPS, within which Staph epidermidis live and reproduce.
Comparing a biofilm to strawberry jam, the seeds would be the bacteria and the rest of the jam would be the EPS. Biofilms make it difficult for neutrophils and antibiotics to reach the bacteria, so the infection is more difficult to treat.
Now, the clinical presentation of catheter-related infections is nonspecific and it includes local and systemic signs and symptoms.
Local signs and symptoms include pain, tenderness, swelling, warmth, erythema, and purulent discharge. Moreover, purulent discharge is actually a strong indicator of infection, but it’s absent in many cases.
On the other hand, systemic signs and symptoms include fever, headache, fatigue, and chills. Also, a person can complain about rigors and chills while receiving infusion.
As far as complications go, bacteria can migrate along the intravenous catheter and enter the bloodstream, causing bacteremia or sepsis.
Alright, now switching gears and moving on to prosthetic devices. Staph epidermidis is a major cause of prosthetic valve endocarditis or short PVE.
Prosthetic valve endocarditis can occur due to direct intraoperative contamination or via hematogenous spread during the first several weeks after the surgery.
Individuals with prosthetic valve endocarditis typically present with fever and valve dysfunction; and the diagnosis is confirmed by positive blood cultures and transesophageal echocardiography.
Transesophageal echocardiography can detect small vegetations on the valve, which mostly consist of platelets, fibrin, and bacteria.
Just like with prosthetic valves, Staph epidermidis can contaminate prosthetic joints at the time of surgery and cause prosthetic joint infections.
Common signs and symptoms of prosthetic joint infections are typically nonspecific and they include fever, pain, joint swelling, erythema and warmth around the joint.
A strong indicator of prosthetic joint infections is the presence of a sinus tract communicating with the prosthesis. Treatment of Staph epidermidis infections is typically done with Vancomycin, but if the bacteria is methicillin-sensitive, Staph epidermidis can also be treated with oxacillin or nafcillin.
Infected medical devices should also be removed. All right, as a quick recap…Staphylococcus epidermidis is a round, gram-positive, facultative anaerobic, non-motile, non-spore forming, catalase positive, coagulase negative, urease-positive and novobiocin sensitive bacteria.
It grows in clusters and it’s the dominant bacteria of the normal skin flora; therefore it is a common contaminant of blood cultures.
But, It’s best known to contaminate and make biofilms on indwelling medical devices, such as intravenous and urinary catheters; and prosthetic devices, like prosthetic heart valves and prosthetic joints.
Treatment relies on antibiotics like Vancomycin, and removing and replacing the infected medical device with a