Listeria monocytogenes isolates from ready to eat plant produce are diverse and have virulence potential

Alva Smith, Jack Hearn, Clare Taylor, Nick Wheelhouse, Maciej Kaczmarek, Edwin Moorhouse, Ian Singleton

Research output: Contribution to journalArticlepeer-review

26 Citations (Scopus)


Listeria monocytogenes is sporadically detected on a range of ready to eat fresh produce lines, such as spinach and rocket, and is a threat to public health. However, little is known about the diversity of L. monocytogenes present on fresh produce and their potential pathogenicity. In this work, fifteen Listeria monocytogenes isolates from the UK fresh produce supply chain were characterised using whole genome sequencing (WGS). Additionally, isolates were characterised based on their ability to form biofilm. Whole genome sequencing data was used to determine the sequence type of isolates based on multi-locus sequence typing (MLST), construct a core single nucleotide polymorphism (SNP) phylogeny and determine the presence of virulence and resistance associated genes. MLST revealed 9 distinct sequence types (STs) spanning 2 lineages (I & II) with one isolate belonging to the ST6 subtype, strains from which have been recently implicated in two large, food-associated L. monocytogenes outbreaks in South Africa and across Europe. Although most of the 15 isolates were different, comparison of core genome SNPs showed 4 pairs of 'indistinguishable' strains (<5 SNPs difference). Virulence profiling revealed that some isolates completely lacked the Listeria pathogenicity island-3 (LIPI-3) amongst other virulence factors. Investigation of the inlA gene showed that no strains in this study contained a premature stop codon (PMSC), an indicator of attenuated virulence. Assessment of biofilm production showed that isolates found in the fresh produce supply chain differ in their ability to form biofilm. This trait is considered important for L. monocytogenes to persist in environments associated with food production and processing. Overall the work indicates that a genetically diverse range of L. monocytogenes strains is present in the UK fresh produce supply chain and the virulence profiles found suggests that at least some of the strains are capable of causing human illness. Interestingly, the presence of some genetically indistinguishable isolates within the 15 isolates examined suggests that cross-contamination in the fresh produce environment does occur. These findings have useful implications in terms of food safety and for informing microbial surveillance programmes in the UK fresh produce supply chain.

Original languageEnglish
Pages (from-to)23-32
Number of pages10
JournalInternational Journal of Food Microbiology
Publication statusPrint publication - 16 Jun 2019
Externally publishedYes

Bibliographical note

Copyright © 2019 Elsevier B.V. All rights reserved.


  • Bacterial Proteins/genetics
  • Codon, Nonsense
  • Drug Resistance, Bacterial/genetics
  • Food Microbiology
  • Food Safety
  • Humans
  • Listeria monocytogenes/classification
  • Listeriosis/microbiology
  • Multilocus Sequence Typing
  • Phylogeny
  • Polymorphism, Single Nucleotide
  • United Kingdom
  • Vegetables/microbiology
  • Virulence/genetics
  • Virulence Factors/genetics
  • Whole Genome Sequencing


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