## Diagnosis of Shigella Infection ### Gold Standard Investigation **Key Point:** Stool culture on selective media (MacConkey or HE agar) followed by biochemical identification and serotyping is the gold standard for confirming Shigella infection. ### Culture Characteristics | Feature | Details | |---------|----------| | **Growth on MacConkey** | Non-lactose fermenting (NLF), colorless colonies | | **Growth on HE agar** | Blue-green colonies (due to H~2~S production) | | **Optimal temperature** | 35–37°C | | **Culture time** | 18–24 hours | ### Biochemical Identification Shigella shows a characteristic pattern: - **Glucose fermentation:** Positive (acid only, no gas) - **Lactose fermentation:** Negative - **Sucrose fermentation:** Variable (depends on species) - **H~2~S production:** Negative (except S. flexneri biovar Y) - **Motility:** Non-motile - **Indole:** Variable - **Methyl red:** Positive - **Voges-Proskauer:** Negative ### Serotyping **High-Yield:** After biochemical confirmation, serotyping using specific antisera identifies the serogroup (A, B, C, or D) and helps in epidemiological tracking and outbreak investigation. ### Clinical Pearl Stool culture has highest sensitivity when collected early in infection (first 3–5 days) and before antibiotic therapy. Multiple samples may be needed if initial culture is negative. ### Why Culture Remains Gold Standard 1. Allows antimicrobial susceptibility testing (crucial for treatment guidance) 2. Enables serotyping for epidemiological purposes 3. Provides isolate for further molecular studies if needed 4. Cost-effective and widely available in resource-limited settings **Mnemonic:** **SHIGELLA** = **S**elective media, **H**istory of dysentery, **I**nvasive organism, **G**old standard is culture, **E**arly sampling, **L**actose **N**on-fermenter, **L**aboratory confirmation, **A**ntibiotics guided by susceptibility
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