A gram-negative bacillus recovered from a child’s urine with recurrent UTIs is oxidase negative, lactose negative, urease positive, and motile. Most likely identification?

• A. Escherichia coli
• B. Klebsiella pneumoniae
• C. Proteus mirabilis
• D. Pseudomonas aeruginosa

Answer: (C)

Biochemical testing of a gram-negative rod from urine uses oxidase status, lactose fermentation, urease production, and motility to distinguish uropathogens. An isolate that is oxidase negative, lactose nonfermenting, urease positive, and motile best matches Proteus mirabilis. Proteus is known for strong motility with swarming, is a non–lactose fermenter, and produces urease, which can raise urine pH and contribute to stone formation. This combination helps separate it from E. coli (lactose fermenter and typically non-swarming), Klebsiella (lactose fermenter and nonmotile), and Pseudomonas (oxidase positive and nonfermenter). Thus Proteus mirabilis fits the profile and clinical context of recurrent UTIs.