A gram-negative rod isolated from urine of a female with recurrent UTI is oxidase negative and urease positive, with A/A on TSI and hydrogen sulfide production; on LIA it shows red/black. Which organism is most likely?

A gram-negative rod isolated from urine of a female with recurrent UTI is oxidase negative and urease positive, with A/A on TSI and hydrogen sulfide production; on LIA it shows red/black. Which organism is most likely?

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

Answer: (A)

This pattern is classic for Proteus mirabilis, a urease-positive, oxidase-negative Enterobacteriaceae commonly associated with urinary tract infections. The oxidase-negative result narrows us to the Enterobacteriaceae rather than Pseudomonas, which is oxidase positive. The organism’s urease activity explains the urinary findings in recurrent UTIs and helps distinguish it from organisms like E. coli (which is typically urease negative). On carbohydrate fermentation tests, this organism shows an A/A result on TSI with hydrogen sulfide production, meaning it ferments glucose (and often other sugars) with acid production in both the slant and butt, and it reduces sulfur to H2S, producing a black precipitate in the butt. On Lysine Iron Agar, the red slant with a black butt (red/black) reflects lysine decarboxylation with concurrent H2S production. The combination of oxidase negative, urease positive, H2S production on TSI, and the red/black pattern on LIA is highly characteristic of Proteus species, particularly Proteus mirabilis, which is also notably associated with UTIs and stone formation due to urease activity. Pseudomonas would be oxidase-positive, while E. coli and Klebsiella typically do not produce H2S in these tests, making Proteus mirabilis the best fit.

This pattern is classic for Proteus mirabilis, a urease-positive, oxidase-negative Enterobacteriaceae commonly associated with urinary tract infections. The oxidase-negative result narrows us to the Enterobacteriaceae rather than Pseudomonas, which is oxidase positive. The organism’s urease activity explains the urinary findings in recurrent UTIs and helps distinguish it from organisms like E. coli (which is typically urease negative). On carbohydrate fermentation tests, this organism shows an A/A result on TSI with hydrogen sulfide production, meaning it ferments glucose (and often other sugars) with acid production in both the slant and butt, and it reduces sulfur to H2S, producing a black precipitate in the butt. On Lysine Iron Agar, the red slant with a black butt (red/black) reflects lysine decarboxylation with concurrent H2S production. The combination of oxidase negative, urease positive, H2S production on TSI, and the red/black pattern on LIA is highly characteristic of Proteus species, particularly Proteus mirabilis, which is also notably associated with UTIs and stone formation due to urease activity. Pseudomonas would be oxidase-positive, while E. coli and Klebsiella typically do not produce H2S in these tests, making Proteus mirabilis the best fit.