A 3-year-old child from rural India presents with acute watery diarrhea for 2 days, fever, and abdominal cramping. Stool microscopy shows RBCs and WBCs. Culture on MacConkey agar grows non-lactose fermenting colonies. The organism is oxidase-negative and produces gas from glucose. Which is the most appropriate confirmatory test to identify enteroinvasive E. coli (EIEC)?

Explanation

## Diagnosis of Enteroinvasive E. coli (EIEC) ### Clinical Context EIEC causes dysenteric diarrhea (bloody stools with WBCs) in children, mimicking Shigella. The organism is non-motile, non-lactose fermenting, and oxidase-negative — making it difficult to distinguish from other pathogenic E. coli and Shigella at the culture stage. ### Why PCR/Hybridization for Invasion Genes (ial) is Correct **Key Point:** EIEC pathogenesis depends on plasmid-encoded invasion genes (ial, ipaH homologues) that allow intracellular invasion and destruction of intestinal epithelium. Detection of these genes is the **gold standard confirmatory test** for EIEC. **High-Yield:** The ial genes encode proteins that mediate bacterial entry into epithelial cells — the defining virulence mechanism of EIEC. PCR or DNA hybridization targeting these genes is: - **Specific:** Distinguishes EIEC from other non-motile, non-lactose fermenting E. coli - **Sensitive:** Detects the plasmid even if it is lost during culture - **Rapid:** Results in hours, not days ### Differential Approach to Pathogenic E. coli | E. coli Type | Virulence Factor | Confirmatory Test | |---|---|---| | ETEC | Heat-labile (LT) and heat-stable (ST) enterotoxins | PCR for *elt*, *est* genes; ELISA for toxins | | EAEC | Aggregative adherence fimbriae (AAF) | PCR for *aaf* genes; HEp-2 cell adherence assay | | EIEC | Invasion genes (*ial*, *ipaH*) | **PCR/hybridization for *ial* genes** | | EHEC/STEC | Shiga toxins (Stx1, Stx2) | PCR for *stx* genes; Shiga toxin ELISA | | EAEC | Enteroaggregative adherence | PCR for *agg3A* | **Clinical Pearl:** EIEC is rare in developed countries but endemic in resource-limited settings. It causes a Shigella-like syndrome (dysentery with fever, tenesmus, and systemic symptoms). ### Why Other Options Are Suboptimal **Serotyping (Option 0):** While O and H antigens exist, they are not specific for EIEC pathotype. Many E. coli strains share the same serotypes, so this cannot confirm the invasive phenotype. **Hektoen Enteric Agar (Option 1):** This is a selective medium for Shigella and Salmonella. While EIEC may grow on it, the medium is not confirmatory — biochemical tests alone cannot distinguish EIEC from Shigella or non-pathogenic E. coli. Shiga toxin detection is for STEC, not EIEC. **Motility and Indole (Option 3):** These are basic biochemical tests. EIEC is non-motile and indole-positive, but these features are shared with many other E. coli strains and do not confirm the invasive phenotype. [cite:Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases Ch 213]