Food Microbiology

Listeria monocytogenes is a human food-borne pathogen that causes serious illnesses including encephalitis, meningitis, septicemia, abortion and still birth but the disease is also manifested as gastrointestinal illness. Listeria monocytogenes is widespread in the environment, has been isolated from various ready-to-eat food products and most of the infections have been found to be foodborne. Molecular typing tools, population genetics and evolutionary studies have played major roles in understanding the transmission, different subtypes, the virulence and evolution of Listeria monocytogenes in different niches. Here, we compare 123 Listeria monocytogenes isolates from New Zealand seafood, horticultural and clinical sources with publicly available international isolates to better understand the characteristics of New Zealand isolates and their relationship with the international isolates. We conducted pulse field gel electrophoresis (PFGE), multi-locus sequence typing (MLST), multi-virulence-locus sequence typing (MVLST) and whole genome sequence analysis (WGS) using single nucleotide polymorphism (SNPs) of NZ isolates and compared them with publicly available typings. PFGE analysis showed that the seafood isolates from NZ grouped separately from the global ones. The minimum spanning tree conducted using seven housekeeping MLST alleles revealed that the NZ isolates formed unique complexes or sequence types (ST) in comparison with the international isolates although some of the seafood isolates grouped closely with international human pathogenic lineage I group strains. Likewise, WGS analysis revealed that most of the seafood isolates clustered with lineage II of the international isolates and some with lineage I while some of the horticultural isolates were lineage III/IV. MVLST also showed unique STs from NZ that were not reported from other countries. Results from the MVLST analysis raises a speculation that the virulence genes may have evolved differently to adapt to NZ conditions and may involve a niche adaptation mechanism.