31 May 2011

New research conducted at a Scottish research institute has given rise to new revelations on how food-borne illnesses are increasingly associated with vegetables and salads. The recent E-coli outbreak in Germany, which was linked to infected cucumbers and has so far claimed 10 lives, could be an indication that fruit and vegetables are ingesting these bacteria as they grow, according to one scientist.

Dr Nicola Holden of The James Hutton Institute, near Aberdeen, is leading research into the presence of bacteria such as Escherichia coli O157:H7 and Salmonella enteric, which cause food poisoning, in fresh fruit and vegetables and has found that washing alone may not be enough as the bacteria can be inside the food itself.

Dr Holden explained, “Food-borne illness has traditionally been associated with undercooked meat products and eggs. However, we have seen a recent rise in the number of outbreaks associated instead with fresh fruit and vegetables; in particular foods that are eaten raw or lightly cooked, like salad vegetables, fresh fruit and sprouted beans. The outbreaks, which are thankfully still very unusual, have been caused by bacteria that we are already familiar with, such as Escherichia coli O157:H7 and Salmonella enterica, but the surprise was finding the bacteria associated with plants and not animals.

“The bacteria are able to get from animal sources on to crops through different routes, most likely in irrigation water or sometimes from slurry spraying, while some contamination can also occur during processing and packaging.”

Dr Holden claims that, though some bacteria land directly on the plants, they are able to grow to much higher numbers around plant roots, despite heavy competition from microbes already in the soil. She said that bacteria can move to the edible parts of the plant where they can cope with the plant’s defence systems and continue to grow in a relatively protective environment as the plants themselves continue to grow until harvest.

Holden has therefore concluded that, “The threat to human health occurs because these bacteria are not simply sitting on the surface of the plant and are particularly difficult to remove post-harvest.”

Fortunately these types of outbreaks are still relatively rare because the majority of fresh produce in the UK is rigorously checked by the producers and only a very small proportion will fall 'though the net'. However, potential problems may arise with fruit, vegetables and salad leaves because the foods are mostly eaten raw and not cooked, unlike meat and egg products.