Which organism is the most likely cause of cystitis in a 25-year-old sexually active woman presenting with dysuria, urinary frequency, and a urine culture positive for Gram‑positive cocci?

Staphylococcus saprophyticus is Responsible for the Cystitis (Answer C)

In a 25-year-old sexually active woman with dysuria, urinary frequency, and Gram-positive cocci on urine culture, Staphylococcus saprophyticus is the most likely causative organism.

Clinical Reasoning

While Escherichia coli causes the vast majority (75-95%) of uncomplicated cystitis cases 1, 2, when Gram-positive cocci are specifically identified on culture, the epidemiology shifts dramatically in this demographic:

• S. saprophyticus accounts for approximately 8% of all acute uncomplicated cystitis cases in young, sexually active women 3
• It is the second most common cause of UTI in this specific population after E. coli 2, 4
• The patient's age (25 years) and sexual activity are classic risk factors for S. saprophyticus cystitis 1

Why Not the Other Options?

Options A, B, and D are incorrect:

• Staphylococcus aureus rarely causes uncomplicated cystitis in healthy women
• Staphylococcus lugdunensis and Staphylococcus hemolyticus are not recognized uropathogens in this clinical context
• These organisms would suggest complicated UTI, catheter-associated infection, or contamination

Option E is incorrect:

• S. saprophyticus is characteristically novobiocin-resistant, which distinguishes it from other coagulase-negative staphylococci
• This is a key microbiologic feature used in laboratory identification
• Novobiocin susceptibility would argue against S. saprophyticus

Important Clinical Pearls

Diagnostic considerations:

• The combination of acute dysuria, frequency, and pyuria in a young sexually active woman establishes the diagnosis of acute cystitis with >90% accuracy 2
• Gram-positive cocci on culture in this demographic strongly suggests S. saprophyticus rather than enterococci or group B streptococci, which are poor predictors of true bladder bacteriuria and often represent contamination or urethral colonization 5

Treatment implications:

• Standard first-line therapies for uncomplicated cystitis (nitrofurantoin, fosfomycin, pivmecillinam) remain appropriate 1
• S. saprophyticus typically responds well to these agents
• No modification of standard empiric therapy is needed based on Gram stain alone