There are some biological topics it can be unpleasant to think about immediately prior to lunch. For some people, it's the realization that bacteria are everywhere – living and growing not only on and in your body but also most likely on that lunch. Insects face the same reality, living in close association with bacteria in and around their bodies.

Researchers are just beginning to unravel how some of these associations work, finding that gut bacteria can: synthesize B vitamins, which are then used by insects; digest complex macromolecules that insects cannot; and otherwise modify the relationship between an insect and its food. As past research has shown that bacteria that cycle between the gut and the environment can reduce the amount of body fat that the fruit fly Drosophila melanogaster has tucked away, Jia-Hsin Huang and Angela Douglas from Cornell University, USA, wanted to try to unravel what the underlying mechanisms behind this waist-shrinking effect might be.

Using lines of flies that were bacteria-free, the researchers first set out to show that infection with particular strains of bacteria as adults could change the fat content of the flies. They found that flies infected with the bacteria Acetobacter tropicalis were much slimmer than their counterparts that were infected with Lactobacillus brevis or remained uninfected.

So how were the bacteria accomplishing this slimming effect on the flies? One possibility was that the flies infected with A. tropicalis were unable to digest as much sugar as the other flies. The researchers compared how well the flies with the different infections were able to digest sugar by measuring how much sugar they ate as well as how much was left undigested in their feces from a clean food source that was not able to be infected with the bacteria. They found that the A. tropicalis-infected flies were actually more able to digest sugar.

Next, the researchers looked at the sugar content of the food the flies were eating. While the researchers had provided all the flies with the same food, the food that was infected with A. tropicalis had a significantly lower sugar content and the researchers concluded that the bacteria were actually consuming the sugar in the food before the flies had a chance to get at it.

Fruit flies in the wild consume high sugar diets that can make it hard to consume enough micronutrients for a balanced diet, so having the right species of bacteria around is important for the insects. And this research shows that having bacteria around can not only change the amount of fat in a fly but also change the way that food is digested, which is probably the case for other animals as well.

While it might make us uncomfortable to think about it, bacteria are everywhere, growing not only on and inside us but also in our food. Understanding the complicated interactions between ourselves, our passengers and our food is only just beginning.

A. E.
Consumption of dietary sugar by gut bacteria determines Drosophila lipid content
Biol. Lett