Comparative evaluation of different chromogenic/fluorogenic media for detecting Escherichia coli O157:H7 in food.

Escherichia coli O157:H7 is a serious and common human pathogen that can cause diarrhoea, haemorrhagic colitis, and haemolytic uraemic syndrome (HUS). This study evaluated the enrichment, detection and confirmation procedures for the isolation of E. coli O157:H7 from raw ground beef and raw drinking milk. The purpose of this investigation was to compare Rainbow Agar O157 (RB; Biolog, Hayward, USA), Biosynth Culture Medium O157:H7 (BCM O157:H7; Biosynth, Staad, Switzerland) and Fluorocult HC (HC; Merck, Darmstadt, Germany) with the conventional Sorbitol MacConkey Agar (SMAC, Merck) using mEC + n (raw ground beef) and mTSB + n (raw milk) enrichment media. Single-path GLISA test (Gold Labeled Immuno Sorbent Assay; Merck) was used as the confirmation test. Growth of 466 strains of gram-negative rods isolated from food samples and 46 known E. coli strains from type culture and other collections (34 E. coli O157:H7 strains and 12 serotypes other than E. coli O157:H7) was examined on the agar media. The E. coli O157:H7 strains could readily be isolated and recognized uniquely by their typical black/grey colonies on RB and blue/black colonies on BCM O157:H7. Examination of the 46 known strains of E. coli reference strains showed false negative results on BCM O157:H7 (3.0%), RB (8.8%), HC (5.9%) and SMAC (5.9%) agars. On BCM O157:H7 no false negative results were found with the typical E. coli O157:H7 (beta-D-glucuronidase and sorbitol negative strains). One of two atypical E. coli O157:H7 strains (beta-D-glucuronidase positive) showed similar colouration to the typical strains and was mis-identified by each of the three media (RB, BCM O157:H7, and SMAC agar media). None of the 60 food samples tested yielded E. coli O157:H7. Examination of the food samples, showed that RB gave the lowest number of false positives. The percentages were RB (2.1%), BCM O157:H7 (3.3%), HC (6.2%), and SMAC (57.3%).