USMLE Answer 10
10. The correct answer is C. 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 (choice B). 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(choice A) 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 (choices D and E) 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.