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

Abstract

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 ACCEPTED MANUSCRIPT A C C E P T E D M A N U S C R I P T2 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.

In-Depth Longitudinal Study of Listeria monocytogenes ST9 Isolates from the Meat Processing Industry: Resolving Diversity and Transmission Patterns Using Whole-Genome Sequencing

“…This sequence type has, however, frequently been found to dominate in raw pork meat ( 21 , 52 , 61 ) and is also commonly found to persist in slaughterhouses ( 14 , 15 , 24 , 62 ) as well as in downstream compartments in the meat processing chain ( 10 , 22 , 26 ). In contrast, several studies report an absence or low abundance of ST9 in other food types and processing environments, such as fish/salmon, dairy, and fresh produce ( 22 , 26 , 31 , 47 , 63 ). These observations suggest that ST9 isolates are highly adapted to survival in meat production environments.…”

Backtracking and forward checking of human listeriosis clusters identified a multiclonal outbreak linked to Listeria monocytogenes in meat products of a single producer

“…Last but not least, pathogenicity is a multifactorial process, and cannot merely be derived from the presence or absence of virulence genes [39]. In summary, the variety of potentially pathogenic strains circulating at the producer, as already described in other food processing plants [40], clearly shows that selectively removing the source of contamination for one specific outbreak cluster is neither sufficient nor sustainable. Instead, the entire L. monocytogenes population in a food processing plant must be controlled and eliminated to not only stop present outbreaks but to also prevent future ones.…”

Virulence potential of Listeria monocytogenes strains recovered from pigs in Spain

“…Importantly, not all L monocytogenes strains are equally virulent; some strains are highly virulent and are more often associated with epidemics, while others are less virulent and are infrequently related to epidemics. [1][2][3] Previous studies have shown that lineage I (or ST 4b) occurs more frequently among clinical isolates than lineage II (or ST 1/2b, ST 1/2a and ST 1/2 c), relative to the frequency of these categories in food. 4 Besides intrinsic L monocytogenes virulence, other extrinsic factors play an equally important role in listeriosis outbreaks, such as individual susceptibility, food category, good hygiene practices and good manufacturing practices, and persistence into the environment or biofilm formation, among others.…”

“…32 Understanding the virulence aspects of L monocytogenes in animals and food supply chains can help regulators and operators design more effective microbial surveillance and prevention strategies. 1 In this sense, more strict control measures, such as a lower limit of bacterial load in food, could be implemented when especially virulent L monocytogenes strains are identified. In the present study, the issue of the general virulence potential of L monocytogenes strains from pig samples in Spain was addressed.…”

“…Although all L monocytogenes strains are currently treated equally for regulatory purposes, some strains are highly virulent and are more often associated with epidemics, while others are less virulent and are infrequently related to epidemics. [1][2][3] Particularly, in human listeriosis outbreaks, isolates belonging to serotype (ST) 4b are over-represented, with a lower number of cases associated with isolates from ST 1/2a, which is usually associated with food and food-related L monocytogenes isolates. 4 5 In 2018, 28 countries reported 2549 conirmed cases of listeriosis in the EU/EEA, with more than 229 associated deaths.…”