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

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.

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.

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.

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.