Samples studied by us showed a total of four bacterial phyla (Fig. 1A). Among them phylum Proteobacteria was highly abundant (82%–90%) followed by Bacteroidetes (3%–9%) and Firmicutes (3%–8%). These three phyla contributed more than 97% of the total bacterial community. All the samples in the present study were found to be dominated by Proteobacteria. Compared to NR polyps, AR polyps exhibited less abundance of Proteobacteria, though this remains the most abundant phylum. Such high abundance of Proteobacteria was also seen in the laboratory-maintained and wild populations of hydra. Interestingly, Franzenburg et al. found newly hatched hydra to be abundant in bacteria belonging to phylum Bacteroidetes. We also found an increase in the abundance of Bacteroidetes in budding as compared to non-budding polyps suggesting a change in the microbial composition of hydra during asexual reproduction.
Principle coordinates analysis (PCoA) plots also showed predominant differences in the microbial composition of NR and AR polyps (Fig. 1B) suggesting the possible influence of budding on the microbiota of hydra or vice versa. The shift in the composition of microbes was also reflected in an altered abundance of different bacterial groups (Table 1). Although microbes belonging to Oxalobacteraceae are the routinely found microbes in hydra, studies in the past have suggested that high abundance of Oxalobacteraceae in hydra polyps inoculated with foreign microbiota. The precise role of Oxalobacteraceae cannot be ascertained with the available data but an increased abundance of microbes belonging to Oxalobacteraceae clearly suggests a change in the microbial composition of hydra during reproduction. Likewise, Pseudomonadaceae, Sinobacteraceae, Rhodobacteraceae, and Cytophagaceae also showed increased abundance in the budding polyps. Microbes belonging to Rhizobiales, on the other hand, showed higher abundance in the non-budding polyps.
Fraune et al. found a correlation between the cellular composition of hydra and its microbial community structure. Disturbance in cellular composition of hydra leads to a change in its microbial community structure. The budding event is characterised by changes in the cellular composition of hydra, especially it is associated with a significant rise in the number of interstitial cells and its derivatives. This could be the possible reason for alteration in the microbiota of hydra during budding. It has been speculated that bacteria or factors derived from bacteria may be necessary for epithelial homeostasis and proliferation leading to budding in hydra. In this study, we have shown for the first time that there is a significant change in the microbial composition of hydra polyps during budding. Our data demonstrate changes in several bacterial groups during budding, which could be directly or indirectly influencing the budding process itself. Moreover, Fraune et al. showed that removal of interstitial cells is associated with the decrease of microbes belonging to Proteobacteria. Based on this study, the increase in the abundance of bacteria belonging to Proteobacteria during budding of hydra observed by us could be due to an increase in interstitial cells.
As is the case for budding polyps, there is no information about the microbial community structure of hydra when it attains sexual maturity. Fortunately, we came across sexually mature male hydra (SMH) in our lab. The sexual maturity can be easily confirmed visibly by occurrence of motile sperm in the testes. When the microbial composition of male polyps was studied; to our surprise, we found significant differences in its microbial composition as compared to both non-budding (non-gonad-bearing) polyps and budding polyps (Fig. 1A and B, Table 1). At the deeper taxonomic level, SMH polyps showed a high prevalence of Oxalobacteraceae, Sinobacteraceae, Rhizobiales, Comamonadaceae, and Methylophilaceae. Franzenburg et al. have shown the temporal variation of microbial community structure of hydra during embryogenesis suggesting the possible influence of microbiota on the development of hydra. In fact, like many other organisms, hydra is also considered as the metaorganism wherein there is a continuous interaction between the host and its microbiota. The present observation on changing microbiota during asexual reproduction and sexual maturity strongly indicates a role of microbes in reproduction in hydra. It may be mentioned that despite several attempts we were unable to get any sexual matured female polyps.