A 32-year-old woman from rural Maharashtra presents with acute bloody diarrhea, fever, and abdominal cramps. Stool culture yields a non-motile, lactose-negative, Gram-negative rod that is oxidase-negative and produces acid from glucose. Which single biochemical test would best differentiate Shigella flexneri from enteroinvasive E. coli (EIEC) in this clinical context?

Explanation

## Differentiating Shigella flexneri from EIEC ### Clinical Context Both organisms present with invasive dysentery (bloody diarrhea, fever, abdominal pain) and share many biochemical features: - Both are non-motile - Both are lactose-negative or slow-fermenting - Both are oxidase-negative - Both ferment glucose with acid only (no gas) This overlap makes them phenotypically similar and requires targeted biochemical testing. ### Key Differentiating Test: Ornithine Decarboxylase **Key Point:** Ornithine decarboxylase (ODC) activity is the single best discriminator between Shigella flexneri and EIEC: - **Shigella flexneri: ODC NEGATIVE** (lacks the enzyme; notably, *S. sonnei* is ODC positive, but *S. flexneri* is ODC negative) - **EIEC: ODC POSITIVE** (produces ornithine decarboxylase, consistent with its E. coli lineage) ### Why Lysine Decarboxylase (Option B) Is Incorrect While it is true that *Shigella* spp. are LDC negative, **EIEC is also LDC negative** — this is one of the key features that distinguishes EIEC from typical *E. coli* (which is LDC positive). Therefore, LDC does NOT differentiate Shigella flexneri from EIEC; both are LDC negative. ### Comparison Table: Biochemical Profile | Test | Shigella flexneri | EIEC | Discriminator? | |------|-------------------|------|----------------| | Motility | Negative | Negative | No | | Lactose | Negative | Negative/slow | No | | Glucose | Acid only | Acid only | No | | Oxidase | Negative | Negative | No | | Lysine decarboxylase | **Negative** | **Negative** | **No** | | Methyl red | Positive | Positive | No | | **Ornithine decarboxylase** | **Negative** | **Positive** | **YES** | | Citrate utilization | Negative | Negative | No | ### Why Other Tests Fail 1. **Methyl red (Option A):** Both are positive (acidic fermentation end-products). Not discriminatory. 2. **Lysine decarboxylase (Option B):** Both Shigella flexneri AND EIEC are LDC negative. Not discriminatory. 3. **Citrate utilization (Option D):** Both are negative. Not discriminatory. ### Clinical Pearl **Clinical Pearl:** EIEC closely mimics Shigella biochemically because it has lost many typical *E. coli* traits (motility, LDC, lactose fermentation). However, EIEC retains ODC positivity, which Shigella flexneri lacks. This single test is the cornerstone of laboratory differentiation. In API 20E or VITEK panels, ODC positivity in a non-motile, lactose-negative, LDC-negative rod strongly favors EIEC over Shigella flexneri. ### High-Yield Fact **High-Yield:** The mnemonic to remember: EIEC "escaped" from E. coli but kept its **O**DC (**O**rnithine decarboxylase). Shigella flexneri is negative for both LDC and ODC. This distinction is tested in NEET PG and is part of standard API 20E identification panels. [cite: Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 16; Mandell, Douglas & Bennett's Principles and Practice of Infectious Diseases 9e Ch 215; Koneman's Color Atlas and Textbook of Diagnostic Microbiology 7e]