Researchers from the University of Missouri are hopeful that a newly developed lab test will detect live Salmonella in poultry and eggs with faster and more accurate results.
Currently, the most common testing method used throughout the food industry to identify Salmonella contamination can take up to 5 days to produce results. Consequently, contaminated food could already be present in grocery stores before Salmonella test results are available. In light of the recent recall of more than half a billion eggs from Wright County Egg and Hillandale Farms in August that sickened hundreds, 5 days could simply be too long to wait for results.
However, in recent research trials employing the new test at the University of Missouri, accurate results were achieved rapidly, in as little as 5 to 12 hours.
Azlin Mustapha, a food scientist and professor at the University of Missouri College of Agriculture, Food and Natural Resources, along with her graduate student, Luxin Wang, developed the new test for Salmonella by modifying a technique called real-time polymerase chain reaction (PCR). The technique has been available for use by the food industry for years and considered to be a quick and reliable method for identifying pathogens in food.
According to Mustapha, traditional PCR technology involves first amplifying a single gene found within the DNA of a particular organism, such as Salmonella bacteria. After being magnified, that specific part of the DNA is then copied thousands to millions of times. This multiplication of genetic material allows for easier detection and identification of bacterial gene sequences in bacteria using visual techniques.
However, the PCR process as it is currently used often produces false-positive results, which could lead to costly but unnecessary recalls of poultry and eggs. The problem, Mustapha states, is that PCR and other “DNA-based methods … do not differentiate between the live and the dead Salmonella.” Thus, the failure of the PCR technique to recognize this distinction has skewed results.
She further explained that “live Salmonella are the ones that can kill consumers, not the dead ones, but false-positives can result in a large number of unnecessary food recalls.”
In order to take advantage of the speed of the PCR method while reducing the frequency of false-positives, Mustapha modified the PCR test by adding a dye called ethidium bromide monoazide (EMA) to the test sample.
EMA enters and is absorbed by dead Salmonella cells where it binds to DNA molecules. Mustapha explains that the absorption of the dye renders the dead Salmonella cells insoluble and, therefore, invisible to a visual study of PCR test results.
In contrast, the dye cannot penetrate live cells. Accordingly, the modification of the traditional PCR testing procedure allows food scientists to easily distinguish between dead and live cells, avoiding false-positive test results.
Earlier, Mustapha developed a similar method to detect E. coli O157 in beef products that has since been adopted by the Missouri Department of Agriculture testing laboratory. Several other testing agencies, including the Food Safety and Inspection Service of USDA, have expressed an interest in implementing the new method in their own laboratories.
By using the modified PCR method, scientists can now quickly isolate the live Salmonella calls, dramatically reducing the overall testing time. “The advantage of this method is that it’s rapid, so you can get the results within 12 hours versus 5 days with the traditional method,” Mustapha noted.
The impetus behind the research was to discover Salmonella contamination as quickly as possible before it enters the food supply, thereby preventing recalls and ensuring the safety of consumers. Mustapha believes that having test results available within 12 hours would enable agencies as well as food companies to accurately test for Salmonella before a product is shipped and put on the market.
Agencies and companies seeking to implement Mustapha’s modified testing method will have to make an initial capital investment in a PCR machine and train personnel to use it. However, Mustapha emphasized that over time, the PCR method will result in significant savings since it requires less labor and time than conventional testing techniques.
According to the Centers for Disease Control and Prevention (CDC), Salmonella bacteria is one of the most common causes of food poisoning in the United States. Salmonellosis, the disease caused by Salmonella, causes diarrhea, vomiting, fever, abdominal cramps and, in severe cases, death.
The CDC estimates that there are 1.4 million Salmonella infections each year. In addition, approximately 400 deaths are caused by acute salmonellosis annually.
PCR technology may serve to detect potential pathogens earlier in the production process allowing for greater food safety in the supply chain.
“Processors and consumers will benefit from the speed and sensitivity of the new test’s results,” said Mustapha. She continued, “This will keep companies from shipping contaminated products, and thus, keep Salmonella infected products out of consumers’ hands.”