Microbes are everywhere. Not only can you find them on that pizza slice from yesterday, they’re also present on your skin and in your gut in large quantities. Don’t worry! Most of these bacteria are beneficial to your health, they can help you digest compounds you normally wouldn’t and train your immune system the difference between good microbes and the bad ones. In fact, animals that are completely sterile are less effective at fighting microbial infections. Making sure that only the right bacteria grow and the bad ones stay out, requires effective management by the immune system. The adaptive immune system, the part of the immune system which is capable of learning, is thought to play a major role in this process. The role of the innate immunity, which is a first barrier and response to pathogens, in this process is still a bit unclear.
If this is the case, would animals without such an adaptive immune system control the microbes that are living on them, and if so, how would they do it? This question has partly been answered by Fraune & Bosch by studying a small organism called Hydra, an animal which is about as distantly related from us vertebrates as you can get, while still being considered animal. These Hydra somewhat resemble their mythological counterpart (hence the name), because of their twelve tentacles. Less mythological is their size: they’re only a few of millimetres long and consist of only a couple of different cell types.
Fraune and Bosch studied two different species of Hydra: Hydra Vulgaris and Hydra Oligactis. They studied individuals that were cultured for more than 30 years in a laboratory, as well as individuals that were isolated directly from several lakes in Germany. By analysing rRNA genes (which are like a bar code for different bacterial species), they identified the bacteria living on the epithelium (‘skin’) of the Hydras. Interestingly, they found that the Hydra Vulgaris and Hydra Oligactis which had been living under the same conditions in the laboratory for 30 years, showed large differences in the kind of microbes living on them. In other words, even though these closely related animals lived in the same environment for a long time, they still retained their own species-specific and characteristic microbial fauna.
Moreover, when Fraune and Bosch analysed the Hydra species from the lakes, they found that the microbes living on Hydra Oligactis in the lake were largely similar to the ones living on Hydra Oligactis in the laboratory. This is striking, since the lakes from which these Hydras were isolated are far from sterile, and contain multiple different bacteria living in the water. So over a long period of time, the Hydra from the laboratory were able to maintain the microbial communities living on their epithelia. They wouldn’t do so if this wasn’t important: Hydra suffer from severe developmental defects when they’re grown in a sterile environment without any bacteria. It seems that keeping the right bacteria in place, and keeping the bad ones out, is just as important for this microscopical animal as it is for us. Learning how they do this, is the obvious next step. Their lab has already made progress on this issue, but I think there’s still a lot more we could learn from these little Cnidaria!
Fraune, S., & Bosch, T. (2007). Long-term maintenance of species-specific bacterial microbiota in the basal metazoan Hydra Proceedings of the National Academy of Sciences, 104 (32), 13146-13151 DOI: 10.1073/pnas.0703375104
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