In our continuous effort to monitor pathogens in the Mediterranean Sea, we examined the microbial communities within the gills of 89 samples, belonging to three common wild fish species (Bluespotted Seabream, Lessepsian Lizardfish and Atlantic Chub Mackerel). The fish were caught several kilometers off the shores of Ashdod at depths of 20-80m, and all appeared to be healthy when sampled. Gills’ tissue was processed and went through 16S rRNA NGS screening, which identified a large variety of potential pathogens – some of which are zoonotic – results which demonstrate the complexity of the relationships between host, organ microbiota and pathogens.

 We further saw that the gills’ microbiome compositions (Fig 2)We found the microbial communities of the Atlantic Chub Mackerel (hereafter, ACM) as the richest of all three (perhaps due to their migratory lifestyle), while that of the Bluespotted Seabream (BSSB) and the Lessepsian Lizardfish (LLF) were shown to be less rich on average and more varied in their richness (Fig 1).

 exhibit species-related affinities with some overlap, especially between the BSSB and the LLF. An interaction network (Fig 3)  was used to find associations between bacteria genera and the extent to which they are associated with the different fish species. It demonstrated that genera bearing strong ties and more characteristic of ACM (with Psychrobacter as its centerpiece), are more diverse and contain less potential pathogenic species. It also expressed strong connections between Vibrio, Photobacterium and Shewanella – all of which contain pathogenic species of economic importance – to the BSSB. When comparing the relative abundance of bacteria genera between the fish species (Fig 4), it was shown that some genera known to contain important fish pathogens make up substantial portions of the gills’ microbiome – especially Photobacterium, in the gills of the BSSB. To further understand the meaning of this we calculated phylogenetic trees of several genera, including that of Photobacterium (Fig 5), and found that roughly a third of the reads within all Photobacterium-associated sequences highly resembled P. damselae – a notorious marine pathogen of zoonotic capabilities.

We went on to search for as many pathogenic agents as could be identified to a high level of certainty at the species level, and managed to find 41 such potential pathogens (the 20 most prevalent of which are presented in Table 1 and Fig 6). These were distributed in differing patterns between the three fish species, perhaps due to differences in species-specific traits such as: habitat, migration habits, trophic level and gills’ microbiome. Combining the results expressed in phylogenetic trees and the data presented in Table 1, we made a comparison between five bacteria genera, showing ratios of pathogenic/non-pathogenic (P/NP) species across all samples combined (Fig 8). This revealed that Vibrio, Streptococcus and Staphylococcus had relatively low P/NP ratios (between 1-4% pathogenic species), in contrast with Photobacterium (31%) and Shewanella (71%; dominated by S. baltica, a significant food spoilage agent). Finally, it was concluded that P. damselae may be a pathobiont (a mutualistic symbiont of pathogenic capabilities) of the BSSB – a hypothesis requiring further research.