Specificity Analysis of a Novel Phage-Derived Ligand in an Enzyme-Linked Fluorescent Assay for the Detection of Escherichia coli O157:H7

An assay using a phage-derived ligand to capture Escherichia coli O15TH7 prior to antibody detection was evaluated for assay specificity. Analysis of 200 strains showed that the assay was highly specific for the O157 serogroup. It detected all the O157:H7 strains including Shiga toxin-producing O157 nonmotile strains as well as O157 non-H7 strains. In addition, the assay detected various O157:H7 phenotypic variants that are not easily detected by routine analytical methods, as well as a rough strain that did not express O157 antigen and therefore is undetectable serologically. The phage ligand assay showed no cross-reactivity to the other E. coli serotypes. Isolates of Salmonella group N and a few Citrobacter freundii strains that cross-reacted with anti-O157 sera also showed cross-reactivity with the phage ligand. However, other strains that cross-reacted serologically with anti-O157 sera were correctly identified as negative with the phage ligand assay, including several strains of E. coli that nonspecifically autoagglutinate latex reagents.