Go with the flow: Spatial regulation of microbiota in Hydra

Muscle contractions playing an unexpected role in microbe host interactions

Background. The freshwater polyp Hydra is an attractive and widely used model organism for studying biological questions in the fields of immunology, evolutionary biology, or neurobiology. An even more unique element of Hydra is their lack of any signs of ageing due to stem cells with endless self-renewal capacity. Moreover, Hydra’s ectodermal epithelial surface, the so-called glycocalyx, is densely packed with a stable bacterial community of various species – a vital characteristic they share with many other species, including humans. In Hydra, the microbes seem to regulate the organism’s spontaneous contractile activity, comparable to gut peristalsis. In humans, disruption of the contraction pattern or dysbiosis leads to gastrointestinal conditions such as irritable bowel syndrome but has also been linked to various other aging-related and end-of-life diseases.


Challenge. Despite the morphological similarities to other species, Hydra’s spontaneous recurrent full body contraction-relaxation cycles (SCs) remain an unsolved mystery, especially since Hydra's SC characteristics do not fit any of the common functions usually linked to such processes in other animals. While it was previously demonstrated that a specific mix of the bacterial cohabitants regulate Hydra’s SCs, their function in contributing to Hydra’s vitality is still unclear.


Solution. The group around Pioneer Campus PI Janna Nawroth and her collaborators from the universities of Kiel and Southern California, respectively, hypothesized that the SCs might generate fluid transport patterns to control and support the growth of Hydra's microbial symbionts. A very plausible hypothesis, as such communities are typically regulated not only by host cells interactions, but also by characteristics of the fluid medium.

Combining high throughput profiling, in vivo flow analysis, and mathematical modelling, the team found that recurrent SCs resulted in maximal shedding of viscous boundary layers near the head's surface, indeed enabling Hydra to transiently reshape the chemical microenvironment in the glycocalyx.

The authors reflected that this exchange could have different functions: (a) facilitating the transport of various compounds, e.g., nutrients and waste, to and from the surface and (b) maintaining a specific spatial colonization pattern along Hydra's body column. Interestingly, they predict that this behavior might be crucial for pathogen defense and help modulate the frequency of Hydra's SCs - suggesting a feedback loop fitting to the previously reported SC regulation by the microbial symbionts.

Most importantly, these findings in Hydra are consistent with microfluidic models mimicking the viscous environment and laminar flow of the gut suggesting that intestinal contractions mix the luminal content, which appears to modulate microbial density and composition.


Conclusion. Like Hydra, the peristaltic gut displays recurrent SCs and a viscosity-dominated flow regime. Given that (a) disturbances of this contractile activity is correlated with various diseases and (b) Hydra's mucous layer is comparable to the mammalian colon, carefully designed and controlled perturbation experiments in Hydra may offer a unique opportunity to study both the mechanisms and the in vivo role of SCs in more complex systems.

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