LINDELL BEACH, B.C. – Producers can reduce the risk of contaminating their crops with E. coli by not harvesting them too early, researchers have found.
“E. coli is actually quite active in the rhizosphere,” said Ron Turco, an agronomy professor at Perdue University who co-wrote a study published this winter in theJournal of Food Protection.
“Any kind of growing plant has a rhizosphere, a region around itself where there are plant carbon materials the plant creates, so E. coli is quite capable of functioning in these regions.”
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He said E. coli is able to live for several weeks around the roots of plants and transfer to the edible portions.
His team added E. coli to soil by applying manure and water treated with manure around the roots of lettuce and radishes.
They observed E. coli’s activity in the plants’ rhizosphere by using a bioluminescent strain that glows when active.
These marked strains allowed researchers to observe bacterial colonization in its natural environment.
E. coli eventually get into above-ground edible surfaces as they colonize the soil around the plant. They can live there for several weeks but die off as the plant matures.
Researchers treated radish and lettuce seeds with E. coli, grew them in an agar-based system and then watched the bacteria rapidly colonize the germinating seeds. The same rapid colonization was seen in soil fed with manure.
E. coli was detected on the edible parts of lettuce within 15 days after establishing in the rhizosphere.
However, the bacteria was not detected on the plants after 27 days, even though it persisted in the bulk soil and the rhizosphere.
“The E. coli die off to a pretty low number after 40 days,” said Turco.
“They do well on young plants, but as the plant ages and the roots dry up, they drop off. If you mix younger plants with older ones, you spread E. coli (to all plants). You need to package plants with pretty similar ages, especially in those pre-packaged mixes.”
E. coli thrive in the rhizosphere because they likes the warmth caused by close proximity to the growing surface and the nutritious plant material leaking from the roots.
Deeper soil is lower in nutrients.
Producers should apply manure to fields well in advance of planting and harvesting. Turco suggested a wait of 90 to 120 days between manure application and harvesting and a minimum of 40 days between planting and harvesting.
However, Turco warned that contamination can also come from other sources. Animals moving through a field, such as dogs, loose livestock and wild animals, can also infect plants.
Turco stressed the importance of not mixing young and old plants and ensuring cleanliness in all stages of seed selection, planting, growing and harvesting
Mussie Habteselassie, an assistant professor of soil microbiology at the University of Georgia, 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 a risk of contamination of the older plants,” said Habteselassie.
“If plants are uprooted during harvest, there is also a possibility of contamination from E. coli living in the rhizosphere.”
Turco is also researching E. coli’s ability to survive in different situations, including water and processed produce.
