Humans share a close relationship with bacteria. There are trillions of microorganisms on our planet, and our bodies play host to millions of them. As scientific inquiry has expanded into this field in recent years after the development of powerful genetic sequencing technologies, researchers have found that the microbes that an organism carries in and on it play an important role in health. When there is an imbalance in the system, disease can result. As such, there is intense research focus in this area, but there are still a lot of unknowns. New work using a simple model organism has provided new insight into the molecular characteristics of this relationship.
Reporting in Nature Communications, scientists at the Zoological Institute Kiel University used the freshwater polyp Hydra as a model to demonstrate how the nervous system of this animal interacts with the microorganisms it hosts, its microbiome. For the first time, it has been shown that nerve cells secrete small molecules that influence the regulation of the growth and composition of certain kinds of helpful bacteria on the body column of Hydra.
"Up to now, neuronal factors that influence the body's bacterial colonization were largely unknown. We have been able to prove that the nervous system plays an important regulatory role here," explained the leader of the research, Professor Thomas Bosch, an evolutionary developmental biologist.
Freshwater polyps are a lasting branch of an ancient part of the animal kingdom (and are described in the video). Their nervous system is made up of only around 3000 neurons, compared to the billions that humans carry. While very simple, there is enough fundamental similarity in the molecular characteristics to extrapolate experimental findings to humans.
The investigators wanted to know whether molecules made by the nervous system, which act as messenger signals, influence the relationship between organisms and their microbiomes. These messenger molecules, called neuropeptides, seem to have an antibacterial effect with impacts the composition and distribution of growing microbes.
In this work, the investigators focused on the development of the nervous system in the freshwater polyp. These animals take about three weeks to grow a complete nervous system. During that time, the microbiome on the surface of the organism changes dramatically, eventually stabilizing. The neuropeptides' antimicrobial effect cuts down the population of Gram-positive types of bacteria over about four weeks. Finally, the standard microbiome composition is reached, one which is dominated by Gram-negative Curvibacter bacteria.
There is a similar effect in other areas where neuropeptides are produced (as they are not made all over the body). The head is one region where neuropeptides are made; for example, there are six times as many Curvibacter bacteria on tentacles than on the head. The researchers suggested that the nervous system has been exerting an influence on the microbiome in addition to other functions for which it is more commonly known.
"The findings are also important in an evolutionary context. Since the ancestors of these animals have invented the nervous system, it seems that the interaction between the nervous system and the microbiome is an ancient feature of multicellular animals. Since the simple design of Hydra has great basic and translational relevance and promises to reveal new and unexpected basic features of nervous systems, further research into the interaction between body and bacteria will, therefore, concentrate more on the neuronal aspects," concluded Bosch.