Culture Media for the Isolation of Diarrhoeagenic Escherichia coli from Foods

The species Escherichia coli contains both diarrhoeagenic and non-diarrhoeagenic strains and it is very important to have methods available which can differentiate between them. Adequate culture methods have been developed for the isolation of verocytotoxin-producing E. coli (VTEC) of serogroup 0 157 from foods. However, at present no single isolation procedure is available for the recovery of all VTEC causing severe human disease. Additionally, there are still no simple sensitive procedures available for the direct cultivation of strains of the other groups of diarrhoeagenic E. coli. The isolation of these organisms will best be accomplished by a combination of culture and molecular biological methods. In this review, some comparative studies of the media described for VTEC, especially VTEC O157, are noted and the difficulties associated with the isolation and enumeration of these organisms considered. Modified tryptone soya broth supplemented with novobiocin or modified E. coli broth supplemented with novobiocin and incubated at 41–42°C are the most appropriate selective enrichments. Injured VTEC O157 cells require pre-enrichment in a non-selective broth. Methods for the isolation of VTEC O157 should include sorbitol MacConkey agar supplemented with cefixime and potassium tellurite as the most effective isolation medium for typical sorbitol non-fermenting VTEC O157, and a second isolation medium not based on the fermentation of sorbitol but, for instance, on β-D-glucuronidase activity. Where the numbers of background flora are low, washed sheep blood agar supplemented with calcium (“EHEC agar”) may be used.