Method to Isolate Escherichia coli O157:H7 from Food

Abstract

A medium for direct overnight isolation of hemorrhagic colitis (HC) strains of Escherichia coli from foods has been developed. It is a direct plating medium incorporating tryptophan (as tryptone), sorbitol, an indicator dye and a fluorogenic substrate for diagnostic purposes. Sodium chloride is included to raise the upper temperature limit for growth and a bile salt concentration lower than that usual for E. coli media but still inhibitory to non-enteric organisms is used. Colonies of HC strains grown overnight at 44.5°C on membrane filters placed on the medium are blue. Subsequent indole staining of the membranes yields red positive colonies. Replicate colonies are confirmed serologically. In contrast, E. coli Type I colonies are yellow and give apparently negative indole reactions. Recovery of HC organisms from artificially contaminated ground beef is ≥90%.