What are the key laboratory methods for diagnosing STEC/EHEC infection and how is serotype O157:H7 typically distinguished?

• A. Detection of Shiga toxin or stx genes; culture on SMAC agar where O157:H7 does not ferment sorbitol; serotyping for O157 and H7
• B. PCR for invA gene; culture on Campy medium
• C. Serology for Shiga toxin antibodies; culture on blood agar
• D. Detection of lactose fermentation on MacConkey; identification by colony color

Answer: (C)

Detecting Shiga toxin production or the presence of stx genes in stool samples, combined with a culture approach that highlights the distinctive O157:H7 phenotype, is the cornerstone of diagnosing STEC/EHEC infections and distinguishing the culprit lineage. Shiga toxins are the defining virulence factor of STEC/EHEC, so assays that detect the toxin itself or the toxin-encoding genes provide direct evidence of infection. Culture on sorbitol-MacConkey plates is used to separate O157:H7 from many other gram-negative enteric bacteria because O157:H7 typically does not ferment sorbitol quickly, yielding colorless colonies, whereas many non-O157 STEC strains do ferment sorbitol and appear differently. After obtaining colonies with the expected SMAC phenotype, serotyping for the O157 antigen and the H7 flagellar antigen confirms the O157:H7 serotype, which has particular public health relevance and outbreak associations.

The other approaches don't fit the standard diagnostic pathway. PCR targeting invA is associated with Salmonella, not STEC/EHEC, and Campylobacter culture relies on selective media that target Campylobacter species. Serology for Shiga toxin antibodies is not used for acute diagnosis because antibodies may be absent or non-specific early in infection and do not pinpoint the causative strain. Culturing on blood agar and looking at lactose fermentation on MacConkey lack the specificity needed to identify STEC/EHEC or distinguish O157:H7.