A 55-year-old diabetic patient presents with a rapidly spreading, crepitant swelling of the perineum with blackish necrotic skin. CT shows gas in the fascial planes. Blood culture grows Klebsiella pneumoniae. Which unique finding on laboratory testing distinguishes hypervirulent Klebsiella pneumoniae (hvKP) from classical KP?

• A Extended-spectrum beta-lactamase (ESBL) production
• B Carbapenem resistance (KPC production)
• C Urease positivity
• D Positive string test (viscous colony forming a string >5 mm) ✓

Explanation

The 'string test' — touching a colony on a plate with a loop and lifting it, with the string of hyperviscous mucus stretching >5 mm — is the bedside phenotypic marker for hypervirulent K. pneumoniae (hvKP), which produces excess capsular polysaccharide (rmpA gene). hvKP causes community-acquired liver abscess with septic metastases (especially in diabetics in Asia), endophthalmitis, and necrotising fasciitis. ESBL and KPC are resistance mechanisms seen in classical KP. Urease positivity is common to all KP.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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