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Abstract

Accumulating evidence indicates that symbiotic microbiota communicate with the host nervous system, thereby modulating its neuronal activity. This modulation is reflected in changes in behaviour and physiological processes that are under neuronal control. This communication has been mainly studied in vertebrate models, whose complexity hampers the complete understanding of this interaction. Therefore, the basic molecular and cellular mechanisms underlying this communication are not completely understood yet. The fresh-water polyp Hydra is a suitable alternative model to study this communication. The presence of an anatomically simple but molecularly complex nerve net that coordinates several behaviours and physiological functions and the presence of a relatively simple and stable associated microbiota, make hydra an attractive model to study the causal effect of symbiotic bacteria and the host neuronal activity. Here we show that the feeding response and the spontaneous contractions of hydra’s body column, both actions that are under the control of the nerve net, are modulated by hydra symbiotic bacteria. Compared to animals harbouring undisturbed microbiota, germ-free animals display shorter feeding response duration. Similarly, germ-free polyps show reduced and less regular spontaneous contraction frequencies. The effects on contraction frequency were partially restored by reconstituting the natural microbiota. Moreover, soluble molecule(s) produced by symbiotic bacteria may be involved in contraction frequency modulation. Our findings suggest that the influence of bacteria on neuronal activity is the outcome of an evolutionary ancient interaction between bacteria and metazoans, opening a window into investigating the basic mechanisms of for example, dysmotility disorders in vertebrates.