Technique detects wide array of pathogens
(PCR), scientists have developed a molecular-based method for
checking food safety.
The technique distinguishes live bacterial cells from dead ones, allowing food safety checkers to identify a whole suite of pathogens in food.
Food contamination scares, regulatory measures and the cost of recalls have driven the demand for better pathogen testing equipment in the market.
Microbiologist Robert Levin and doctoral student Shishan Wang at the University of Massachusetts Amherst say the method could help food processors avoid the costs of having to make massive recallsof meat carrying such pathogens as E. coli.
The new method uses a variation of PCR techniques, which scientists use to make lots of copies of a small, specific stretch of DNA. PCR generates large quantities of DNA from tiny samples. It canbe used to detect very small quantities of pathogens.
However PCR just copies the designated DNA. It doesn't indicate whether the DNA came from a cell that was dead or alive, information that food testers need when testing samples for organisms thatmake people sick.
Using PCR, the researchers developed a technique to test seafood for the DNA of Vibrio vulnificus, a disease-causing bacterium from the same family as those that cause cholera.
Levin and Wang treated their bacteria samples with ethidium bromide monoazide (EMA), a chemical that winds its way in between the strands and building blocks of a DNA molecule. EMA will insertitself only into the DNA of the damaged cell membranes of dead or dying bacterium.
After dosing the bacteria with EMA, the researchers zapped their samples with high-intensity visible light. This caused the EMA to form strong, cross-linking bonds with the dead DNA it got tangledup in.
The bonds prevent the DNA molecules from separating, so they can't be copied during PCR. Only DNA from live cells will be copied, alerting the testers to the presence of living bacteria.
"Once you've determined the optimum concentrations of EMA you can completely inhibit amplification of DNA from dead cells," Levin stated in research published in the Journal ofMicrobiological Methods.
The scientists have worked out the protocols for testing for V. vulnificus. The method can be used to test for other harmful bacteria after some minor adjustments are made, they say.
