In a urine culture showing more than 100,000 col/mL, what test is best for differentiating S. epidermidis from S. saprophyticus?

The differentiation between Staphylococcus epidermidis and Staphylococcus saprophyticus is crucial in microbiology, particularly in the context of urinary tract infections, where S. saprophyticus is a common uropathogen, especially in young women. Both of these bacteria are coagulase-negative staphylococci, making them similar in some characteristics, which is why an effective differentiation test is necessary.

Novobiocin susceptibility is the definitive test for this purpose. S. saprophyticus is known to be resistant to novobiocin, while S. epidermidis is generally sensitive to this antibiotic. Therefore, conducting a novobiocin susceptibility test allows clinicians and microbiologists to accurately identify S. saprophyticus in a clinical specimen, especially when there are high colony counts of coagulase-negative staphylococci.

The other tests mentioned, such as lactose fermentation, urease production, and catalase activity, are not reliable or specific for distinguishing between these two species in this context. Lactose fermentation is not relevant for these Staphylococcus species, urease production does not effectively differentiate the two, and while both are catalase-positive (like all staphylococci), this characteristic does not provide any distinguishing information in this scenario.