Detection of Escherichia coli Serotype O157 in beef carcasses and faecal material.

The aim of this paper is to describe a methodology applicable to the isolation of viable Escherichia coli O157 from beef carcasses and faecal samples and specifically to overcome problems of background flora outgrowth, which can reduce sensitivity of the test. 90 samples were collected in a slaughterhouse of province of Ravenna from December 2001 to January 2003, namely carcass samples and rectum content of 45 animals. 2 x 2.5 cm fragments (tick < 0.5) were excised aseptically from rump, flank, brisket and neck; faecal material was collected using a spoon after incision of rectum in the tripery. The entire pool of tissue fragments from each carcass (equivalent to 20 cm2) and 25 grams of faecal material were used to isolate Escherichia coli serotype O157 with the Laboratory Procedures MFLP-80 and MFLP-90 (Directorate's-Health Canada's). Faeces from cattle contain an huge number of bacteria, other than E. coli, that can survive/multiply in presence of Tryptone Soya broth added with bile salts and novobiocin (20 mg/L). Immunomagnetic enrichment procedure has proved useful to capture specifically E. coli O157, but other organisms were found in a great number, thus reducing the sensitivity of the test; besides the diffusion of the fluorescent pigments in the Phenol Red Sorbitol Agar with MUG made impossible to detect beta-glucoronidase-negative colonies. HC agar with MUG proved to be more selective so it decreased background bacteria outgrowth. Besides the use of sterile swab instead of glass spatula and the technique of streaking helped very much in detection of isolated colonies on both agar media.