One of the major themes in biology is how we can be really alike in lots of ways and really different in others. For example, we share a huge amount of our DNA with mice, and in lots of ways we’re similar, but in lots of ways we’re really different too. One way in which humans are all kind of the same, but also a little different from each other is in the population of microflora, the microbes that live in and on us all.
As we talked about earlier, our guts are full of microbes that are digesting the bits of food we can’t digest ourselves. In return for this food supply, these microbes are making some useful vitamins for us. The same thing is going on in other mammals, including mice. By and large humans and mice have some of the same types of bacterial strains resident in them. But, relatively small, shifts in bacterial populations among different humans and among different mice can have outcomes on traits including weight.
Our guts contain high numbers of bacteria from two bacterial phyla, the Bacteroidetes, which are Gram-negative, and Firmicutes, which are Gram-positive. Scientists observed that obese humans had a higher percentage of Firmicutes relative to Bacteroidetes than leaner humans did.
Firmicutes are a little better at making use of the extra food going through the intestines than the Bacteroidetes bacteria are.
It makes sense that obese individuals’ microbiota might be better at getting extra energy from food. Since the bacteria are making compounds that get used by their human hosts, humans harboring extra Firmicutes might indeed be expected to have more weight. What wasn’t clear was which came first, the obese human, or the fattening bacteria. In order to investigate, scientists at Washington University in Saint Louis looked at similar bacteria in normal and obese mice. If you’ve never seen an obese mouse, here you go:
What these scientists saw was that the normal and obese mice had the same shifts in Bacteroidetes relative to Firmicutes that humans had. They were then able to do an experiment.
In the first step, they took some bacteria from both obese and lean mice. Then, they got some "germ-free mice" that didn’t have any bacteria in their guts at all. They inoculated the germ-free mice with the bacteria from either obese or lean mice and studied the development of the (formerly) germ-free mice. They saw that mice inoculated with bacteria from the obese mice became fatter than mice that had gotten bacteria from leaner mice.
These experiments underline how even though we all have, similar types of bacteria in our system small differences can lead to noticeable changes.