P3-07 Comparative Evaluation of a New Selective Chromogenic Agar for the Isolation of Non-O157 STEC Serovars in Conjunction with Immunomagnetic Separation and Acid Enrichment

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Hari Prakash Dwivedi, bioMérieux, Hazelwood, MO
Gregory Devulder, bioMérieux, Hazelwood, MO
Jennifer Bick, bioMérieux, Hazelwood, MO
Introduction: Non-O157 Shiga toxin-producing Escherichia coli (STEC) are one of the most important foodborne pathogens in meat products. The selective isolation of non-O157 STEC colonies on agar media is critical for their confirmatory identification. chromID EHEC® is a new selective chromogenic agar for the selective isolation of non-O157 STEC colonies from foods.

Purpose: Comparative evaluation of chromID EHEC agar and modified Rainbow® agar for the selective isolation of non-O157 STEC from artificially inoculated beef samples.

Methods: A panel of 30 strains of O26, O45, O103, O111, O121 and O145 serovars was evaluated for their selective isolation on this new chromogenic media and compared to modified Rainbow agar. Briefly, 375 g samples of fresh raw beef were spiked with 25-100 CFU of different strains and following enrichment either immunomagnetic separation (IMS) or a combination of IMS and acid treatment was performed according to USDA-MLG guidelines. Isolation of non-O157 STEC colonies was performed on both media followed by their confirmation using PCR targeting stx and eaegenes.

Results: Sensitivity of the new chromogenic agar was 100% following the IMS and acid treatment as all tested strains were successfully isolated from the beef samples. Confirmed colonies for the tested strains ranged between 28-100% for the new media and 0-100% for modified Rainbow agar. Some of the strains such as E. coliO103:H11 (BAA-2215) were not recovered on modified Rainbow agar. Acid treatment was helpful in enhancing the specificity of the non-O157 STEC isolation procedure for both media.

Significance: Due to high sensitivity, desired specificity and differential colony coloration, the chromID EHEC agar could be implemented for the selective isolation of non-O157 STEC from foods. The new chromogenic media supported the growth of all non-O157 STEC strains studied ensuring their recovery from contaminated food samples.