A 39-year-old woman comes to the emergency department due to 3 days of fever, dysuria, and left flank pain. The patient has a history of multiple urinary tract infections and had a ureteropelvic junction calculus removed 2 years ago. Last month, she was treated for vaginal candidiasis. The patient does not use tobacco, alcohol, or illicit drugs. Her mother has thyroid cancer. Temperature is 38.3 C (100.9 F), blood pressure is 120/70 mm Hg, and pulse is 110/min. Left costovertebral angle tenderness is present. Laboratory results are as follows:
| Leukocytes | 13,000/mm3 |
| Urinalysis | |
| pH | 8.5 |
| Blood | positive |
| Leukocyte esterase | positive |
| Bacteria | many |
| White blood cells | 100+/mm3 |
Imaging reveals an enlarged left kidney with perinephric fat stranding and a nonobstructive renal calculus. Which of the following organisms is most likely to be seen in this patient's urine culture?
This young woman with acute fever, dysuria, flank pain, costovertebral angle tenderness, leukocytosis, pyuria, and perinephric fat stranding likely has acute pyelonephritis. Urinary infections typically arise from fecal contamination of the vaginal introitus with subsequent ascension of organisms into the urinary system. The most common pathogen is Escherichia coli (~80%), but Proteus mirabilis, Klebsiella pneumoniae, and Staphylococcus saprophyticus are also frequently isolated.
Although urine culture is required for definitive identification, the presence of urinary alkalization (pH >8) raises suspicion for a urease-producing bacterium such as Proteus mirabilis (most commonly) or Klebsiella pneumoniae. Urease splits urea into ammonia and carbon dioxide; ammonia then converts to ammonium and alkalinizes the urine. High urine pH reduces the solubility of phosphate, raising risk for development of struvite stones (magnesium ammonium phosphate).
(Choices A, B, C, F, and G) Candida, Citrobacter freundii, Enterococcus faecalis, Pseudomonas aeruginosa, and Staphylococcus aureus cause urinary tract infections primarily in hospitalized patients or those with certain risk factors (eg, urinary anatomic abnormality, obstruction, immunocompromise, pregnancy). This outpatient with a nonobstructive stone is at low risk for these pathogens. In addition, none of these organisms produce urease; therefore, urinary alkalization is less likely.
(Choice D) Although Escherichia coli is the most common cause of urinary tract infection, the organism is not urease-producing. Urinary alkalinization is atypical.
Educational objective:
Proteus mirabilis is a urease-producing bacterium that commonly causes urinary tract infections. Urease generates ammonium, resulting in urine alkalinization (pH>8). This decreases the solubility of phosphate, dramatically increasing the risk of urinary calculi with struvite stones (magnesium ammonium phosphate).