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Tracking a “Bad News” Bacterium

Microbiologist Manan Sharma (center) and student Sean Ferguson (left) use a specialized microscope to observe whether fluorescent E. coli cells are internalized into roots of baby spinach plants, while microbiologist David Ingram (right) prepares spinach tissue for observation.

Microbiologist Manan Sharma (center) and student Sean Ferguson (left) use a specialized microscope to observe whether fluorescent E. coli cells are internalized into roots of baby spinach plants, while microbiologist David Ingram (right) prepares spinach tissue for observation.

This post is part of the Science Tuesday feature series on the USDA blog. Check back each week as we showcase stories and news from USDA’s rich science and research portfolio.

Here’s a vision fit for a nightmare:  a “family reunion” of every type of Escherichia coli.

But in fact, there are hundreds of strains of E. coli, including ones that live in our intestines and help us digest our food.  Only a few types, like E. coli O157:H7, cause foodborne illness.

Still, food safety experts know it’s possible for some of these disease-causing strains to survive in the environment and contaminate leafy greens grown in contaminated soil.  That’s why scientists with USDA’s Agricultural Research Service (ARS) tackled the task of pinpointing the odds for possible internal contamination of a plant via its root system.

They modified several E. coli strains to contain a gene for fluorescence, which allowed them to track the pathogen’s travels in baby spinach plants.  They put the fluorescence gene in a specific location within the chromosomal structure of the E. coli where it wouldn’t interfere with any essential metabolic functions or stress responses of the cells.  This meant the cells were more likely to survive and glow under stressful conditions in the plants.

The scientists confirmed that disease-causing E. coli could survive in the soil for up to 28 days, and that the E. coli cells were able to migrate into the roots of the spinach plants.

They also examined baby spinach plants over 28 days after germination to see whether any of the E. coli strains were taken up past the roots and into the plant’s interior structures.

The bottom line:  At day 28, there was no evidence that the E. coli had been “internalized” in the leaves or shoots of the spinach plants grown in soil.  This means that although E. coli can survive in soil, it’s highly unlikely that foodborne illness would result from the bacterium becoming internalized through plant roots into leafy produce.

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