Recovery of E. coli O157 strains after exposure to acidification at pH 2.

AIMS

Rapid detection and selective isolation of E. coli O157:H7 strains have been difficult owing to the potential interference from background microflora present in high background food matrices. To help selectively isolate E. coli O157H7 strains, a useful plating technique that involved acidifying the cultures to pH 2 was evaluated with a large number of E. coli O157:H7 strains to ensure response to treatment was consistent across strains.

METHODS AND RESULTS

Escherichia coli O157, 46 strains including ATCC 35150, were acidified to pH 2 following enrichment and plated onto Tryptic Soy Agar + 0.6% Yeast Extract (TSA-YE) and Sorbitol MacConkey Agar + cefixime and tellurite (CT-SMAC). Samples were enumerated and modest decreases in plate counts were observed on TSA-YE media, with a greater reduction observed on CT-SMAC.

CONCLUSIONS

The acid-resistant character of E. coli O157:H7 is a consistent trait and may be used for improved isolation of the organism from mixed cultures.

SIGNIFICANCE AND IMPACT OF THE STUDY

There was little difference observed between the commonly used laboratory strain E. coli O157:H7 35150 and 45 other strains of E. coli O157 when subjected to acidifying conditions prior to plating, demonstrating that an acid rinse procedure was equally effective across a wide variety of E. coli O157 strains and broadly applicable for isolating unknown strains from food samples.