A 25-year-old woman presents to the emergency department with a 3-day history of fever, chills, and painful urination. She reports increased urinary frequency and urgency but denies flank pain or vaginal discharge. Her medical history is unremarkable, and she is sexually active with one partner. Vitals signs are temperature 38.3°C (100.1°F), pulse 89 beats/minute, and blood pressure 108/65 mmHg. On physical examination, there is suprapubic tenderness to palpation. Urinalysis shows pyuria and a positive nitrite test. Urine culture grows >10^5 CFU/mL of gram-negative bacilli with a metallic sheen on Eosin Methylene Blue agar.
Which of the following is the most likely causative organism?
The correct answer is:
A) Escherichia coli
The patient’s symptoms of fever, dysuria, frequency, and urgency, with pyuria, bacteriuria, and positive nitrites, indicate acute uncomplicated cystitis. Urine culture showing gram-negative bacilli with metallic sheen on Eosin Methylene Blue agar points to Escherichia coli, the most common cause of community-acquired urinary tract infections (70–95% of cases).
Answer choice B: Klebsiella pneumoniae, is incorrect. Klebsiella pneumoniae can cause urinary tract infections, particularly in hospitalized patients, but forms mucoid colonies on an Eosin Methylene Blue agar without the characteristic metallic sheen of Escherichia coli. It is less common in community settings.
Answer choice C: Pseudomonas aeruginosa, is incorrect. Pseudomonas aeruginosa is associated with complicated or hospital-acquired urinary tract infections, showing green pigment on culture and no metallic sheen on Eosin Methylene Blue. It is rare in uncomplicated community-acquired cases.
Answer choice D: Proteus mirabilis, is incorrect. Proteus mirabilis causes urinary tract infections with alkaline urine and urease production but shows swarming motility on agar, not a metallic sheen on Eosin Methylene Blue agar.
Answer choice E: Staphylococcus saprophyticus, is incorrect. Staphylococcus saprophyticus causes urinary tract infections in young women but is a gram-positive coccus and does not grow with a metallic sheen on Eosin Methylene Blue agar, which is selective for gram-negative bacilli.
Key Learning Point
Escherichia coli is the leading cause of acute uncomplicated cystitis, identified by gram-negative bacilli with a metallic sheen on Eosin Methylene Blue agar. Urinalysis with nitrites/pyuria and culture confirms the diagnosis.
