A 5-year-old boy presents three days after eating undercooked minced beef patties, now with bloody diarrhoea followed by pallor, oliguria and petechiae. Peripheral smear shows fragmented red cells and the platelet count is 42,000 per microlitre. Which laboratory finding would best confirm the etiological agent?

• A Serum antibody against lipopolysaccharide detected by Widal agglutination
• B Positive string test on colony subculture
• C Growth of lactose-fermenting pink colonies with metallic sheen on EMB agar
• D Growth of colourless colonies on sorbitol-containing MacConkey agar ✓

Explanation

The child has haemolytic uraemic syndrome following enterohaemorrhagic Escherichia coli O157:H7 infection. This serotype characteristically does not ferment sorbitol, so it forms pale, colourless colonies on sorbitol MacConkey agar, distinguishing it from most commensal E. coli strains which ferment sorbitol and appear pink. The metallic sheen on EMB agar is shown by ordinary lactose-fermenting E. coli and does not identify O157:H7, while the string test relates to Vibrio species.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.