Species-Specific Bacterial Symbionts in Hydra: Are Particular Microbial Communities Selected for and Maintained by the Host?

Cnidarians have a simple tissue grade organisation with a limited number of cell- and tissue-types, and mucus as the sole physical barrier between epithelial tissue and a habitat now known to support large numbers of microbes, including potential pathogens.  Without the benefit of defensive phagoctytes, what evidence exists for the selection of particular symbiotic bacteria within the Cnidarian surface tissue epithelium?

Interested in the evolution of metazoan microbial communities, Fraune and Bosche (2007) compared the microbiota of two closely related basal metazoan species: Hydra oligactis and Hydra vulgaris.  In order to examine microbiota evolution over time, hydra polyps were obtained both direct from the wild, and from cultures maintained in laboratories for over 30 years.  Microbial communities were surveyed via various means, including molecular methods (Restriction Fragment Length Polymorphism (RELP) analysis of 16S rRNA), Transmission Election Microscopy (TEM), phase contrast microscopy, and Fluorescent In-Situ Hybridisation (FISH).

RELP patterns indicated that the microbial communities of the two Hydra species were markedly different:  almost all lab-cultured H. oligactus samples shared one RELP pattern, whereas 16 different patterns were observed in H. vulgaris samples; this is a surprising result when considering the uniformity of living conditions (e.g. food, temperature) which these individuals experienced over the proceeding decades, and the authors cite selective constraints as being the probable cause (although care should be taken when interpreting such results, see below).  This result was echoed when wild polyps of both species where analysed: one dominant RELP pattern observed in H. oligactus samples (though different to that of cultured samples), and more diversity observed in the patterns of H. vulgaris samples.  It appears interspecies differences between wild and cultured polyps are far less pronounced than that between the microbial communities of the different hydra species.

Furthermore, species-specific phylotypes were observed via phylogenetic analysis of RELP derived sequences (phylotypes being defined as sequences of ≥97% similarity).  A particularly interesting α-proteobacteria phylotype specific only to H. oligactus was confirmed as a bacterial endosymbiont (via TEM and FISH techniques), previously unreported as extant in Hydra. Again, species’ differences were found to be larger than interspecies:  although no H. vulgaris samples were found to contain any bacterial endosymbiont, all cultured H. oligactus polyp epithelial cells and c. 20% of wild H. oligactus polyp cells were observed as containing this type of α-proteobacteria phylotype.  The authors conclude that the stark differences between the microbial communities of the two species of Hydra, together with the maintenance of a specific type of microbiota over time, suggests that selective pressures applied by the Hydra epithelium help to structure these microbial communities.

This study provides compelling evidence that cnidarian hosts do indeed have a role in shaping their microbiotas; some other investigations have found similarly species-specific microbial communities in other Anthozoans, including some analyses of the threatened stony schleractinians that build coral reefs.  However, observations of such clear cut species-specific differences in anthozoan microbiotas are not always evident, with many studies providing conflicting results; different molecular methods have their own advantages and disadvantages in the assessment of microbial diversity (e.g. the variability inherent to the RELP technique), which must be kept in mind when such investigations are interpreted. 

In addition, various abiotic and biotic factors have been suggested as having an important role in the structuring of microbial communities, including location, temperature, disease, and the influence of bacteriophage; these factors, alongside punitive selective pressures from the host itself, should be considered when designing further investigations into interactions between microbial communities and the metazoans which they inhabit.

Fraune, S. and Bosch, T. C. G. (2007) Long-term maintenance of species-specific bacterial microbiota in the basal metazoan Hydra. PNAS 104, pp 13146-13151.

http://www.pnas.org/content/104/32/13146