The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms.  A new study by scientists at Purdue University found that E. coli bacteria thrive in this region, critical to plant growth.

Scientists conducted the study by adding E. coli bacteria to soil by applying manure and manure-treated water to the rhizosphere of lettuce and radish plants.  The study showed that the E. coli bacteria eventually made their way to the aboveground surfaces and leaves of the plants, where it can live for up to 40 days.  Scientists observed bacterial activity in the rhizosphere using a bioluminescent E. coli created by Bruce Applegate that glows when active. Applegate, a co-author on the project, is an associate professor in the food science and biological sciences departments at Purdue.

"E. coli is actually quite active in the rhizosphere. They’re eating something there – probably plant exudates," said Ron Turco, a professor of agronomy and co-author of the study published in the November issue of the Journal of Food Protection.  "In actual field application, you pick up other things that are all around," Turco said. "You don’t just get the plants that are 40 days old. An animal getting loose in a field could also contaminate plants."

Mussie Habteselassie, Turco’s former postdoctoral researcher and now an assistant professor of soil microbiology at the University of Georgia’s Griffin campus, said harvesting practices in manure-treated fields can be critical for produce crops.

"If you harvest young and old plants together or mix them after harvesting, there is risk of contamination of the older plants," Habteselassie said. "If plants are uprooted during harvest, there is also a possibility of contamination from E. coli living in the rhizosphere."

Lessons from the study:  "Producers should apply manure to fields well in advance of planting and harvesting. Turco said a wait of 90-120 days between manure application and harvesting, with a minimum of 40 days between planting and harvesting, should minimize the chance of E. coli contamination."