Blood cultures from a patient with enteric fever grow Salmonella Typhi. The Widal test shows anti-O titre 1:160, anti-H titre 1:320 (baseline in the community is 1:80). Ciprofloxacin disk zone is 24 mm (CLSI susceptible ≥31 mm), and nalidixic acid disk zone is 10 mm (resistant). What is the clinical interpretation?
- A Fully ciprofloxacin-susceptible S. Typhi — treat with standard ciprofloxacin dose
- B XDR Salmonella Typhi — use meropenem only
- C Nalidixic acid resistant S. Typhi (NARST) with reduced ciprofloxacin susceptibility — use azithromycin or cefixime ✓
- D Multi-drug resistant S. Typhi — use chloramphenicol as first-line
Explanation
Nalidixic acid resistance in S. Typhi (NARST) indicates chromosomal mutation in gyrA gene, which reduces ciprofloxacin affinity despite the zone appearing in the susceptible range by old CLSI criteria. CLSI now uses MIC ≤0.06 µg/mL as the fluoroquinolone-susceptible breakpoint for Salmonella; NARST strains typically have ciprofloxacin MIC 0.125–1.0 µg/mL (intermediate/reduced susceptibility), predicting clinical failure. Current Indian/WHO guidelines recommend oral azithromycin (10 mg/kg/day for 7 days) or cefixime for uncomplicated NARST enteric fever. XDR refers to resistance to all first-line agents plus fluoroquinolones AND cephalosporins.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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