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Harmful bugs may lurk within leaves

Healthy salad greens could be contaminated with bacteria that cause food poisoning, despite thorough rinsing. New research shows that harmful bugs can enter a lettuce plant through its roots and end up in the edible leaves. Although uncommon, food poisoning caused by eating plants can occur. Vegetables that are fertilized with animal manure, which can contain pathogens, pose the biggest threat. Raw salad vegetables are now washed after harvesting to reduce the risk of contamination.

Food Microbiologist Karl Matthews and colleagues at Rutgers University in New Jersey investigated whether bacteria were getting inside the lettuce, rather than just sitting on the leaves. The team grew lettuce in manure inoculated with E. coli O157:H7.

After sterilizing the plant's surface with bleach, the researchers still found bacteria within the internal tissues that are for water transport. Lettuce leaves could be infected by simply irrigating plants with bacteria-inoculated water, despite the fact that foliage did not come into direct contact with the water. Small gaps in growing roots are a known port of entry for plant pathogens, and may allow E. coli to get in.

"If it is found in saleable plants, it presents us with a new vehicle for E. coli O157:H7," says Tom Cheasty, who directs E. coli O157:H7 surveillance at the UK Public Health Laboratory Services in London. But he adds that, for the moment, there is no compelling reason to treat salad with suspicion.
Maanda Mandavha

Reference: Hilborn, E D. et al. A multistate outbreak of E. coli O157:H7 infections associated with consumption of mesclun lettuce.

Published by the Directorate: Food Control
Department of Health
Private Bag X828
0001 Pretoria

 

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