How is the E. coli O157:H7 strain identified in the laboratory?

How is the E. coli O157:H7 strain identified in the laboratory?

• A. Oxidase positive test.
• B. Non-sorbitol fermentation on SMAC agar with detection of Shiga toxin genes (stx) by PCR or immunoassay.
• C. Gram-positive cocci in pairs on Gram stain.
• D. Growth on MacConkey agar with lactose fermentation.

Answer: (B)

Identifying pathogenic E. coli O157:H7 hinges on its distinctive combination of metabolism and virulence markers. On Sorbitol-MacConkey agar, most E. coli strains ferment sorbitol, which gives colored colonies, but this O157:H7 lineage generally does not ferment sorbitol, producing colorless colonies. That non-sorbitol fermentation pattern provides a practical screening signal for this specific strain. To confirm its identity and its virulence potential, laboratories look for Shiga toxin genes (stx) by PCR or detect Shiga toxin by immunoassay. The presence of these toxins is a defining feature of the pathogenic O157:H7 strain, so combining a non-sorbitol result on SMAC with direct detection of stx gives both a discriminatory phenotype and a confirmation of virulence. Other options don’t fit because they don’t provide that specific combination. E. coli is a Gram-negative rod, not Gram-positive cocci in pairs, so that result would be inconsistent. E. coli is typically oxidase negative, so an oxidase-positive test would not identify this organism. On ordinary MacConkey agar, E. coli generally ferments lactose, and while O157:H7 can appear lactose-nonfermenting in some cases, lactose fermentation on MacConkey is not a reliable or specific identifier for this strain.

Identifying pathogenic E. coli O157:H7 hinges on its distinctive combination of metabolism and virulence markers. On Sorbitol-MacConkey agar, most E. coli strains ferment sorbitol, which gives colored colonies, but this O157:H7 lineage generally does not ferment sorbitol, producing colorless colonies. That non-sorbitol fermentation pattern provides a practical screening signal for this specific strain.

To confirm its identity and its virulence potential, laboratories look for Shiga toxin genes (stx) by PCR or detect Shiga toxin by immunoassay. The presence of these toxins is a defining feature of the pathogenic O157:H7 strain, so combining a non-sorbitol result on SMAC with direct detection of stx gives both a discriminatory phenotype and a confirmation of virulence.

Other options don’t fit because they don’t provide that specific combination. E. coli is a Gram-negative rod, not Gram-positive cocci in pairs, so that result would be inconsistent. E. coli is typically oxidase negative, so an oxidase-positive test would not identify this organism. On ordinary MacConkey agar, E. coli generally ferments lactose, and while O157:H7 can appear lactose-nonfermenting in some cases, lactose fermentation on MacConkey is not a reliable or specific identifier for this strain.