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

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

Answer: (D)

Novobiocin susceptibility testing differentiates Staphylococcus saprophyticus from other staphylococci. Staphylococcus saprophyticus is resistant to novobiocin, while many other Staphylococcus species are susceptible. In the disc diffusion setup, you place a novobiocin-impregnated disk on a lawn of the organism and incubate; a clear zone of inhibition means susceptibility, whereas little or no inhibition indicates resistance. If the organism shows resistance to novobiocin, this pattern points to Staphylococcus saprophyticus, which is clinically important because it is a common cause of uncomplicated urinary tract infections, especially in young, sexually active women. The other organisms listed are identified by different tests, not by novobiocin resistance.