A urine culture grows Klebsiella pneumoniae. The Kirby-Bauer disc diffusion shows reduced inhibition zones for ceftazidime and cefotaxime compared to controls. Double disc synergy test (DDST) using clavulanate-augmented discs shows ≥5 mm increase in zone diameter. Which resistance mechanism is confirmed and which antibiotic is definitively active?
- A AmpC hyperproduction; meropenem is active
- B OXA-48 carbapenemase; ceftazidime-avibactam is active
- C MCR-mediated colistin resistance; tigecycline is the only active agent
- D ESBL (extended-spectrum beta-lactamase) production; meropenem (carbapenem) is the definitive agent ✓
Explanation
The double disc synergy test (DDST) confirms ESBL production when a clavulanic acid-containing disc enhances the inhibition zone of extended-spectrum cephalosporin discs by ≥5 mm, because clavulanate inhibits ESBLs (CTX-M, SHV, TEM extended-spectrum variants). ESBL-producing organisms are resistant to all penicillins, cephalosporins, and aztreonam; carbapenems (meropenem, imipenem, ertapenem) remain the treatment of choice. AmpC is not inhibited by clavulanate (DDST would be negative). Carbapenemase (OXA-48, KPC) would require a carbapenem-specific test (modified carbapenem inactivation method, mCIM).
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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