Nourishing Lodger
The composition of the intestinal flora has a significant influence on digestion. Perhaps microorganisms even control whether a person is fat or thin. The human genome has been deciphered. But most of the genetic material in our body is still unknown. Because it's not human at all, it belongs to microorganisms. And since the tiny lodgers have such a big impact on us, scientists like Jeffrey Gordon of Washington University in St. Louis are demanding that not only our genome be decoded, but also the "microbiome" - the entire genetic make-up of all microorganisms living in humans must become. A daunting task.
"We are superorganisms that contain not only human cells, but also bacterial cells and archaea. In adults, the number of bacterial and archebacterial cells is ten times that of humans," explains Gordon. "The genes of these 10 to 100 trillion microorganisms significantly outnumber our genes. They form an important part of our genetic landscape and give us traits that we didn't have to evolve ourselves."
Gordon and his colleagues don't just deal with genetics. Using "germ-free" mice, they investigated how bacteria and archaea living in the gut influence nutrient absorption. "We are superorganisms that contain not only human cells"
(Jeffrey Gordon) These rodents are born with no bacteria in their digestive tract and are kept in a sterile environment so that no microorganisms can colonize them. The researchers compared the germ-free mice's food intake and weight to normal mice, which closely resembled them genetically.
The result: The "common" rodents ate 29 percent less than the germ-free ones, but carried around 42 percent more body fat. The bacteria and archaea therefore ensured that the animals could utilize nutrients that the germ-free mice excreted undigested. In fact, many microorganisms that are part of the gut flora can process various forms of polysaccharides that humans and mice cannot digest on their own.
The scientists then concentrated on two specific microorganisms that also live in the human intestine: the bacterium Bacteroides thetaiotaomicron and the archae Methanobrevibacter smithii. To do this, they first expose germ-free mice to the bacterium, the archae, or both microorganisms at the same time. B. thetaiotaomicron can initiate a fermentation process that breaks down indigestible polysaccharides in humans and mice into smaller, digestible sugars. M. smithii breaks down hydrogen and other substances produced during the process, slowing it down. So the microorganisms work together to break down the polysaccharides.
This was noticeable in the mice: if bacteria and archaea were present in the gut, the gene activity of B. thetaiotaomicron changed significantly. In this case, the bacteria preferentially break down fructans. This class of polysaccharides includes, for example, inulin, which is added to skim milk yoghurt, among other things, because it gives it a creamy texture. Due to the greater appetite of the bacteria for fructans, the mouse obviously also had more energy available. Because the rodents, which had both microorganisms in their intestines, formed significantly more fat pads than the mice, which only carried one type of microbe around with them.
Microorganisms can therefore lead to weight gain - in previously germ-free mice. However, the experiments say little about the influence of the microorganisms on the weight of a person whose intestinal flora consists of several hundred types of bacteria."The presence of M. smithii increased the effectiveness of the digestive system. It remains to be seen whether archae can be manipulated to improve overall digestion," says Gordon. "And will be interesting to see if fat people have more M. smithii in their gut than thin people." Figuring out the latter will be part of the decoding of the microbiome. A small part.
