USMLE® Step 1 style questions USMLE
A 27-year-old woman comes to the clinic because of burning during urination for the past two days. She had a similar episode a month ago that resolved with one dose of antibiotics. Urine culture now shows a novobiocin-resistant organism that does not reduce nitrate. Which of the following is the most likely causative agent of this patient's symptoms?
With Staphylococcus saprophyticus, sometimes called Staph saprophyticus, “staph” means grapes, “coccus” means round-shape, while “saprophyticus” refers to organisms that grow on decaying organic material.
So, Staphylococcus saprophyticus are round bacteria that tend to live in grape-like clusters, and are commonly found on contaminated meat products.
Now, a little bit of microbe anatomy and physiology.
Staph saprophyticus 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 live with or without oxygen.
Staph saprophyticus is catalase positive, so it makes an enzyme called catalase.
We can use this to differentiate Staph saprophyticus 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 saprophyticus, it makes hydrogen peroxide dissociate into water and oxygen, making the mixture foam.
Staph saprophyticus 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 saprophyticus, urease makes 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 Staphylococcus species, Staph saprophyticus and its close relative, Staph epidermidis, 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 saprophyticus or Staph epidermidis, the fibrin doesn’t clump up.
Finally, to distinguish Staph saprophyticus from Staph epidermidis, the novobiocin test is done.
This is when a disk imbued with Novobiocin, an antibiotic, is added to the culture.