A scientist at Plant Research International has claimed that salmonella bacteria can spread within lettuce plants grown in infected soil as well as on the leaves. His work was undertaken as part of a doctoral thesis at Wageningen University in The Netherlands.

Michel Klerks investigated the physiological and molecular interactions between salmonella bacteria and lettuce varieties. He discovered that salmonella can move to the roots of the lettuce plant. The bacterium then reproduces and spreads itself on the plant.

Bacterium spread does not lead to visible differences between healthy and infected soil-grown plants. Research showed salmonella was not only found on the plant but also within the plant itself. Internal reproduction and spreading increase the risk of food poisoning through lettuce consumption.

Lettuce can be infected by contaminated manure, the research states. Past studies showed the risk of salmonella infection can be reduced by decreasing the amount of pathogens in dairy cattle manure, achieved by feeding cattle more hay or straw.

Breeding new lettuce varieties resistant to the pathogens may reduce the risk of infection, Klerks says. Hygiene measures after harvest are of little use because washing does not remove the bacteria from internally infected vegetables.

Klerks also focused on the development of more sensitive and more accurate molecular detection methods to determine small numbers of salmonella bacteria and pathogenic E. coli bacteria. Testing manure or infected soil for the extent of salmonella and/or E. coli infestation before seedlings are planted out enables a good estimate of the risk of crop contamination.

The methods can also be applied after harvest in the routine diagnosis of these pathogens in the food production chain. The analytical time required for testing food products by means of molecular methods is reduced to two days whereas current standard procedures require five days.