‘Unwashable’ bacteria hide in produce

PURDUE (US) — Washing the outside of produce may not be enough to remove harmful food pathogens. Researchers have discovered Salmonella and E. coli living inside plant tissues.

Two new studies in the Journal of Food Protection and Food Research International found E. coli 0157:H7 in tissues of mung bean sprouts and Salmonella in peanut seedlings after the plants’ seeds were contaminated with the pathogens prior to planting.

Researchers say the seeds could have been contaminated before or after planting through tainted soil or water.

“The pathogens were in every major tissue, including the tissue that transports nutrients in plants,” says Amanda Deering, postdoctoral researcher in food science at Purdue University and co-author of the studies.

Finding pathogens inside plants has been challenging because tests require slicing off pieces of the plants, which can move the bacteria from the outside to the inside or vice versa. It becomes difficult to know where a pathogen might have been before the plant was cut.

“The results are often imprecise because the methods allow bacteria to move,” says co-author Robert Pruitt, professor of botany and plant pathology.

Deering used a fixative to freeze the location of the bacteria in the plant tissues before slicing samples. Antibodies labeled with fluorescent dye were used to detect the pathogens, a process called immunocytochemistry.

“This shows us as close to what was in the plant when it was living as possible,” says Deering, who was able to count hundreds of bacteria in almost every type of tissue.

“The number of bacteria increased and persisted at a high level for at least 12 days, the length of the studies.”

Proper sanitization would eliminate Salmonella and E. coli from the surface of foods, but not inner tissues, but cooking those foods to temperatures known to kill the pathogens would eliminate them from inner tissues.

The U.S. Department of Agriculture’s Agricultural Research Service funded their work.

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