Which organism is a Gram-negative rod that is an indole-positive lactose fermenter and MUG positive in urine culture?

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Multiple Choice

Which organism is a Gram-negative rod that is an indole-positive lactose fermenter and MUG positive in urine culture?

The main concept here is how biochemical tests in a urine culture narrow down the organism. A Gram-negative rod that rapidly ferments lactose points to several common uropathogens, but adding the indole test and the MUG test distinguishes them. Escherichia coli fits all three traits: it is a Gram-negative rod, it ferments lactose, and it is indole-positive due to the enzyme tryptophanase. It also commonly shows a positive MUG test, because E. coli produces β-glucuronidase that cleaves MUG to yield fluorescence under UV light. This combination—Gram-negative rod, lactose fermentation, indole positivity, and MUG positivity—points most specifically to E. coli in urine culture. Other lactose-fermenters like Klebsiella and Enterobacter are typically indole-negative, which helps differentiate them. Proteus species are often indole-negative as well (though some exceptions exist). The indole and MUG results together make E. coli the best match.

The main concept here is how biochemical tests in a urine culture narrow down the organism. A Gram-negative rod that rapidly ferments lactose points to several common uropathogens, but adding the indole test and the MUG test distinguishes them.

Escherichia coli fits all three traits: it is a Gram-negative rod, it ferments lactose, and it is indole-positive due to the enzyme tryptophanase. It also commonly shows a positive MUG test, because E. coli produces β-glucuronidase that cleaves MUG to yield fluorescence under UV light. This combination—Gram-negative rod, lactose fermentation, indole positivity, and MUG positivity—points most specifically to E. coli in urine culture.

Other lactose-fermenters like Klebsiella and Enterobacter are typically indole-negative, which helps differentiate them. Proteus species are often indole-negative as well (though some exceptions exist). The indole and MUG results together make E. coli the best match.

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