Marine organisms are facing a dual and cumulative threat, i.e. pathogenic microorganisms reinforced by global warming. Bacteria belonging to the genus Vibrio are one of the most common pathogens in oceans and are responsible for massive death of molluscs. However, despite this ecological and economical concern, virtually nothing is known about the behaviour of bacterial individuals during infection. Does every single cell of an isogenic pathogen behave the same, or is the bacterial population composed of different sub-populations, behaving differently to divide the labour and conquer its new ecological niche. This “divide and conquer” strategy is the focus of a new field of research in microbiology, but this field remains typically restricted to mammal pathogens. The aim of this project will be to study the behaviour of model pathogens like Vibrio harveyi or Vibrio tapetis, infecting the abalone or the clam respectively, working at the single cell level to account for the phenotypic heterogeneity. The goal will be to create Vibrio bioreporters, targeting relevant virulence genes and to monitor in vivo the expression of these genes during the different stages of infections, at the single cell level. This strategy will allow to better understand the behaviour of marine pathogens, and will deepen our understanding on the extent of microbial phenotypic heterogeneity during microbial infections.