Which feature best distinguishes Shigella species from enteroinvasive E. coli (EIEC) in a patient presenting with acute bloody diarrhea and abdominal cramps?

Explanation

## Distinguishing Shigella from EIEC ### Key Differentiating Feature **Key Point:** **Motility and presence of flagella** is the best feature that distinguishes *Shigella* species from enteroinvasive *E. coli* (EIEC). *Shigella* species are **non-motile and lack flagella**, whereas EIEC strains are **motile and possess flagella** (being derived from *E. coli*, which is typically peritrichously flagellated). ### Detailed Comparison | Feature | Shigella | EIEC | |---------|----------|------| | **Motility / Flagella** | **Non-motile; no flagella** | **Motile; flagella present** | | **Lactose fermentation** | Non-fermenter | Also typically non-fermenter (or very late/weak) | | **Invasiveness** | Invasive (plasmid + chromosomal genes) | Invasive (plasmid-mediated) — **shared feature** | | **Enterotoxin (LT/ST)** | Not produced | Not produced by EIEC — **shared feature** | | **Indole production** | Negative | Variable | | **Lysine decarboxylase** | Negative | Negative | ### Why the Other Options Are Incorrect - **Option A (Invasiveness):** Both *Shigella* and EIEC invade intestinal epithelial cells via plasmid-encoded invasion proteins (IpaA–D). This is a **shared** feature and does not distinguish them. - **Option B (Enterotoxins LT/ST):** EIEC does **not** produce the classic heat-labile (LT) or heat-stable (ST) enterotoxins associated with ETEC. Neither organism produces these toxins, so this is not a distinguishing feature. - **Option D (Lactose fermentation):** While *Shigella* is classically non-lactose-fermenting, EIEC strains are **also non-lactose-fermenting** (or at best very late/weak fermenters). This is therefore a **shared** characteristic and is not a reliable discriminator between the two. ### Clinical Pearl **Clinical Pearl:** In the microbiology laboratory, motility testing (hanging drop or semi-solid agar) readily distinguishes *Shigella* (non-motile) from EIEC (motile). This is the single most reliable phenotypic test to separate these two biochemically similar organisms. Both appear as non-lactose-fermenting, oxidase-negative, gram-negative rods on MacConkey agar, making lactose fermentation an unreliable discriminator. ### High-Yield Fact **High-Yield:** *Shigella* species have lost their flagellin genes through evolution, rendering them permanently non-motile. EIEC, being *E. coli* strains that have acquired invasiveness plasmids, retain their flagella and motility. This evolutionary difference is the cornerstone of laboratory differentiation (Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 6th ed., Ch. 7; Jawetz, Melnick & Adelberg's Medical Microbiology, 27th ed.).