V.vulnificus is an aquatic bacterium and an important human pathogen, classified into three biotypes. BT3 appears to be rather clonal and geographically restricted to Israel, where it caused an outbreak of wound infections. To understand the bacterium's evolution, the genome of environmental strain VVyb1(BT3) was compared to the available BT1 genomes revealing higher similarity (90%) to the clinical strains. 4% of the genes were unique to VVyb1(BT3), while 29 of them had homologies among Shewanella. In an environmental survey we succesfully co-isolated Shewanella and V.vulnificus from the same niche. However, molecular analysis revealed that some of these considered ‘unique’ genes are harbored by non-sequenced BT1 strains, rather than by Shewanella. This led to the hypothesis that BT3's genome was created as yet-undiscovered event, based on the core genome of BT1 strain that gained a rather small number of genes by horizontal gene transfer from its natural environment, leading to high virulence and biotype change. Thus, in the next step the virulence of BT3 and of its potential “ancestor” - an identified BT1 candidate strain, was examined. Using mice model, BT3 strain was found to possess a relatively high virulence level that is similar to the tested BT1 strain, supporting their evolutionary relatedness. Still, qPCR of few genes and preliminary RNAseq results showed high variation between the two biotypes, suggesting that changes in gene expression might also lead to the formation of new biotype. Finally, multi-strain full-genome comparison study together with transcriptome analysis may answer the unresolved evolution of this pathogen.
