Pathogenicity testing: The Sereny test (guinea pig keratoconjunctivitis model) is classically used to detect which diarrheagenic category of Escherichia coli based on invasiveness?

Introduction / Context:
Diarrheagenic E. coli strains are grouped by pathogenic mechanisms: EPEC (adherence/effacement), ETEC (toxins LT/ST), EHEC/VTEC (Shiga-like toxins), and EIEC (invasion). The Sereny test historically assesses invasive capacity by inducing keratoconjunctivitis in guinea pig eyes.

Given Data / Assumptions:

• Sereny test involves inoculating the conjunctival sac with bacteria.
• Positive result indicates epithelial invasion and inflammation.
• Animal-use caveats acknowledged; the test is mainly of historical/teaching interest.

Concept / Approach:
EIEC shares invasion mechanisms with Shigella (including plasmid-mediated invasion). Hence, EIEC and Shigella give positive Sereny reactions, whereas EPEC, ETEC, and EHEC/VTEC generally do not, as their pathogenesis does not require epithelial invasion in the same way.

Step-by-Step Solution:
Recall each E. coli pathotype mechanism.Match invasive phenotype to EIEC.Identify Sereny test as an invasion assay.Select enteroinvasive E. coli.

Verification / Alternative check:
Cell culture invasion assays (e.g., HeLa, Henle) and PCR for invasion genes (ipaH) provide modern confirmation parallel to the historical Sereny test.

Why Other Options Are Wrong:

• EPEC: attachment/effacing, not invasive.
• ETEC: enterotoxin-mediated secretory diarrhea, noninvasive.
• VTEC/EHEC: toxin-mediated; invasion is not the primary mechanism.

Common Pitfalls:
Confusing cytotoxicity (toxins) with invasion; Sereny specifically reflects epithelial invasion causing keratoconjunctivitis.

Final Answer:
enteroinvasive E. coli