A 35-year-old man from Mumbai presents with acute watery diarrhea, abdominal pain, and fever (38.5°C) for 2 days. He had attended a family wedding 2 days ago where he consumed contaminated food. Stool culture yields a gram-negative, non-motile, non-lactose-fermenting rod that is oxidase-negative and produces acid from glucose without gas. Biochemical tests confirm Shigella sonnei. Which of the following is the MOST appropriate antimicrobial therapy for this patient?

Explanation

## Antimicrobial Therapy for Shigella Infection **Key Point:** Fluoroquinolones (especially ciprofloxacin) are the first-line empiric therapy for Shigella in India due to widespread resistance to older agents. ### Antimicrobial Resistance Patterns in Shigella | Agent | Efficacy | Resistance Status | Notes | |---|---|---|---| | **Ampicillin** | Historically effective | High resistance (>50% in India) | No longer recommended | | **Ciprofloxacin** | Excellent | Low resistance (<5%) | First-line for invasive disease | | **Tetracycline** | Moderate | High resistance (>40%) | Outdated; poor CNS penetration | | **TMP-SMX** | Moderate | High resistance (>30%) | Resistance increasing; not first-line | | **Ceftriaxone** | Excellent | Low resistance | Alternative if fluoroquinolone contraindicated | | **Azithromycin** | Good | Emerging resistance | Second-line option | ### Why Ciprofloxacin Is the Correct Choice 1. **Empiric coverage:** Ciprofloxacin provides excellent coverage against Shigella and other common enteropathogens (Salmonella, Campylobacter, ETEC). 2. **Resistance pattern:** Resistance to ciprofloxacin in Shigella remains <5% in most Indian centers, making it highly reliable. 3. **Rapid symptom resolution:** Fluoroquinolones reduce duration of diarrhea and bacterial shedding more effectively than older agents. 4. **Dosing:** 500 mg BD for 3 days is the standard regimen for uncomplicated Shigella dysentery in immunocompetent adults. 5. **Systemic absorption:** Ciprofloxacin achieves good tissue and intracellular concentrations, important for an intracellular pathogen. **High-Yield:** In India, Shigella shows >50% resistance to ampicillin and TMP-SMX. Fluoroquinolones are the empiric choice for acute dysentery. **Clinical Pearl:** Antimotility agents (loperamide, diphenoxylate) should be AVOIDED in Shigella dysentery as they increase the risk of toxic megacolon and systemic complications. **Mnemonic:** **FQ-First** — Fluoroquinolones (ciprofloxacin, ofloxacin) are the first-line agents for Shigella in resource-limited settings with high resistance to older drugs. ### Rationale for Rejecting Other Options **Ampicillin:** Resistance rates exceed 50% in India; no longer reliable for empiric therapy despite historical use. **Tetracycline:** Outdated choice with high resistance; poor intracellular penetration and longer duration of therapy (7 days vs. 3 days). **TMP-SMX:** Resistance increasing (>30%); inferior to fluoroquinolones for reducing symptom duration.