The relationship between animals and their intestinal bacteria is even more complex than previously thought. It is known that the host benefits from substances that the bacteria secrete. And the bacteria live on the food in the intestine and on each other’s breakdown products.
But sometimes there is a third food relationship: there are intestinal bacteria that only survive on substances that the host produces itself. Swiss researchers discovered this in an ingenious, multidisciplinary study with honey bees. They wrote about it this week in Nature Microbiology.
The intestines of almost all animals are chock full of bacteria. For example, we humans have ten times as many bacteria in our intestines as cells in our body: a total of about 100 trillion individuals from more than five thousand species, together accounting for a kilo or two. These bacteria are useful: they break down components of our food that we cannot digest ourselves, such as cellulose, they produce valuable substances such as vitamin K and they keep harmful bacteria under control. In turn, the bacteria benefit from a stable, comfortable living environment and from all that food in the intestine.
But does the host itself also contribute substances to this metabolic exchange? This is very difficult to investigate, the Swiss write, because of the ‘overwhelming’ amount of substances in our intestines, from food to bacterial breakdown products and our own waste products.
Only sugar water
They therefore looked for a simpler animal model. They found that in the honey bee (Apis mellifera). It only has about twenty types of bacteria in its intestine. The bee itself can survive on nothing but sugar water. At the same time, he has at least one type of intestinal bacteria that cannot digest sugar. This offers opportunities for interesting experiments, the Swiss thought.
They bred bees without intestinal bacteria, by allowing them to hatch from the egg in a sterile environment. They only fed those bees sterile sugar water. They then looked at what happened when different combinations of bacteria were allowed to colonize the bee intestine.
They soon discovered that a complex network of relationships is created in the intestine: the different types of bacteria feed each other with all kinds of substances that they have made themselves from sugar. But during one of the experiments the Swiss came across something strange. In that experiment, only one type of bacteria lived in the bee intestine: Snodgrassella alvi (named after the American entomologist Robert Snodgrass, who discovered this species in bees). That bacteria cannot convert sugar. Yet this species survived for a long time in the bee intestine.
Chemical analysis of the intestinal juice showed that it contains all kinds of acids, including citric acid and malic acid, which the bees apparently make themselves – after all, they were only fed sugar water and one type of bacteria. S. alvi they didn’t have to expect anything, because they can’t digest sugar. Could the bacteria possibly live on these acids?
To investigate this, the Swiss ‘labeled’ the glucose molecules in the sugar water by replacing the natural, relatively light carbon atoms with their slightly heavier isotopes. With an advanced measuring instrument (nanoscale secondary ion mass spectrometryor nano-SIMS) they were then able to demonstrate that these heavier isotopes first ended up in the bee, then in the intestinal juice acids, and then in the continuously dividing and growing bacteria.
There are probably many more bacteria that live on substances excreted by their host, the Swiss write – also in other animal species. They think that through these substances, hosts can ‘steer’ the bacterial composition in their intestines towards the most favorable possible cooperation. They want to investigate that principle further.