2009-2010 - Bacterial Pathogens and Generic E. coli in Fresh Leafy Green Vegetables

Executive Summary

The Food Safety Action Plan (FSAP) aims to modernize and strengthen Canada's food safety system in order to better protect Canadians from unsafe food and ultimately reduce the occurrence of foodborne illness.

Leafy green vegetables have been reported to be responsible for numerous outbreaks of foodborne illness worldwide. The Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO) has ranked leafy green vegetables as the highest priority among fresh fruits and vegetables in terms of microbiological hazards. After harvest, leafy vegetables are only subject to minimal processing (trimming, cutting, sanitizing, washing and packaging) and are often consumed raw. As such, pathogens introduced during any step of production may not only survive but also multiply. The bacterial pathogens Salmonella and Escherichia coli (E. coli) O157:H7 have been related to the majority of the global outbreaks of foodborne illness associated with leafy vegetables. In addition, the bacterial pathogen Listeria monocytogenes (L. monocytogenes) has been identified as the primary pathogen of concern in ready-to-eat (RTE) foods including fresh-cut RTE leafy vegetables due to its wide distribution in the environment and its ability to grow under refrigeration temperatures.

Considering these factors and their relevance to Canadians, leafy vegetables have been selected as one of the priority commodity groups of fresh fruits and vegetables for enhanced surveillance under the FSAP. During five annual microbiological targeted surveys (2008/09 - 2012/13), over 10,000 leafy vegetable samples will be collected from retail locations and tested for the presence of various pathogens of concern.

This targeted survey (2009/10) focussed on bacterial pathogens of concern and generic E. coli (as an indicator of fecal contamination). The main objectives of this survey were to generate baseline surveillance data on:

  • the presence and distribution of bacterial pathogens of concern: E. coli O157:H7/NM (non-motile) and Salmonella species (spp.) in whole leafy vegetables and in pre-packaged, fresh-cut, RTE leafy vegetables (hereafter referred to as "fresh-cut leafy vegetables");
  • the presence, distribution, and levels of generic E. coli in whole and fresh-cut leafy vegetables; and
  • the presence, distribution, and levels of the bacterial pathogen L. monocytogenes in fresh-cut leafy vegetables.

In this survey, a total of 4250 samples including imported and domestic, conventional and organically grown, whole and fresh-cut leafy vegetables were collected and tested for the bacterial pathogens E. coli O157:H7/NM, Salmonella spp. as well as generic E. coli. E. coli O157:H7 and E. coli O157:NM were not detected in any of the samples. Salmonella spp. and generic E. coli (>100 colony forming units (CFU)/g) were not detected in 99.9% of the samples. A total of five samples (0.1%) were found to be unsatisfactory: two samples (0.05%) due to the presence of Salmonella with the other three samples showing high levels of generic E. coli (> 1000 CFU/g). In addition, an elevated level of generic E. coli was found in another sample (0.02%). This sample was assessed as investigative and requiring further evaluation, as the E. coli counts were elevated but below the unsatisfactory threshold of 1000 CFU/g.

All fresh-cut leafy vegetable samples (1850) were also tested for L. monocytogenes. L. monocytogenes was not detected in 99.6% of the samples. A total of seven samples (0.4%) were found to be unsatisfactory due to the detection of the pathogen. However, the levels of contamination were below 100 CFU/g when enumeration was performed, a low level generally considered to pose very little risk in this type of food.

To assist in the food safety investigations, pulsed field gel electrophoresis (PFGE) patterns (i.e., DNA typing) of L. monocytogenes and Salmonella, as well as serotypes of Salmonella, were identified from the respective isolates of the positive samples.

All unsatisfactory samples were subject to food safety investigations and appropriate follow-up activities were conducted by the Canadian Food Inspection Agency (CFIA). Two product recalls resulted from the referral of the unsatisfactory samples and subsequent investigations. It is important to note that there were no reported illnesses associated with consumption of any of the products in this survey.

Results of the 2009-10 survey indicate that bacterial pathogens were not detected in the majority of the leafy vegetable samples, including fresh-cut RTE leafy vegetable samples, tested in this survey. A very small fraction of the leafy vegetable samples was found to be contaminated with bacterial pathogens or high levels of generic E. coli. The results suggest that not all leafy vegetables in the Canadian market were produced under Good Agricultural Practices (GAPs) and/or Good Manufacturing Practices (GMPs), processed and maintained under sanitary conditions during production and processing, and/or stored at appropriate refrigeration temperatures. The preliminary findings also could be indicative that contamination of leafy vegetables with pathogenic micro-organisms does occur and may be a potential source of foodborne illness in Canada. Results of the three remaining microbiological targeted surveys on leafy vegetables will be released annually upon completion.

The complete text of this report is available upon request.

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