To that end, SDSU food microbiology professor Sanjeev Anand, PhD, and doctoral student Neha Neha, MS, are focusing specifically on the recovery potential of any Listeria cells injured in a variety of ways.
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Collaborating on the project is Gemechis Djira, PhD, an associate professor in the SDSU Department of Mathematics and Statistics, who is creating regression models.
“Our risk assessment models use both product matrix parameters and environmental considerations, such as storage temperature, duration, pH, and water activity, in addition to the potential levels of cross-contamination from the environment,” Dr. Anand relates.
This is important, Dr. Anand says, because recent cases of Listeria in frozen foods that have resulted in recalls, ranging from frozen vegetables to ice cream bars, have reinforced the need for better methods of gauging the risk of foodborne pathogen contamination in processing plants.
“Listeria contamination has been recently traced to niches in the food processing environment that harbor the bacteria,” he points out. “For example, Listeria contamination in one commercial ice cream plant was traced to bacteria on the spout of an ice cream freezer. This is not surprising, since Listeria is a cold-loving microorganism. Pasteurization and cooking kill this organism, but the bacteria can grow at temperatures 40 degrees Fahrenheit and above in refrigerators and can even survive freezing.”
Neha says that, although injured Listeria cells are not known to cause illness, they may have the ability to recover and repair themselves.
To better understand the risk from injured cells, she looked at the organism’s behavior in different types of ice cream mixes with total solid levels ranging from 36 percent to 45 percent. She spiked the samples with three levels of a nonpathogenic Listeria strain before pasteurization.
“Results showed that injured Listeria cells did not recover in the ice cream mix itself under the normal conditions of mix handling,” she reports. “Studies are currently underway to evaluate the influence of any handling abuse on the recovery potential of injured cells.”
To address the issue of cross-contamination in the manufacturing environment, the next step is to determine how Listeria builds up in the environment, what characteristics make this possible and how it resists cleanup. This phase of the study starts later in 2018, Neha notes.
“We want to understand what characteristics make it possible for Listeria to persist and recover,” Neha relates.
She is planning to do whole genome sequencing of the bacteria, with the goal of understanding the gene expression that leads to colonization. “This should be beneficial in comparing any resident strain of Listeria, which can form resilient biofilms in the harborage sites and is difficult to eradicate,” Neha says.
“If we gain more knowledge about injured cells and integrate that information among the variables for the product and the environmental site, we should be able to design more robust risk-assessment models,” Neha predicts. “Additionally, examining injured Listeria at a molecular level will help scientists worldwide design novel cleaning techniques that can eliminate the bacteria in the manufacturing environment and prevent their persistence.”
New Lab Assay
“Solus One Listeria is an assay offering next-day detection of Listeria species in environmental samples,” says Ray Wakefield, CEO of Solus. “This assay provides a negative or a presumptive positive result from a single enrichment step in less than 25 hours.”
The whole process is a selective enrichment followed by an immunoassay, Wakefield explains. “If you break this down, the enrichment incubation is 22 hours and the immunoassay is 2 hours and 45 minutes, hence a total of 24 hours and 45 minutes,” he elaborates.