Evaluation of Bile Salt No. 3, Novobiocin, and Cefixime-Tellurite in Enrichment and/or Selective Plating Media for Detection of Escherichia coli O157:H7 from Ground Beef


Abstract: Efficacy of commonly used selective supplements for detection of E. coli O157:H7 from ground beef was evaluated for suppression of sorbitol positive flora while supporting recovery of sorbitol negative flora. Selectivity of bile free modified E. coli broth (b-mEC) was not improved by the single addition of bile salts no. 3 (1.12 mg/ml), novobiocin (20 µg/ml), cefixime (0.05 µg/ml)-tellurite (2.5 µg/ml), or their combinations. The sorbitol positive flora was more resistant to the selective agents than sorbitol negatives. Sorbitol MacConkey agar and E. coli O157:H7 agar had almost equal selectivity. Adding cefixime (0.05 µg/ml)-tellurite (2.5 µg/ml) to the selective plating media highly enhanced the selectivity, and a general sorbitol based plating medium without cefixime-tellurite is not appropriate for this purpose due to its nonselectivity. As a result, a bile-free enrichment broth such as b-mEC and a sorbitol based selective plating medium containing cefixime-tellurite appeared to be reliable.

Keywords: E. coli O157:H7, ground beef, bile salt, novobiocin, cefixime-tellurite

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