Deep sea vents home to human pathogenic Vibrios

Vibrio spp. are ubiquitous in the aquatic environment. Technological advances have allowed for the characterisation of many genomic sequences, shedding light on how this genus can survive in a range of habitats. Vibrios can grow in sediments, the water column, and even around hydrothermal vents.

This study conducted by Hasan et al. (2015) has described how a novel species of Vibrio was isolated and characterised, with reference to phylogeny, from deep sea hydrothermal vents. The genomic, functional and phylogenetic analyses carried on this new species, Vibrio antiquarius, revealed the presence of genes associated with symbiosis of a host, adaptation and virulence factors commonly seen in Vibrio species pathogenic to humans.

In 1999 during dives in submersibles along the East Pacific Rise, four samples of water were collected from the surrounding hydrothermal vents and sulphide chimneys. Four mesophilic bacterial isolates were cultured; three of which were Vibrios, one being Shewanella algae. One isolate, Vibrio EX25, was shown to be genetically similar to V. parahaemolyticus and V. cholerae, both of which are human pathogens. However, this strain was phylogenetically distant enough to be described as a separate species, V antiquarius.

The entire genome was sequenced, which showed that V. antiquarius contains >70 genomic islands (in particular, pathogenic islands) and many insertion sequences within the genome. These factors are suggested to give this species, along with other Vibrios, the ability to adapt faster or have quicker responses to changing environmental conditions.

 V. antiquarius has a number of genes allowing for survival in the deep sea, however some were sequenced that are not present in other species: one gene encoding for alkyl hydroperoxide reductase (the most abundant enzyme in the endosymbiont of Riftia pachyptila), multicopper oxidase (an essential gene for manganese oxidation), and delta-9 fatty acid desaturase (essential for growth under immense pressure).

This species of Vibrio has a type III secretion system (T3SS), which enables injection of effector proteins directly into eukaryotic cells of the host. This system allows V. antiquarius to utilise virulence factors; T3SS genes causing diarrhoea as in a cholera infection, RTX toxin and type IV pilin pilA which encodes for proteins expressed during human infection. Along with these genes and those that are ubiquitous, V. antiquarius is able to thrive in the deep sea hydrothermal vent habitat, likely in close association with the inhabiting animals.

A question is raised however, as to the wider use of these pathogenicity genes – could they provide some unknown ecological function? Vibrios can be involved in providing ecological pathways for other organisms (V. fischeri provides bioluminescence in bobtail squid, for example), and so can we write off Vibrios as only the cause of disease, or as a means of ensuring stability in the environment? This paper provides a new perspective on how Vibrios fit into the ecology of the deep sea, and may open up new research options into determining their wider role in the aquatic environment, as a pathogen and ecological stabiliser.

Hasan, N.A., Grim, C.J., Lipp, E.K., Rivera, I.N., Chun, J., Haley, B.J., Taviani, E., Choi, S.Y., Hoq, M., Munk, A.C. and Brettin, T.S., 2015. Deep-sea hydrothermal vent bacteria related to human pathogenic Vibrio species.Proceedings of the National Academy of Sciences, 112(21), pp.E2813-E2819.

http://www.pnas.org/content/112/21/E2813.short