A new method for detecting disease-causing bacteria in food could eventually help keep tainted products from landing on diners’ plates.
The new test was able to detect much lower levels of Salmonella bacteria in beef and poultry than two commonly used methods. It also proved equally accurate at identifying disease-causing strains of the organism while ignoring benign ones that may have been present. The research was published in a recent issue of the Journal of Food Safety.
“People think that nowadays pathogen testing has reached its limit or it’s already pretty much saturated, but there are still many things we could do to improve this technology,” said Dr. Soohyoun Ahn, an assistant professor of food science at the University of Florida and the study’s senior author.
Bacteria and other microbes in food cause an estimated 48 million illnesses in the United States each year, according to a 2011 report from the U.S. Centers for Disease Control. Ten percent of the infections with a known cause were due to Salmonella, leading to roughly 19,000 hospitalizations and 378 deaths, more than any other food-borne contagion. Those with weakened immune systems, such as the elderly and children under age five, are particularly likely to require medical intervention.
Ahn’s method adds two more steps to a standard test called an ELISA that is used to detect substances such as bacterial cells in laboratory samples. Ahn first added a process already employed in food testing that uses a magnet to bring together magnetic beads that have picked up bacteria from the sample. This technique, known as immunomagnetic separation (IMS), concentrates the bacteria and allows the test to detect smaller populations. Her method also utilizes another sensitivity-boosting procedure called tyramide signal amplification that is not currently used for testing food products.
“Tyramide signal amplification is a very robust detection method, and I am not surprised that it was demonstrated to be more sensitive than prior methodologies,” said Dr. Kevin Roth, head pathologist at the Columbia University Medical Center and editor-in-chief of the American Journal of Pathology. Roth, who was not involved in the study, added that it is “capable of enhancing signals by orders of magnitude over conventional detection methods.”
Ahn first detailed her procedure in a 2014 paper in the Journal of Food Protection, which showed that it could detect much lower levels of E. coli bacteria in beef and milk compared to the standard assay with IMS but no signal amplification.
The new study focused on reducing the amount of time that Salmonella bacteria from food samples need to grow before Ahn’s test can detect them with high accuracy. Bacterial samples taken from food are usually allowed to grow before tests are run, a process known as enrichment. A more responsive method that can detect smaller amounts of bacteria would require less enrichment time, and her test was able to achieve the same sensitivity with just 12 hours of enrichment as ELISA did with 24 hours.
Because properly cooking food kills bacteria, detecting Salmonella in raw meat is not necessarily a cause for concern. But manufacturers of ready-to-eat meals containing meat have to be more vigilant, and while they test their products before shipping them, no test is perfect. A more sensitive method like Ahn’s could identify food containing lower levels of all sorts of bacteria, thereby catching more contaminated products before they reach grocery stores.
New methods of testing require extensive validation, so if Ahn’s technique is eventually adopted, it will not happen anytime soon. If it is one day approved, it would be added to the arsenal of testing methods used by food manufacturers and the government.
“Pathogen detection is a really sensitive issue,” Ahn said. “We are talking about public health.”