Pathogen persistence from paddock to plate (VG16042)

What was it all about?

The risk of contamination of fresh produce with human pathogens from irrigation water and soil amendments is usually managed by withholding periods that define the time between application and harvest. These exclusion periods are based on published data, however most research has been conducted in the United States or Europe where temperatures, UV radiation and soil types are very different to Australian conditions.

The project team examined factors that potentially affect contamination of vegetables by human pathogens. This included benchmarking surveys examining what human pathogens are present on fresh vegetables, in irrigation water, in manure and in soil amendments used by growers. Trials examined die-off rates of human pathogens in manure amended soils and on leaves after irrigation with contaminated water.

The survey showed that human pathogens were rarely found on Australian fresh vegetables. Pathogenic bacteria were found on less than 1 per cent of 5,533 samples, with most at levels unlikely to cause illness. Bacteria were also uncommon in samples of manure/compost used on farms and in irrigation water.

Project trials examined pathogenic die-off rates of bacteria added to poultry litter or cattle manure and incorporated into soil used to grow lettuces. Under these conditions, the average population of E. coli in soil plus poultry litter fell by > 99 per cent within 20 days. In soils amended with cattle manures, E. coli also declined rapidly in two trials conducted in spring and summer, however populations persisted during autumn. The data indicate that E. coli was generally reduced to below or close to detectable levels within 50 days, while in some cases Salmonella spp. could persist in soils up to 60 days. Three of 200 lettuces were contaminated with E. coli at harvest, and none were positive for Salmonella spp.

Trials also examined how quickly bacteria died on lettuce leaves after irrigation with contaminated water. E. coli and Salmonella were undetectable after two days on intact vegetables but could survive at least six days on damaged lettuces. Even damage occurring four days before irrigation increased survival of E. coli on lettuce. If water quality is poor or unknown, a 48-hour withholding period between irrigation and harvest significantly reduces the risk that vegetables will be contaminated at harvest. However, longer withholding periods are needed if plants have been physically damaged.

ACT NOW

Read these factsheets, articles and guides prepared by the project team, which can be found on the Fresh Produce Safety Centre website:

• Journal article – Persistence of human pathogens in manure-amended Australian soil used for production of leafy vegetables
• Factsheet – Reducing food safety risks from pre-harvest water (for vegetable producers)
• Factsheet – Reducing food safety risks from manures (for vegetable producers)
• Reducing food safety risks from pre-harvest water (for supply chain managers and regulators)
• Using manures to grow vegetables – a guide to reducing risk (for supply chain managers and regulators)
• How safe are my soils? published in WA Grower, Spring 2018 on p.34