Evaluation of Different Immuno-magnetic Beads and Selective Plating Media for the Confirmation of Shiga Toxin-producing Escherichia coli Detected by Molecular Methods in Beef Trim Enrichment Broths

Introduction: Shiga toxin-producing Escherichia coli of serogroups O26, O45, O103, O111, O121 and O145 are a testing concern for the beef industry and regulators.  Numerous tests are available that attempt to predict the presence of these pathogens.  However, potential positive results require culture confirmation entailing immuno-magnetic separation of the targeted pathogen and plating to differential media capable of aiding laboratories in rapidly identifying suspect colonies.  Therefore, several commercial products have become available for use in culture confirmation.

Purpose: To compare culture confirmation rates using IMS beads available from Abraxis, BioControl, Dynal, LabM and Romer Labs plated onto six different STEC selective agars (modified Rainbow, R&F STEC media, chromID EHEC media, CHROMagar STEC, washed sheep’s blood agar, and STEC Differential Agar).

Methods: Culture confirmation using each IMS bead-media combination was performed on 54 STEC potential positive beef trim enrichments (9 enrichments per O group).

Results: Variations were observed in bead-media combinations in the number of confirmations, levels of background colonies, and number of non-confirming suspect colonies present.  Overall BioControl (52%), Abraxis (46%), and Romer (45%) IMS beads confirmed a greater (P < 0.05) number of positives than the numbers confirmed by Dynal and LabM beads (41 and 43%, respectively).  However, within a serogroup, Dynal-O103, Abraxis-O111, and LabM-O121 beads confirmed more positive cultures than the respective BioControl beads.  Modified Rainbow and sheep’s blood agars confirmed the greatest numbers of samples overall (57 and 53%, respectively), which were significantly greater (P < 0.05) than R&F and STEC differential agars (44 and 38%, respectively). Within serogroup differences were again observed, with equal numbers of O45 confirmed on R&F STEC agar as on modified Rainbow agar.

Significance: The optimal IMS bead and selective media combinations have been identified for use in STEC culture confirmations. (Mentioning product names neither constitutes guarantee/warranty, nor implies approval to exclusion of others.)