Evaluation of a microplate latex agglutination method (Verotox-F assay) for detecting and characterizing verotoxins (Shiga toxins) in Escherichia coli

The performance of a commercial microplate latex agglutination assay, the Verotox-F assay, was compared with that of the Vero cell assay for the detection and characterization of Escherichia coli verocytotoxins (VTs). Culture filtrates of 68 VT-positive E. coli strains (65 human isolates 33 of serotype O157:H7/H-, 32 of non-O157 serotypes] and 3 reference strains) and 104 VT-negative strains (100 human isolates and 4 reference strains) were investigated. The toxin phenotypes and genotypes of the 68 VT-positive isolates were VT1 only (18 strains), VT2 and/or VT2c (33 strains), and VT1 plus VT2 (17 strains). The Verotox-F assay involved incubation of serial dilutions of culture filtrates with equal volumes of latex particles sensitized with anti-VT1 antibody or anti-VT2 antibody in 96-well microtiter plates with appropriate controls and examination for latex agglutination after 20 to 24 h. Compared to the results of the Vero cell assay, the Verotox-F assay was 100% sensitive and 100% specific for the detection of VTs in culture filtrates and correctly identified the toxin types of all 68 VT producers. By checkerboard titration with purified toxins, the sensitivity of the Verotox-F assay was found to be 14 pg (0.7 ng/ml) for VT1, 12 pg (0.6 ng/ml) for VT2, and 350 pg (17.5 ng/ml) for VT2c; this sensitivity is comparable to that of the bioassay. The anti-VT2 latex reagent detected both VT2 and VT2c and did not cross-react with VT1. The anti-VT1 reagent showed a low-level cross-reaction with VT2c only at levels (>/=4.5 microg/ml) that were about 1,000-fold higher than those found in culture filtrates. We conclude that the Verotox-F assay is highly sensitive and specific for the detection and characterization of VTs in culture filtrates of human E. coli isolates. The test is rapid, reliable, and easy to perform; its results are easy to interpret; and it should allow testing for VT to become more widely performed.