Our gastrointestinal system helps us obtain nutrients from food. However, we aren't the only ones that benefit from our gut: a diverse community of microorganisms call the gut home and can help us digest our food. These tiny companions can also produce compounds that send signals to the brain, potentially affecting its development. This is particularly influential in early life, when microbes populate the gut and the brain develops. The community of microbes that populate the early gut depends on the microbes present in the environment, so an animal's gut microbes can look quite different depending on where they grew up. A properly developed and functioning brain is important for survival – which is particularly critical for amphibians whose populations are declining. With this in mind, scientists Kyle Emerson and Sarah Woodley at Duquesne University, USA, wondered whether the brains of developing tadpoles could be influenced by the microbes found in the water where they're living.
To test this, the researchers raised northern leopard frog (Lithobates pipiens) tadpoles in either natural pond water, or pond water that had been sterilized to reduce the diversity of microbes present in the water. After keeping tadpoles in both types of water as they grew for 4 weeks, the researchers then looked at the amphibians’ gut microbes by sequencing the bacterial DNA. They found that the bacterial communities in the guts of tadpoles raised in the natural pond and sterilized pond water were distinct: there were fewer species of bacteria in the gut of tadpoles raised in sterilized pond water. But did this shift in gut bacteria impact the tadpoles’ behaviour?
To assess this, the researchers transferred tadpoles one at a time to a small tank and recorded how they reacted when a new and mysterious object – a glass vial – appeared in the tank. The tadpoles raised in normal water were unfazed by the object, swimming around normally; however, the tadpoles raised in the sterilized water took a more cautious approach around the unfamiliar object, swimming less. Even when the researchers tried to grab the tadpoles’ attention by placing their favourite snack – frog brittle – in the vial, the developing amphibians showed little interest. In contrast, the tadpoles with a greater variety of bacteria in their gut were more excited by the presence of food, swimming around more and faster. It appears that the microbes present in the guts of these tadpoles influenced how they behaved when faced with either a strange object or a familiar delicious one.
To determine how the microbial content of the pond water impacted the tadpoles’ growth and development, the researchers measured the body and brain size of the youngsters. Those raised in the sterilized pond water were heavier and had larger brains. Emerson and Woodley suggested this could be related to how efficiently the tadpoles with a wider assortment of gut bacteria digested their food. While measuring different brain regions, they also found that tadpoles raised in sterilized pond water had narrower medullas, a region of the brain that regulates processes such as breathing, showing that brain development was altered by gut bacteria, not only in terms of the size, but also its shape.
Changing the microbial community in the water where these tadpoles were living not only impacted the microbial community living in their gut, but also ultimately altered their development and behaviour. Understanding how the bacterial community in an animal's rearing environment affects their life is particularly important when they are facing increasing challenges in the wild, such as amphibians. We can't underestimate the power of these tiny bacterial influencers: they don't need social media to shape the minds of these tadpole youths, they do it simply by existing in the gut.