Previous PageTable Of ContentsNext Page

Survival and possible transmission of pathogens from sewage sludge (biosolids) to lettuce plant tissue

Jacqui Horswell, V. Ambrose and Tom Speir

ESR Limited, PO Box 50-348, Kenepuru Science Drive, Porirua, Wellington, New Zealand. Email:


Food poisoning causing bacteria, are increasingly being discovered in fresh produce, including lettuce. It is not fully understood how pathogens are able to get inside plants, but, one hypothesis proposes that plants can become contaminated when grown in fields fertilised with inadequately treated manure. If proved, this could have serious implications for the application of biosolids to farmland where food crops may be grown.

We investigated health risks associated with growing lettuce plants from seeds, in a soil/biosolids mix contaminated with 105 E.coli 0157:H7 and Salmonella typhimurium per gram of mix. The pathogens were genetically modified with reporter genes to facilitate their visualisation in the system. Following surface sterilisation, no E.coli or S.typhimurium could be recovered from the surface of the seedlings. However, in macerated tissue samples taken 5 days after planting, E.coli and S.typhimurium were detected, at levels of 24 and 17 MPN per gram of lettuce, respectively, suggesting that there was transmission of pathogens from contaminated biosolids to lettuce plants. In a further experiment, surface contamination of lettuce seedlings also led to internalisation of the pathogens. Here, 106 E.coli or S.typhimurium was applied to the surface of lettuce seedlings and the tissue was sterilised and macerated. Both pathogens were found inside the lettuce tissue up to 27 days after contamination of the plants. Although the levels of E.coli and S.typhimurium used in this study were far greater than what may normally be expected in treated biosolids, even a low level of contamination could present a human health risk

Previous PageTop Of PageNext Page