Knowing that the pathogen occurs naturally in soil, the researchers asked hundreds of scientists across the country to collect soil samples from generally undisturbed places in the natural world, such as the off-trail areas of state and national parks. From these samples, the group developed a nationwide atlas of 1,854 listeria isolates, representing 594 strains, and 12 families of the bacteria, called phylogroups.
“As we’re trying to figure out the risk of getting Listeria from soil and different locations, our group created a more systematic way of assessing how frequently different Listeria are found in different locations,” says senior author Martin Wiedmann, PhD, a food safety and food science professor in the Cornell University College of Agriculture and Life Sciences in Ithaca, N.Y. “We’ve studied Listeria in places as diverse as New York, Colorado, and California, but before this atlas, [it] was difficult to make comparisons and assess Listeria diversity in different locations.”
Lead author Jingqiu Liao, PhD, who worked in Dr. Wiedmann’s laboratory as a graduate student, is now a post-doctoral researcher at Columbia University. She found Listeria present across a wide range of environmental circumstances. This bacterium is controlled mainly by soil moisture, salinity concentrations and molybdenum—a trace mineral found in milk, cheese, grains, legumes, leafy vegetables, and organ meats.
“The goal of this work was to systematically collect soil samples across the United States,” says Dr. Liao, “and to capture the true large-scale spatial distribution, genomic diversity, and population structure of Listeria species in the natural environment.
“With whole genome sequencing and comprehensive population genomics analyses,” Dr. Liao says, “we provided answers to the ecological and evolutionary drivers of bacterial genome flexibility—an important open question in the field of microbiology.”
Dr. Liao says that this work can serve as a reference for future population genomics studies and will likely benefit the food industry by locating Listeria contaminations that may have a natural origin.
If the pathogen is found in a processing facility in the western U.S., for example, and that facility had used ingredients from a distant state, Dr. Wiedmann says, “Knowing the genomic information of Listeria isolates and their possible locations across the U.S., we can better narrow the origins to a specific region. You can use this information almost like a traceback; it’s not always proof, but it leads you to evidence.”
