[Virulence of Listeria welshimeri]

Virulence Characterization of Listeria monocytogenes, Listeria innocua, and Listeria welshimeri Isolated from Fish and Shrimp Using In Vivo Early Zebrafish Larvae Models and Molecular Study

Listeriosis is one of the most notable foodborne diseases and is characterized by high rates of mortality. L. monocytogenes is the main cause of human listeriosis outbreaks, however, there are isolated cases of disease caused by other species of the genus Listeria. The aim of this study was to evaluate strains of L. monocytogenes (n = 7), L. innocua (n = 6), and L. welshimeri (n = 2) isolated from fish and shrimps for their virulence based on the presence of virulence genes and the in vivo Danio rerio (zebrafish) larvae models. A total of 15 strains were analyzed. The zebrafish larvae model showed that the larvae injected with L. monocytogenes strains were characterized by the lowest survival rate (46.5%), followed by L. innocua strains (64.2%) and L. welshimeri (83.0%) strains. Multiplex PCRs were used for detection of selected virulence genes (luxS, actA2, prfA, inlB, rrn, iap, sigB, plcB, actA, hlyA), the majority of which were present in L. monocytogenes. Only a few virulence-related genes were found in L. welshimeri, however, no correlation between the occurrence of these genes and larval survival was confirmed. This research highlights the importance of the potential impact that Listeria spp. strains isolated from fish and shrimps may have on consumers.

Whole-genome sequence of Listeria welshimeri reveals common steps in genome reduction with Listeria innocua as compared to Listeria monocytogenes

We present the complete genome sequence of Listeria welshimeri, a nonpathogenic member of the genus Listeria. Listeria welshimeri harbors a circular chromosome of 2,814,130 bp with 2,780 open reading frames. Comparative genomic analysis of chromosomal regions between L. welshimeri, Listeria innocua, and Listeria monocytogenes shows strong overall conservation of synteny, with the exception of the translocation of an F(o)F(1) ATP synthase. The smaller size of the L. welshimeri genome is the result of deletions in all of the genes involved in virulence and of "fitness" genes required for intracellular survival, transcription factors, and LPXTG- and LRR-containing proteins as well as 55 genes involved in carbohydrate transport and metabolism. In total, 482 genes are absent from L. welshimeri relative to L. monocytogenes. Of these, 249 deletions are commonly absent in both L. welshimeri and L. innocua, suggesting similar genome evolutionary paths from an ancestor. We also identified 311 genes specific to L. welshimeri that are absent in the other two species, indicating gene expansion in L. welshimeri, including horizontal gene transfer. The species L. welshimeri appears to have been derived from early evolutionary events and an ancestor more compact than L. monocytogenes that led to the emergence of nonpathogenic Listeria spp.