In vitro and in vivo characterisation of Listeria monocytogenes outbreak isolates

Listeriosis is an important food-borne disease responsible for high rates of morbidity and mortality. L. monocytogenes has been the cause of several foodborne outbreaks and its ability to adapt and survive in a wide range of stress conditions makes it difficult for food producers to eradicate. Many L. monocytogenes are avirulent, but have the ability to increase their virulence if exposed to environmental stresses. The goal of this study was to use phenotypic assays and whole genome sequencing to elucidate possible links between food related stress resistance and virulence phenotypes in L. monocytogenes strains originating from different sources. Four L. monocytogenes isolates from sweetcorn and one isolate from a raw milk (control) were sequenced and characterised using a range of phenotypic assays. The four L. monocytogenes sweetcorn isolates displayed a significant increase for in vitro adhesion and invasion of epithelial cells, and a high level of colonisation for in vivo liver and spleen. In addition, the four L. monocytogenes isolates displayed an advanced ability to form biofilms, resist heat stress and resist a mixture of antimicrobials, which we have tested in this study. Investigation of the four L. monocytogenes sweet corn isolates at the genomic level identified Single Nucleotide Polymorphisms, which may have a role in the phenotypes we have observed within this study, particularly in response to survival properties within the environment or in terms of adhesion and invasion. We highlight the importance of combining whole genomic sequencing strategies in conjunction with phenotypic strategies as a key strategy in the investigation against all outbreak foodborne pathogens.