Purpose: Evaluation of the new latex agglutination assays for the identification of STEC colonies on selective chromogenic medium from pure cultures and artificially inoculated beef samples.
Methods: A panel of 110 strains of target STEC serogroups and 40 non-target strains isolated on selective chromogenic (chromID EHEC) and non-selective (TSAB) media were tested using the latex assays to study inclusivity, exclusivity, and time to agglutination. A portion of colonies were tested with negative controls to rule out any autoagglutination. For the inoculation study, 325-g samples of raw beef were individually spiked with 25-100 CFU of 30 STEC strains (five per serogroup) and following enrichment and immunoconcentration, colonies were isolated on the chromogenic medium. Up to five typical colonies were analyzed using the latex assays and positive colonies were confirmed using the serogroup-specific PCR. Uninoculated samples were confirmed negative before the experiments.
Results: The latex assays showed 100% inclusivity with all 110 STEC strains resulting in a distinguished visible agglutination within 10-20 sec from both media. None of the 40 non-target strains showed any cross-reactivity with the latex. In the inoculated beef samples study, characteristic colonies isolated on chromogenic medium from each sample resulting in a positive agglutination reaction were confirmed to target STEC using serogroup specific PCR. The blue colored latex made it easy to read and interpret the agglutination reaction.
Significance: Due to high level of specificity, recombinant phage protein based latex assays eliminate the possibility of cross-reactivity with non-target isolates. Further, the highly sensitive latex assays provide quick and easy-to-read high intensity agglutination reactions with target antigens.