Bacteria to cure depression? A study is trying it

Very often on these pages we have discussed how our genome and that of any other living organism is a mosaic of pieces integrated from other living beings, often (but not always) through viral vectors that have made DNA sequences derived from viruses, microorganisms, fungi, plants and animals transit from one Kingdom to another. Without microorganisms, the bulk of this continuous genetic reshuffling, essential for the evolution of species to proceed at a speed sufficient to generate the complexity we see around us, probably would not exist; in this sense, microorganisms, and viruses above all, have truly physically shaped nature and our own species. On the other hand, it has long been known that microbes, and particularly bacteria, have also shaped the physical world and the biosphere since life began: the very oxygen we breathe was first produced by microbes, which created the atmosphere we know today (in the process killing off a large part of the biosphere that existed at the beginning of life, for which oxygen was toxic), but the soil and all of the earth’s geochemistry are also the product of of individual species or, much more often, of entire microbial communities that collaborate to create the environment in which we live. Even the climate, as we have already discussed here, and in particular rainfall, is strongly influenced by particular species of plant pathogenic bacteria, which have evolved refined systems to condense snow or rain through specific proteins, thus managing to land from the atmosphere in optimal for your life cycle.

However, it is in our intestines that the action of microorganisms affects us daily and closely. In that environment, the supply of proteins, fats and carbohydrates that the bacteria can feed on is regular and abundant; at the same time, the microbial ecosystem that thrives there cooperates in an indispensable way in the digestion of food and in making vitamins and minerals available to our body. In the intestinal community, viruses also play an important role: in particular, specialized viruses such as certain certain types of phages keep the bacteria in our body under control, preventing their unregulated growth and constituting an important limiting selection factor towards more harmful strains. But there is much, much more.

Meanwhile, the intestinal microbiome plays a critical role in the maturation of our immune system, both in its innate and adaptive component, while simultaneously our immune system finely regulates the symbiosis between us and the microbial populations we carry inside. Most importantly, we’ve known for years that 90 percent of the gut microbiome are capable of producing neurotransmitters such as dopamine and serotonin., which play a key role in regulating human moods. Because this ability is unique to bacteria that live in animal bodies, it appears that these microbes have evolved over millions of years to create chemical messengers that allow them to communicate and influence the mood and behavior of their hosts.

The evolutionary reason why bacteria produce chemicals that improve our moods could be for example to make us more likely to be gregarious, therefore providing microorganisms with the opportunity to colonize other hosts. Not surprisingly, when comparing the microbiomes of volunteers diagnosed with depression with those of non-depressed people, two types of bacteria – Coprococcus And Dialist – which were common in the guts of healthy participants, but absent in the depressed, are demonstrably capable of producing antidepressant substances. Although this is not definitive proof, it should be considered together with the numerous other data produced in animal models that indicate that the intestinal microbiome is indeed able to alter the behavior of rats and mice. In particular, “germ-free” rodents, produced by cesarean delivery in sterile environments, are unable to recognize other mice and show behavior similar to human anxiety and depression. It is possible to restore more normal behavior by introducing certain strains of bacteria into their intestines. Perhaps one day we will treat depression the way it was done in mice, by modulating the makeup of our own gut flora; in 2024, in particular, we will have the first results of some clinical trials that have precisely this objective.

Let’s think about it. Not only did we all evolve from bacterial cells; not only did our earliest ancestors incorporate entire bacterial cells, which today form the mitochondria of our cells; not only does our genome carry traces of continuous integration of virus DNA; not only do we absorb essential nutrients only thanks to our intestinal flora; not only that, finally, our immune system is helped in its maturation by the multitude of bacteria that we acquire from birth. The fact is that even our own brains, perhaps our own thoughts and almost certainly our moods are influenced by the billions of cells and by the vast variety of genetic and biochemical programs that dwell in the intestinal ecosystem.. “I am vast, I contain multitudes,” wrote Walt Whitman; and this phrase, in light of what we are learning, has never seemed so true.