September 14, 2009

Food scientists from the University of Missouri have developed a new technique to detect live E. coli cells in ground beef.

The two-step technique, called a real-time polymerase chain reaction (PCR), is a quick and reliable method for detecting and identifying pathogens in food.

The researchers have successfully tested the technique on ground beef, chicken and eggs. Testing takes about 12 hours, as opposed to older methods, which require up to two days for results.

PCR is unable to differentiate live from dead microbial cells, and the detection of the harmless dead cells may result in false-positive findings, which could lead to unnecessary product recalls, said the scientists.

But real-time PCR, which is used to amplify and simultaneously quantify a targeted DNA molecule, enables both detection and quantification of a specific sequence in a DNA sample.

The procedure follows the general principle of polymerase chain reaction; its key feature is that the amplified DNA is quantified as it accumulates in the reaction in real time after each amplification cycle.

To prevent false-positive findings, researchers stain samples with a dye called ethidium bromide monoazide (EMA), which cannot penetrate live cells but can enter dead cells where it binds to DNA molecules, making them insoluble and therefore invisible to PCR tests, said Azlin Mustapha, associate professor of food science at U of M.