Other tests rely on DNA markers, but these also can take days to process, he said. That's a problem because by the time test results come back, products may already be in food suppliers' warehouses or on store shelves, he said.
Last summer, for example, a Georgia company recalled nearly 37,000 pounds of precooked chicken products that may have been contaminated by Listeria. The chicken products had been distributed to warehouses in Georgia and Arkansas, as well as to grocery stores in Maryland and New York, when the recall was issued.
"To overcome the time delay and allow for rapid detection before foods are shipped, you need to be able to detect a lower number of the pathogen cells at the processing plant," Geng said.
The ability to detect L. monocytogenes at low levels is essential because most of the foods susceptible to Listeria contamination are ready-to-eat products, which are cooked or otherwise processed for human consumption before they make it to a grocer's shelves.
"Since precooked meats have already been processed, the bulk of microorganisms that were present in the raw product have been eliminated," Bhunia said.
"We don't expect high numbers of microorganisms in processed products, so we need to be able to detect extremely low levels of contamination."
Detection at low levels also is important for another reason, Bhunia said.
"Listeria can grow at refrigeration temperatures, so if a product has a level of Listeria low enough to evade detection when it's tested at the processor, that Listeria still can grow in the home refrigerator to a level that makes it infective to people at risk."
Contact: Jennifer Cutraro