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Doctoral thesis2017Open access

Is your salad safe to eat? : aspects of foodborne zoonotic bacteria in ready-to-eat leafy vegetables and mixed-ingredient salads

Söderqvist, Karin


Ready-to-eat (RTE) leafy vegetables or mixed-ingredient salads are a popular part of the modern healthy diet. Contamination of these products with bacterial pathogens can occur during any step in the production chain and, since there is no step that kills pathogens during the production of RTE salads, a completely safe final product can never be guaranteed. In fact, almost 10% of RTE mixed-ingredient salads from Swedish retail outlets tested in this thesis were contaminated with foodborne pathogens or presumptive pathogens. Listeria monocytogenes was isolated from two out of 141 samples analysed. The other findings included detection of virulence genes present in pathogenic Yersinia enterocolitica and shiga toxin-producing Escherichia coli (STEC), but these could not be culturally confirmed. In growth trials, it was found that mixing RTE baby spinach with cooked chicken (representing a mixed-ingredient salad) strongly influenced growth of inoculated L. monocytogenes, pathogenic Y. enterocolitica and E. coli O157:H7 gfp+ during storage under temperature abuse (15 °C). Mixed-ingredient salad also supported growth of L. monocytogenes under storage conditions recommended for this product in Sweden (8 °C for three days). The estimated risk of listeriosis was 16-fold higher on consuming a mixed-ingredient salad stored at 8 °C at the end of shelf-life, or 200 000-fold higher when the salad was stored at 15 °C, compared with consumption on the day of inoculation. Hence, preventing temperature abuse during storage is of critical importance in mitigating the risk of foodborne listeriosis from these mixed-ingredient salads. The microbiota of RTE baby spinach and mixed-ingredient salad during the growth trials was studied by Illumina 16S rRNA amplicon sequencing. This molecular method revealed changes in the bacterial communities during storage at 8 or 15 °C and correlations were observed between viable counts of inoculated strains and abundances of some taxonomic orders. However, this method was not useful in identifying human pathogens in the salads, even when these were present in high numbers that can cause disease in humans.


food safety; bacterial contamination; microbiota; Listeria monocytogenes; baby spinach; growth potential; microbial contamination; microbiota; Yersinia enterocolitica; Escherichia coli; temperature abuse

Published in

Acta Universitatis Agriculturae Sueciae
2017, number: 2017:5
ISBN: 978-91-576-8783-8, eISBN: 978-91-576-8784-5
Publisher: Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences