Which laboratory method is commonly used to detect Shiga toxin genes in Escherichia coli O157:H7?

• A. PCR or immunoassay for Shiga toxin genes (stx).
• B. Gram stain.
• C. Catalase test.
• D. Oxidase test.

Answer: (A)

Detecting Shiga toxin genes relies on finding the genetic instructions that enable toxin production, specifically the stx1 and stx2 genes carried by pathogenic E. coli such as O157:H7. PCR is the standard approach because it is fast, sensitive, and highly specific for these genes, providing direct confirmation that the toxin genes are present in the isolate or sample. Immunoassays can detect the Shiga toxin protein itself, which shows the toxin is being produced, but that tests for the toxin rather than the gene; it complements genetic testing but does not identify the genes. Routine phenotypic tests like Gram stain, catalase, or oxidase do not assess virulence genes, so they aren’t used to detect Shiga toxin genes.