A 45-year-old man with cirrhosis and ascites presents with spontaneous bacterial peritonitis (SBP). Ascitic fluid culture grows a gram-negative rod that ferments lactose, produces indole, and is methyl red positive but Voges-Proskauer negative. The organism is resistant to ampicillin but susceptible to third-generation cephalosporins. Which of the following additional findings would MOST reliably differentiate this isolate from Klebsiella pneumoniae in the clinical laboratory?

Identification of E. coli vs. Klebsiella pneumoniae

Clinical Context

The patient presents with SBP, a common complication in cirrhotic patients. The organism described has characteristics consistent with Enterobacteriaceae: gram-negative rod, lactose fermentation, and third-generation cephalosporin susceptibility. However, the indole positivity is a key clue.

Biochemical Differentiation

Why the Correct Answer is Best

Option 0 (Correct Answer): Positive urease + mucoid morphology is the gold standard differentiator for K. pneumoniae. If this isolate is urease-negative with non-mucoid colonies, it confirms E. coli. The urease test is rapid (4 hours), highly specific, and the mucoid phenotype reflects the thick polysaccharide capsule characteristic of K. pneumoniae—a virulence factor that makes it more pathogenic in SBP and more resistant to antibiotics.

Why Each Distractor is Wrong

Option 1 (Negative motility + positive citrate): While K. pneumoniae is non-motile and citrate-positive (and E. coli is motile and citrate-negative), the organism here is already indole-positive, which rules out K. pneumoniae before these tests are even needed. These tests are less discriminatory in this context because indole already separated them.

Option 2 (Ornithine decarboxylase positive + lysine decarboxylase negative): This pattern is actually reversed from what would differentiate them. E. coli is positive for both ornithine AND lysine decarboxylase; K. pneumoniae is negative for ornithine but positive for lysine. This option describes a pattern that doesn't match either organism reliably and is not a standard differentiating test.

Option 3 (Positive indole + negative capsule): The isolate is already described as indole-positive in the stem, so repeating indole positivity is redundant. Moreover, "negative capsule production" is not a standard laboratory test—capsules are visualized microscopically (India ink) or inferred from mucoid morphology, not routinely tested as a discrete assay.

High-Yield Summary

Mnemonic: "IMVIC" for Enterobacteriaceae differentiation:

In this case, indole positivity already points to E. coli, but urease negativity + non-mucoid morphology provides the most clinically relevant and rapid confirmation.

Koneman's Textbook of Diagnostic Microbiology Ch 6