Vibrio vulnificus is an opportunistic pathogen responsible for wound infections and septicaemia following ingestion of contaminated seafood, and has the highest reported mortality rate for seafood-related diseases. It is an autochthonous inhabitant of coastal marine environments where it is exposed to predation by heterotrophic protozoa. Protozoan predation on bacteria acts as a selective force, leading to evolution of antiprotozoal mechanisms that may also function as virulence factors.
V. vulnificus isolated from environmental and clinical samples were exposed to predation by heterotrophic protists. Planktonic cells were exposed to Tetrahymena pyriformis, and biofilms exposed to the surface feeding Acanthamoeba castellanii. V. vulnificus numbers in the planktonic phase were determined by plate counts and spectrophotometry while biofilm biomass was assessed by crystal violet staining. The health and number of protozoa were determined by microscopy. Different environmental and clinical isolates exhibited unique grazing defence mechanisms.
An oyster isolate showed grazing resistance and toxicity to T. pyriformis in planktonic phase. In order to investigate the potential factors involved in the toxicity, the cell free supernatant of V. vulnificus was used to determine the presence of secreted toxic factors affecting T. pyriformis. The secreted toxic factor was determined to be iron-dependent, heat stable, proteinase-resistant and had a molecular mass of <10 kDa. The iron dependent grazing resistance was further investigated by analysing the transcriptomic profile of the V. vulnificus under iron-replete and -deplete conditions. In total 255 genes were differentially expressed in iron-deplete conditions in comparison to the iron-replete condition including genes related to ammonia and formate production, biotin, purine and pyrimidine biosynthesis, phosphate uptake, copper transfer and virulence master regulators.