Abstract:

We aimed to improve the selectivity of lauryl tryptose (LST) broth and HC (hemorrhagic colitis Escherichia coli strains) agar for the detection of E. coli O157:H7 in frozen ground beef samples. The selectivity of LST broth was superior to that of LST containing novobiocin (20 mg/l) (LST + n), but LST containing novobiocin, cefixime (0.05 mg/l) and tellurite (2.5 mg/l) (LST + nct) was found to be unsuitable as an enrichment medium. The sorbitol positive strains were significantly more resistant to novobiocin than were the sorbitol negative strains (P < 0.05). As a result, the enrichment of ground beef samples in a selective broth without antibiotics such as LST, and then spread plating a subsequent amount from this original enrichment culture and the first three 10-fold serial dilutions onto a rhamnose and sorbitol based selective medium such as cefixime-tellurite HC medium (CTR-HC) was found to be recommendable as a routine cultural method. Transferring the metallic sheen property of eosin methylene blue (EMB) agar to CTR-HC agar can represent an alternative to the available media, and this will shorten the analysis time by 1 day. Use of salicine or 4-methylumbelliferyl-b-D glucuronide appears unnecessary in routine cultural techniques.

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