Direct inoculation into media containing bile salts and antibiotics is unsuitable for the detection of acid/salt stressed Escherichia coli O157:H7

The efficiency of selective enrichment broths for the recovery of low numbers of acid/salt stressed Escherichia coli O157:H7 was determined. Stressed cultures were diluted to low levels and recovered in tryptone soya broth with added bile salts, to make modified tryptone soya broth, and buffered peptone water with various combinations of antibiotic supplementation including novobiocin, acriflavine and a mixture of vancomycin, cefsulodin and cefixime (VCC) at 37 °C and 42 °C. Significantly fewer stressed cells, in some cases as little as 0·3% of the starting population, were recovered by all the selective enrichment broths containing bile salts or VCC antibiotics compared to the non-selective controls. The use of such enrichments to recover low numbers of stressed E. coli O157:H7 may result in failure to detect the organism. Parallels with salmonella methodology are made and the need for a non-selective pre-enrichment stage in E. coli O157:H7 methods discussed.