Salmonella Typhi bacteremia is confirmed from a blood culture in a 22-year-old. Disk diffusion shows susceptibility to ciprofloxacin (zone 30 mm) but resistance to nalidixic acid (zone 10 mm). What does this nalidixic acid resistance indicate about treatment?
- A The isolate can be treated safely with standard-dose ciprofloxacin
- B The organism is susceptible to all fluoroquinolones since the zone for ciprofloxacin is large
- C High-dose amoxicillin is the recommended alternative
- D Nalidixic acid resistance indicates likely reduced fluoroquinolone susceptibility (DRFS); ceftriaxone or azithromycin should be preferred ✓
Explanation
Nalidixic acid resistance (NalR) in Salmonella Typhi is a surrogate marker for decreased fluoroquinolone susceptibility (DRFS), caused by point mutations in the gyrA gene (Asp87 → Asn/Gly, Ser83 → Phe). NalR Typhi isolates may appear susceptible to fluoroquinolones by standard disk diffusion yet exhibit treatment failure with ciprofloxacin due to elevated MICs (MIC 0.125–1 µg/mL — within susceptible range but reduced). CLSI and WHO recommend that NalR S. Typhi be treated with ceftriaxone or azithromycin rather than fluoroquinolones. Standard-dose ciprofloxacin should be avoided.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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