Chapter 16 Review of media for the isolation of diarrhoeagenic Escherichia coli

Abstract

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 enterohaemorrhagic E. coli (EHEC) of serogroup O157 from foods. However, at present no single isolation procedure is available for the recovery of all EHEC. 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 paper some comparative studies of the media described for EHEC, especially EHEC O157, are noted and the difficulties associated with the isolation and enumeration of these organisms considered. Modified trypticase 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 EHEC O157 cells require pre-enrichment in a non-selective broth. Methods for the isolation of EHEC O157 should include sorbitol MacConkey agar supplemented with cefixime and potassium tellurite as the most effective isolation medium for typical sorbitol-non-fermenting EHEC O157 and a second isolation medium not based on the fermentation of sorbitol but, for instance, on β-D-glucuronidase activity. Where the background flora is low, washed sheep blood agar supplemented with calcium (“EHEC agar”) may be used.