Washing salad greens thoroughly may not be enough to ensure that they are free of bacteria before eating them, according to food microbiologist Karl Matthews and colleagues at New Jersey#;s Rutgers University, for harmful bacteria can travel up the plant#;s roots to end up inside the leaves.

Matthews embarked on the new research after an outbreak of poisoning caused by the potentially deadly O157:H7 strain of Escherichia coli that was connected with pre-washed lettuce (lettuce washed after harvesting to reduce the risks of contamination).

The researchers grew lettuce in manure containing E. coli O157:H7, to see if the bacteria would be present inside the lettuce, as well as on the leaves. Once grown, they sterilized surface of the plants with bleach before testing for the bacteria, which they discovered was present in the plant#;s internal tissues.

The findings suggest that lettuce leaves could be infected with the deadly bacteria even if they do not physically come into contact with the bacteria – it could, for example, simply be present in the water used to irrigate the plants. Matthews warns in the Nature journal: “If you¹re using surface irrigation there¹s still a chance that the edible portion of the plant can be contaminated.”

The researchers suspect that the bacteria are able to enter the growing roots of the plants through small gaps.

Matthews does acknowledge that poisoning by pre washed salad leaves is rare, and explained that the concentration of bacteria is higher in the laboratory than in the fields; his group is now investigating whether other types of bacteria could contaminate salad plants grown on farms.