​The fate of surrogate verocytotoxic E. coli contaminating the rhizospheres of root vegetables during processing and retail and wholesale distribution

Summary

This project was funded by Food Standards Scotland and Food Standards Agency to provide better understanding of the survivability of E. coli at all stages along the vegetable production and distribution chain.

In recent years there have been a number of high-profile foodborne disease illness outbreaks that have been associated with fresh produce. In particular, in 2011 epidemiological data linked an outbreak of E. coli O157 in Great Britain to the handling and preparation of vegetables that had contaminated soil on the surface. Therefore FSS and FSA wanted to investigate the extent to which E. coli could survive on the surface of soil contaminated vegetables and the potential risk to consumer health.

There are many types of E. coli and not all of them are harmful to humans. However the strains that produce a toxin called verotoxin are pathogenic and are called verocytotoxic E. coli (VTEC), of which E. coli O157 is one type. This study used a naturally occurring non-pathogenic E. coli O145 that has previously been implicated in foodborne illness associated with eating fresh produce. It was used to contaminate vegetables (potatoes, carrots and leeks) a week before harvest. The project looked at pre and post-harvest consequences of contaminated manure and irrigation water accidentally being applied to the crops and assessed the impact of contamination as the vegetables progressed through the production chain. For example, the spread of pathogens to wash water from produce that had been contaminated.

The outcomes of the study showed that after a worst case scenario situation (i.e. escaped cattle getting into the field or irrigation water becoming contaminated) occurring, E. coli O145 could be isolated from both potatoes and leeks after harvest. However when replicating commercial processes of brushing, washing and storage they eliminated or significantly reduced the contamination on the vegetables. E. coli O145 was more likely to be recovered from vegetables stored at a constant refrigerated temperature (retail) compared with crops stored at ambient temperatures (wholesale), possibly due to the bacteria becoming stressed and dying during temperature fluctuations.

The study also assessed the effect of handling vegetables by gloved or ungloved harvest workers with poor hygienic practices. The results confirmed the importance of workers adopting a good handwashing technique following a visit to the bathroom and that when gloves were worn there was lower contamination passed to the vegetables. The study also recommended that surfaces such as flush handles and door latches are cleaned regularly.

The results from this project will be used by Food Standards Scotland to inform future risk assessments and food safety advice in relation to fresh produce.