Listeria monocytogenes (L.monocytogenes) is a bacterium that has been widely detected in the environment and plants, including leafy vegetables. It has the potential to become a human pathogen, affecting young people, older people and pregnant women. Every year in Australia a number of people die as a result of L. monocytogenes infections. Supermarkets displaying due diligence have set very tight specifications that vegetable growers must comply with. This, in turn, has led to growers having to pre-test their products for the presence or absence of L. monocytogenes as a condition of supply to supermarkets. The problem, however, is that with L. monocytogenes testing, there are a number of tests available and often the grower is confused as to which test should be used. More importantly the major problem is that growers have no idea how to reduce the incidence of L. monocytogenes in the field and they have no idea how this bacterium enters farming land to contaminate their crops. What is needed is an information package on this issue for the grower.

This project had three aims. The first aim was to understand how L. monocytogenes enters a vegetable farm from the environment. The second aim was to understand if a rapid detection method can be of assistance to the grower and to make recommendations on what method the grower should be asking for when getting produce tested by laboratories. The third aim of this project was to produce an information sheet on testing and on ways to reduce L. monocytogenes issues on farm.

In order to understand how L. monocytogenes enters the farm, a number of samples were taken from three vegetable growing sites in Victoria and two vegetable growing sites in Queensland. Samples of produce, soil, water and manures found on farms were taken and tested for the presence or absence of L. monocytogenes, in most cases when positives were detected the numbers of L. monocytogenes were quantified.

The results of this project suggest a possible pathway for how L. monocytogenes contamination occurs in vegetable farms. L. monocytogenes was found to be present in high numbers in silage and fermented baled hay, which are both, in turn fed to and ingested by ruminants; this passes through the animals usually without causing infection to them. The L. monocytogenes remains trapped within the matrix of the faeces and disperse only when the faeces becomes dry in hot weather (a process assisted by wind and heat). The dust carrying the L. monocytogenes is then spread by strong winds covering large distances, where it can contaminate vegetables through the dust particles. Leafy vegetables (e.g. curly parsley) that can trap dust more effectively usually show higher levels of detection than smooth leaf vegetables, such as cos lettuce. The results of this study clearly show that after windy and hot days the level of detection on leafy vegetables can potentially increase.

Testing for L. monocytogenes by growers should be based on the Australian standards and any L. monocytogenes detected must be enumerated. Information based on the findings of this project and recommendations for reducing the incidence of L. monocytogenes on produce before delivery to markets are discussed in easy to understand terms.

More work needs to be carried out to determine the virulence of the subtypes of L. monocytogenes found in vegetables, as these are more than likely to be of low virulence as their origin is from plant matter and not infected animals.

This project has been funded by Hort Innovation, using the research and development levies listed below and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.

• Vegetable Fund