A 26-year-old patient presents with dysuria and cloudy urine. Urine culture shows Gram-negative rod, large gray colonies on BAP, pink on MAC, oxidase positive, TSI A/A, motile, citrate green, MUG positive, Indole positive (red). Which organism is most likely?

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Multiple Choice

A 26-year-old patient presents with dysuria and cloudy urine. Urine culture shows Gram-negative rod, large gray colonies on BAP, pink on MAC, oxidase positive, TSI A/A, motile, citrate green, MUG positive, Indole positive (red). Which organism is most likely?

Interpreting a pattern of biochemical tests to identify a urine isolate is being tested here. The organism is a Gram-negative rod that forms lactose-fermenting colonies (pink on MAC) and shows an A/A result on TSI, indicating acid production from glucose and lactose. It is motile, which fits many Enterobacterales. The Indole test is positive, producing a red color, which reflects tryptophanase activity common to Escherichia coli. The MUG test is positive, showing beta-glucuronidase activity typical of E. coli and helping distinguish it from other lactose fermenters. Citrate utilization is negative (green), matching E. coli, which does not use citrate as a sole carbon source under these conditions. Taken together, this specific combination—lactose fermentation, motility, indole positivity, MUG positivity, and citrate negativity—most strongly points to Escherichia coli as the culprit.

Interpreting a pattern of biochemical tests to identify a urine isolate is being tested here. The organism is a Gram-negative rod that forms lactose-fermenting colonies (pink on MAC) and shows an A/A result on TSI, indicating acid production from glucose and lactose. It is motile, which fits many Enterobacterales. The Indole test is positive, producing a red color, which reflects tryptophanase activity common to Escherichia coli. The MUG test is positive, showing beta-glucuronidase activity typical of E. coli and helping distinguish it from other lactose fermenters. Citrate utilization is negative (green), matching E. coli, which does not use citrate as a sole carbon source under these conditions. Taken together, this specific combination—lactose fermentation, motility, indole positivity, MUG positivity, and citrate negativity—most strongly points to Escherichia coli as the culprit.

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