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From: The eUpdate, 1.11.11

New Test May Speed Salmonella Detection

Method differentiates live and dead DNA

A lab test developed by researchers at the University of Missouri may speed the detection of Salmonella in poultry and eggs, generating accurate results in as little as five to 12 hours, compared to as many as five days, the time required by the most common current Salmonella identification methods.

Real-time polymerase chain reaction (PCR) is a tool commonly used by industry to detect pathogens in food, but it cannot tell whether the sample genetic material that it amplifies to detect and identify bacteria within a food source is living or dead. A dead Salmonella bacterium is of no danger to a consumer, so real-time PCR can yield a number of false positive results, delaying accurate identification of contaminated food.

To take advantage of PCR’s speed but improve its accuracy, Azlin Mustapha, PhD, an associate professor of food science at the University of Missouri in Columbia, added an extra step: She dyed the entire sample with a substance called ethidium bromide monoazide (EMA). Unlike live Salmonella cells, the dead ones absorb the dye, rendering them insoluble and undetectable by PCR.

“Dead cells have holes and pores within the membrane through which the stain can enter and then bind the DNA,” Dr. Mustapha said. “But if you look at a healthy cell, the membrane wall is usually intact, and the stain is unable to penetrate. That binding prevents the DNA of dead Salmonella cells from being amplified in the subsequent PCR step.”

Dr. Mustapha, who had previously developed a method to detect E. coli 0157 in beef, which has since been adopted by state agriculture officials, has received inquiries regarding the new test from health officials as well as industry in the U.S. and overseas. “The advantage of this method is that it’s rapid, allowing accurate testing for Salmonella before a product is shipped to the market,” she said.

“Processors and consumers will benefit from the speed and sensitivity of the new test's results.”

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