A 35-year-old woman presents to her gynecologist with complaints of burning on urination for the past 2 days.Dipstick test of her urine demonstrates marked positivity for leukocyte esterase, but no reactivity for nitrite. Urineculture later grows out large numbers of organisms. Which of the following bacteria are most likely to beresponsible for this patient's infection?

Enterococcus faecalis

The correct answer is B. The positive leukocyte esterase test indicates the presence of neutrophils in the urine, suggesting a bacterial infection. The nitrite test exploits the fact that most Enterobacteria (gram-negative enteric rods) are able to form nitrite from nitrate; thus, the nitrite test is used to diagnose urinary tract infections. One limitation of this method is the fact that enterococci (gut streptococci) do not produce nitrite from nitrate, but can nonetheless cause urinary tract infections. Enterococcal urinary tract infections are often nosocomial and classically acquired in the intensive care unit, although they can occur in other settings. Enterobacter sp. (choice A), Escherichia coli(choice C), Klebsiella pneumoniae(choice D), and Pseudomonas aeruginosa(choice E) can cause urinary tract infections and would usually be picked up by the dipstick for nitrites. False-negative results might still be seen with these organisms if the infection was light, the bladder had been recently emptied prior to collection, and the urine was new and had not yet grown enough bacteria to produce a positive result.