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)

The key concept is identifying pathogenic E. coli O157:H7 by its metabolic behavior on selective media plus virulence gene detection. On Sorbitol MacConkey agar, most E. coli ferment sorbitol and produce colored colonies, but O157:H7 typically does not ferment sorbitol, so its colonies remain colorless. This sorbitol non-fermentation is a practical initial screen. To confirm the strain and its virulence, laboratories look for Shiga toxin genes (stx) using PCR or immunoassays, since O157:H7 is a Shiga toxin–producing E. coli (STEC). Together, the sorbitol non-fermenting phenotype on SMAC and the presence of stx genes provide a reliable identification pathway for this strain. The other options don’t fit: oxidase tests are negative for E. coli, the organism is a Gram-negative rod (not Gram-positive cocci in pairs), and growth on MacConkey with lactose fermentation does not distinguish O157:H7, which is defined by sorbitol non-fermentation and toxin gene presence.