Novel Agglutination Assays Using Phage Ligand Proteins to Identify the Top 7 Shiga Toxin-producing Escherichia coli (STEC) Serogroups

Introduction: Shiga toxin-producing Escherichia coli (STEC) are an important cause of foodborne illness.  E. coli O157:H7 is implicated in the majority of the outbreaks and Hemolytic Uremic Syndrome cases. However, other serotypes such as O26:H11, O103:H2, O145:H28, O111:H8, O121 have also been implicated. As a result, the food industry needs fast, sensitive and complete methods for STEC detection to ensure a safe food supply.

Purpose: This study was designed to evaluate latex agglutination assays using phage ligand proteins for the top 7 STEC serogroups.

Methods: The latex test was performed by mixing a colony taken directly from a selective agar with a drop of the blue latex suspension. A positive agglutination is obtained within 1 minute of mixing. For each serogroups, the inclusivity study was performed on 20 specific strains and the exclusivity on 33 non-target strains from other E. coli serogroups or other gram negative or gram positive bacteria.

Results: The inclusivity study showed a high specificity: all target organisms produced positive results in less than one minute. The exclusivity study showed that none of the latex assays cross reacted with the other E. coli serogroups or bacterial species tested.

Significance: These rapid, sensitive and reliable agglutination assays can be used for the identification of presumptive isolated STEC colonies from selective media. They can be an economic, rapid and sensitive alternative to PCR for identification of the positive colonies to be further studied for the presence of eae and stx virulence genes, as described in the USDA MLG 5B.03 protocol.