Vibrios infecting marine invertebrates: new insights into vibrio virulence and their adaptive gene reservoirs – VIBRIOGEN

Vibrios are bacteria responsible for a significant proportion of infections affecting marine invertebrates and represent the major cause of mortality in farmed marine species. They are able to colonize many habitats, existing as planktonic forms, in biofilms or associated with various hosts. This high adaptability is linked to their capacity to generate genetic diversity, in part through lateral gene transfer, but also by varying gene expression control. Thus, in vibrios, the evolution of virulence is intimately linked to genomic plasticity, transcriptomic versatility and the resulting ecological fitness.

In contrast to species that are pathogenic to humans, little data are available regarding invertebrate pathogens. A few results have been gathered for some species, especially as far as colonization and invasion processes are concerned; however, recent advances in genomics and transcriptomic profiling alongside the establishment of domesticated animal strains will help to make up for our lack of knowledge of “non model” systems leading to new and original scientific questionings.

Vibrio splendidus has been associated with many oyster mortality episodes in France. The high genetic diversity of V. splendidus strains isolated from diseased animals, the absence of obvious links between virulence and taxonomic markers, the moderate virulence of most pathogenic isolates but the existence of cooperation between pathogenic isolates observed in experimental infections suggest that virulence in V. splendidus results from a combination of factors, each associated with different strains in the population. Thus, understanding virulence in V. splendidus requires an analysis of the pan-genome of the population, in order to distinguish the genetic elements shared by all strains (core genome) from those that are strain specific (accessory genome). The hypothesis of “metavirulence” must be validated through a functional demonstration ex vivo and in vivo of cooperation between strain specific genes leading to pathogeny and the deciphering of the interactions underlying agonism between strains.

Vibrio nigripulchritudo currently threatens the economic viability of shrimp farming in New Caledonia. This epidemic correlates with the emergence of a pathogen lineage, identifiable by MLST, suggesting that at least some of the virulence genes are carried on the chromosomes. In this lineage, only highly pathogenic strains (HP) carry two plasmids, both of which are required for virulence. The linkage between taxonomic markers, presence of the plasmids and high pathogenicity suggest that interplay between replicons is the basis for high virulence. Understanding the nature of these interactions requires the genome wide comparison of strains with variable levels of virulence, so as to identify genetic elements specific to HP strains before demonstrating their importance in the pathogeny process in vivo.

The aim of our project is to investigate virulence as an emerging property of strains or population in vibrios infecting marine invertebrates. Virulence is viewed not only as the result of acquiring new genetic resources thanks to genomic plasticity but also and foremost through their interactions, be it at the level of the dialogue between replicons or between strains within the population. To this aim, we will use a combination of population, comparative and functional genomic approaches, together with transcriptomics in addition to more classical bacterial genetics, experimental pathology and cellular biology approaches.

Fundamentally this project will help to elucidate the repertoire of genetic elements involved in pathogenicity. In addition, through an improved knowledge of the vibrio core genome, it will lead to a better understanding of evolution of the vibrio radiation. From a more applied point of view, the elucidation of virulence mechanisms is a prerequisite to devising prophylactic methods to fight infectious agents.