A 20-year-old patient presents with dysuria and suprapubic pain. A midstream urine sample is cultured. Gram stain shows Gram-positive cocci in clusters. Blood agar shows white opaque colonies. Catalase bubbling. Coagulase negative. Novobiocin: growth ~20 mm. Which organism do you suspect?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

This scenario hinges on recognizing a common cause of uncomplicated urinary tract infections in young, sexually active women and using a couple of biochemical tests to differentiate Staphylococcus species.

Gram-positive cocci in clusters that are catalase positive point to Staphylococcus. The coagulase test distinguishes Staphylococcus aureus (coagulase positive) from coagulase-negative staphylococci. Since the organism is coagulase negative, it’s not S. aureus.

Among coagulase-negative staphylococci, novobiocin susceptibility differentiates the two most relevant urinary pathogens: Staphylococcus epidermidis is inhibited by novobiocin (susceptible), while Staphylococcus saprophyticus is resistant (grows despite the disk). The result here shows growth around the novobiocin disk, indicating resistance, which fits Staphylococcus saprophyticus.

Putting this together with the clinical picture—dysuria and suprapubic pain in a young woman—the most likely organism is Staphylococcus saprophyticus. S. epidermidis is more associated with device-related infections, and S. lugdunensis, although a coagulase-negative staph, has other test patterns that don’t match as cleanly here.