Which diagnostic clue most strongly supports Staphylococcus saprophyticus as the cause of a female outpatient urinary tract infection?

Which diagnostic clue most strongly supports Staphylococcus saprophyticus as the cause of a female outpatient urinary tract infection?

• A. Novobiocin resistance
• B. Coagulase positivity
• C. Gram-positive cocci in clusters with beta-hemolysis
• D. Urease production

Answer: (A)

Novobiocin resistance is the strongest clue pointing to Staphylococcus saprophyticus in a young woman with an outpatient urinary tract infection. S. saprophyticus uniquely resists the antibiotic novobiocin, while other common staphylococci—such as S. epidermidis, which can cause UTIs but is typically novobiocin sensitive—do not. Therefore, when urine isolates grow in the presence of a novobiocin disk (no or minimal inhibition), it supports S. saprophyticus as the etiologic agent. In contrast, coagulase positivity and Gram-positive cocci in clusters with beta-hemolysis are features of Staphylococcus aureus, not saprophyticus, so they point away from the correct organism. Urease production is associated with other urinary pathogens like Proteus and some Klebsiella species and is not diagnostic for Staphylococcus saprophyticus.

Novobiocin resistance is the strongest clue pointing to Staphylococcus saprophyticus in a young woman with an outpatient urinary tract infection. S. saprophyticus uniquely resists the antibiotic novobiocin, while other common staphylococci—such as S. epidermidis, which can cause UTIs but is typically novobiocin sensitive—do not. Therefore, when urine isolates grow in the presence of a novobiocin disk (no or minimal inhibition), it supports S. saprophyticus as the etiologic agent.

In contrast, coagulase positivity and Gram-positive cocci in clusters with beta-hemolysis are features of Staphylococcus aureus, not saprophyticus, so they point away from the correct organism. Urease production is associated with other urinary pathogens like Proteus and some Klebsiella species and is not diagnostic for Staphylococcus saprophyticus.