CFU data from Vibrio alginolyticus strains, resulting from a controlled infection experiment using juvenile pipefish

Filamentous vibriophages represent a massive repertoire of virulence factors which can be transferred across species boundaries, leading to the emergence of deadly pathogens. All filamentous vibriophages that were characterized until today were isolated from human pathogens. Considering frequent horizontal gene transfer among vibrios, we predict that other environmental isolates, including non-human pathogens also carry filamentous phages, of which some may encode virulence factors. The aim of this study was to characterize the phage repertoire, consisting of prophages and filamentous phages, of a marine pathogen, Vibrio alginolyticus. To do so, we sequenced eight different V. alginolyticus strains, isolated from different pipefish and characterised their phage repertoire using a combination of morphological analyses and comparative genomics. We were able to identify a total of five novel phage regions (three different Caudovirales and two different Inoviridae), whereby only those two loci predicted to correspond to filamentous phages (family Inoviridae) represent actively replicating phages. Unique for this study was that all eight host strains, which were isolated from different eukaryotic hosts have identical bacteriophages unique for this ecotype, suggesting a clonal expansion of this strain after the phages had been acquired by a common ancestor. We further found that co-occurrence of two different filamentous phages leads to within-host competition resulting in reduced phage replication by one of the two phages. We found virulence genes encoded on one of the two filamentous phages. The close phylogenetic relationship between filamentous phages from the present study with phages isolated from the human pathogen V. parahaemolyticus, suggest frequent movement of filamentous phages between different Vibrio species, which may have led to the emergence of the pathogenicity of the strains from the present study.