Abstract:

A differential agar for the further detection of presumptive Escherichia coli O157 strains among typical sorbitol negative and cefixime-tellurite resistant strains was investigated. For this purpose, in parallel to colonies of 8 reference strains, 1130 suspect colonies picked from the surface of cefixime-tellurite (CT)-sorbitol MacConkey (SMAC) (CT-SMAC) agar plates of food samples (white cheese, raw milk and ground beef) were transferred to differential agar plates. After a 6-h incubation period at 42 ºC, only 47 (4.15%) of the 1130 colonies appeared as typical E. coli O157 on the differential agar plates (100 x 15 mm) gridded into 25 numbered sections. Salicin (S), rhamnose (R) and cellobiose (C) were tested separately and together in combination with MUG. The best results were obtained from the combined use of the 3 in SMAC medium (SRC-MUG-SMAC). When violet red bile lactose agar (VL) was used in the same manner as SRC-MUG-SMAC, only 16 (1.41%) of the 1130 colonies remained typical. In this procedure, no biochemical test was required for further identification. The use of an SRC-MUG-SMAC plate immediately after the colony selection step appeared to be more discriminative than the addition of these sugars and MUG to the primary isolation media. As a result, the testing of presumptive E. coli O157 colonies using a differential agar plate including MUG and 3 (or more) sugars that are not fermented by this organism is a rapid, easy, precise and relatively inexpensive isolation procedure, and it is particularly ideal for use in routine screening laboratories.

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