This image shows colonies of Listeria cornellensis growing on an Oxford medium, one of the media typically used for isolating Listeria. (Credit: Henk C. den Bakker)

5 new kinds of Listeria may make food safer

The discovery of five new species of a group of bacteria called Listeria could lead to better ways to detect pathogens in food.

To date, of the 10 previously known species of Listeria, only two are pathogenic to humans. Listeria monocytogenes is the main cause of Listeriosis, which causes illness in more than a thousand people—and death in nearly 250—each year in the United States through infected deli meats, seafood, and produce.

The new study, published online in the International Journal of Systematic and Evolutionary Microbiology, suggests that all five new species are benign.

The research was part of a larger study to examine the distribution of such foodborne pathogens as Listeria, E. coli, and Salmonella in agricultural and natural environments. Samples were taken from fields, soil, ponds, and streams in New York, Colorado, and Florida.

“Doing studies on natural diversity in produce fields helps us develop better and more precise tests to make food safer,” says Martin Wiedmann, professor of food science at Cornell University and the paper’s senior author.

To identify the five new species, researchers applied molecular biology methods, including full genome sequencing. The findings have implications for understanding the evolution of what makes Listeria monocytogenes pathogenic.

“The most recent common ancestor (of L. monocytogenes and closely related nonpathogenic species) was a pathogen, and that makes it difficult to reconstruct the evolution of pathogenicity in Listeria,” says Henk den Bakker, a research associate in Wiedmann’s lab and the paper’s first author.

But the five new species add more evidence to the existence of four distinct evolutionary branches of Listeria.

“Now we see the evolutionary tree has a couple of new branches, which gives us a nice dataset to reconstruct what happened on a genomic level during the evolutionary transition from a free living ancestor to a pathogen,” den Bakker says.

The study has economic implications, since tests to identify Listeria cost about $25 each, and large food and produce plants run thousands of tests, spending up to half a million per year. To identify L. monocytogenes, food producers and researchers first look for the presence of more common Listeria species.

But tests may only work to identify Listeria species that are very similar to L. monocytogenes.

“If you find a species that grows at different temperatures, or feeds on different sugars, then running the same test is not a good idea,” Wiedmann says. “This helps us to develop better, rational tests that makes our food supply safer.”

The US Department of Agriculture funded the study.

Source: Cornell University

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