Section: USMLE

10) A 47-year-old man with a history of sickle cell disease has had numerous hospitalizations requiring the placement of IV lines. The patient has poor peripheral venous access, and a catheter is placed in right subclavian vein. The patient subsequently develops right arm discomfort and swelling and a temperature of 40.1 C with chills. Multiple blood cultures are taken, and gram-positive cocci are isolated. The organism is catalase positive and grows on mannitol salt agar, but does not turn the agar yellow; the colonies are gamma-hemolytic on a sheep blood agar plate. Which of the following organisms is the most likely cause of this patient’s symptoms?

1. Enterococcus faecalis
2. Staphylococcus aureus
3. Staphylococcus epidermidis
4. Streptococcus agalactiae
5. Streptococcus pyogenes

The patient has developed bacteremia; the description of the causative agent is consistent with a staphylococcal organism (catalase positive, gram-positive cocci that grow on mannitol salt agar). The organism is most likely Staphylococcus epidermidis as it was not able to ferment mannitol and was not hemolytic. Both of these characteristics tend to rule out Staphylococcus aureus. Two other tests that are commonly used are coagulase production and excretion of DNAse from colonies. S. aureus is positive in both tests, S. epidermidis is negative.

Enterococcus faecalis might grow on the mannitol salt agar as it is relatively haloduric but these organisms are catalase negative. The enterococci are extremely variable in hemolytic ability so this characteristic is not useful in species identification.

Both streptococcal organisms are catalase negative and beta-hemolytic on sheep blood agar plates. Also, neither would grow on the mannitol salt agar. Streptococcus pyogenes is sensitive to growth inhibition by bacitracin, whereas Streptococcus agalactiae (group B streptococci) is not.