Vibrios: A Big Family?

Vibrios have undoubtedly been a recurrent theme in our lectures this year: from their importance as symbionts in the Hawaiian squid Euprymna scolopes to their role as potential pathogens, vibrios occupy a wide variety of geographical and ecological niches. Especially the close association of vibrios to zooplankton due to the presence of a specific chitinase may have facilitated their widespread distribution. But what about the genetic relationship between vibrios occupying different niches:  did adaptive radiation lead to genetic segregation or do Vibrio populations have a panmictic population structure?  

Hoffmann et al. (2012) aimed to clarify Vibrio population dynamics within natural marine environments by studying genetic relatedness and phylogenies of both sponge-derived (SD) and shallow, coastal water Vibrio populations. Sponges have a diverse microbiota (e.g. see Robyn’s blog “Shedding light on the function of marine sponge microbiota”), including vibrios, and occupy a range of benthic habitats ranging from shallow waters to 900 metre depth. Vibrio species have been reported to be involved in both secondary metabolism and synthesis of antibacterial compounds in sponges; however it remained unclear how these SD Vibrio species are related to the nearshore counterparts.

Hoffmann et al. used a variety of state-of-the-art techniques to elucidate this relationship between different Vibrio populations. Basically Vibrio strains very isolated from sponge samples and cultured in highly selective agar. PCR, 16 rRNA gene sequencing, 16S-23S rRNA intergenic spacer region (ISR), multi-locus sequence analyses (MLSA), as well as phylogenetic analyses were then used to investigate the degree of relatedness and to construct phylogenies.

In accordance with previous studies, they found that most SD Vibrio isolates (74%) clustered within the Vibrio Harveyi clade. The MLSA approach was applied on these isolates to further distinguish the phylogenetic relatedness of SD and the coastal reference Vibrio strains. Results pointed towards strong evidence for extensive recombination among SD vibrios and their nearshore counterparts strains, isolated from a variety of globally distinct locations, which supported the hypothesis that SD and shallow-water vibrios are not two disparate populations, but members of a larger single panmictic group. Moreover there was evidence for horizontal gene transfer occurring between both groups, suggesting that DNA may be exchange even across these expansive niches.

Overall these findings may help to explain the wide distribution of vibrios, but particularly interesting are the implications of this study on the movement of marine pathogens such as the highly pathogenic V. parahaemolyticus O3:K6 clone, which spread from Calcutta in India to South-East Asia, Atlantic and Gulf coast of the United States, Europe, Africa and South America. In fact, the panmictic population structure could be a critical component of the global distribution of the O3:K6 pandemic strain. Like Vezzulli (see Vicky’s comment on my blog “Early warning systems for Vibrio disease risks”), Hoffmann et al. suggested that the association of vibrios with copepods could provide a means by which vibrios traverse disparate niches, from the near-shore pelagic to the deep benthic habitats, which is supported by the copepod life cycle overlapping these two regions.  

I think this is a very interesting study which links together several themes covered in previous blogs: sponge microbiota (authors briefly mention in their conclusion that sponges may support Vibrio speciation events), horizontal gene transfer, vibrios and chitin, and vibrios as human pathogens. 

Hoffmann, M., Monday, S. R., McCarthy, P. J., Lopez, J. V., Fischer, M. & Brown, E. W. 2012. Genetic and phylogenetic evidence for horizontal gene transfer among ecologically disparate groups of marine Vibrio. Cladistics, 1–19.