The novobiocin susceptibility test is used for the identification of which organism?

• A. Corynebacterium diphtheriae
• B. Streptococcus pneumoniae
• C. Staphylococcus capitis
• D. Staphylococcus saprophyticus

Answer: (D)

The test relies on a simple pattern of antibiotic response to separate closely related staphylococci. Staphylococcus saprophyticus is distinct in its resistance to novobiocin, so when a 5 µg novobiocin disk is placed on a staphylococcal isolate, lack of a clear zone of inhibition points to S. saprophyticus. In contrast, other coagulase-negative staphylococci, such as Staphylococcus capitis, are usually susceptible and show a noticeable inhibition zone. This makes the novobiocin susceptibility test especially useful for distinguishing S. saprophyticus in cases like UTIs, where it’s a common pathogen. The other organisms listed are identified by different tests and would typically not be characterized by novobiocin resistance alone.