USMLE® Step 1 style questions USMLE
A 75-year-old woman presents to the emergency department for evaluation of right knee pain for the past two days. Medical history is notable for hypertension, type II diabetes mellitus, and knee osteoarthritis status post-bilateral total knee arthroplasty. Temperature is 38.5 °C (101.3 °F), blood pressure is 142/89 mmHg, pulse is 115/min, respirations are 20/min and SpO2 is 99% of room air. On physical examination, the right knee appears swollen and is tender to palpation. An arthrocentesis is performed and cultures reveal a Gram-positive coccus which is catalase-positive and coagulase-negative. Which of the following organisms was most likely identified?
Content Reviewers:Viviana Popa, MD
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 skin normal 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.