Immunomagnetic Method Helps Isolate E Coli

A combination of a newly developed immunomagnetic separation method with a specific chromogenic substrate medium provides a highly sensitive solution for the isolation of Escherichia coli O157:H7.

The new method, called Captivate O157, uses specific antibody-coated magnetic beads for the isolation of E coli O157:H7 from pharmaceutical, food, animal feed, beverage, or environmental samples. This strain is one of the most important food-borne pathogens of animal origin. Associated primarily with self-limiting diarrhea, it can also lead to much more serious conditions. Conventional isolation methods can give rise to false-negative results due to the problem of competing flora obscuring E coli O157:H7 colonies.

Captivate O157 was developed by International Diagnostics Group Ltd. (IDG, Bury, UK). The recommended protocol employs enrichment in modified tryptone soy broth (MTSB), followed by separation with Captivate prior to sub-culture of the O157-bead complex on sorbitol MacConkey agar, supplemented with cefixime and potassium tellurite. E coli O157:H7 appear as pale translucent colonies on this medium. Their identity can be confirmed using latex agglutination kits or appropriate antisera.

Immunomagnetic separation offers a way of rapidly extracting E coli O157:H7 from non-target bacteria, resulting in improved rates of isolation. Other benefits include a 10-fold concentration of E coli O157:H7 and easy-to-visualize separation of beads. The Captivate separation is highly strain-specific and results in less than 1% carry-over of non-O157 E coli-H7 from mixed cultures.



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