A urinary isolate of Escherichia coli produces a zone of inhibition of 18 mm with ceftriaxone (breakpoint ≥26 mm susceptible by CLSI). The double disc synergy test (DDST) shows a 'keyhole' enhancement of the zone between amoxicillin-clavulanate and ceftazidime discs. What is the clinical implication for treatment choice?
- A ESBL-producing E. coli; carbapenems are the drugs of choice for serious infections ✓
- B AmpC-overproducing strain; cefepime is the preferred choice
- C KPC-producing strain; ceftazidime-avibactam is required
- D Hypervirulent strain; polymyxin-based therapy is indicated
Explanation
DDST (also called clavulanate synergy test) demonstrating expansion or keyhole distortion of the cephalosporin inhibition zone towards the amoxicillin-clavulanate disc is the phenotypic hallmark of ESBL production. ESBLs (most commonly CTX-M-15 in India) hydrolyse extended-spectrum cephalosporins and are inhibited by clavulanate in vitro. Despite this in vitro susceptibility, CLSI/ESCMID guidelines recommend treating serious infections with carbapenems (imipenem, meropenem, ertapenem) because ESBLs frequently cause inoculum effect failure and clinical failures with cephalosporins even when MICs appear susceptible.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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