“Understanding is a two-way street” –Eleanor Roosevelt
Last week, we talked about lipopolysaccharide (LPS), which our immune system responds to with alacrity. LPS is the detritus of pathogenic bacteria, and it triggers a clarion call to pull up the drawbridge and man the ramparts.
This connection between pathogens and the immune system is undeniable. It is one of the strongest reactions our body can make in response to bacterial invaders. But this fight has gone on for millennia, and over time the battle has become exquisitely baroque.
It’s Complicated
When the immune system discovers LPS, it can brutally attack bacteria in the intestines, dramatically altering the microbiome. The immune system lacks subtlety. When a bacterium enters a gut cell, the immune system doesn’t politely reach in and pluck it out. Instead, it destroys the cell outright, killing both the intruder and the hostage. The gut lining is a single layer thick, so dead cells can leave a gap. Thus, in its exuberant crusade to kill rogue bacteria, the immune system can open up leaks in the gut.
Bacteria that produce LPS can then breach the gut lining and enter the bloodstream. From there, they can take their pick of tissues to colonize. These displaced gut microbes spur the immune system into further action, escalating the war and leading to systemic inflammation. This becomes a feedback loop where bacteria and immune cells take turns making us sick.
So does the microbiome shape the immune response, or does the immune system shape the microbiome? The answer is yes. In its zeal to kill pathogens, the immune system can inadvertently open the floodgates, creating a negative feedback loop.
It would all end well if the immune system could wipe out the pathogens and declare victory. But as long as we diligently feed those pathogens with junk food, an endless battle is guaranteed.
The Gut-Brain Axis is Also a Cycle
There’s more: The gut and the brain are connected by the vagus nerve. Communications from the gut microbiome travel up the vagus to alert the brain to irregularities. You are not always directly aware of problems in your gut, but they often manifest as anxiety or depression.
The vagus, however, is a two-way channel, and the brain can talk back to the gut, affecting hormones and the immune system. So once again, the back and forth makes it tricky to show causality.
These problems are not due to experimental flaws; they are intrinsic to the way the microbiome has taken up residence in our bodies. There is an essential circularity to our cohabitation with microbes. That cycle can be virtuous or vicious. And for better or worse, it is pretty stable.
However, it is still possible to disrupt the cycle. While it is difficult to block the immune system, it is easy to modify the microbiota: Simply stop feeding the bad bacteria and start feeding the good. That means easing sugar out of the diet and substituting prebiotic fiber. Within a few days, the ecology of the gut should diversify and become healthier.
Fiber-consuming bacteria produce butyrate, a small molecule that nourishes and heals your gut lining, plugging the leaks. With bacteria back where they belong, the immune system will stand down, and your precious tissues will no longer be under attack.
In Sum
In biology, nothing is simple. Evolution works by continuously layering new features over the existing chassis. The concept of recruiting your own set of microbes to protect you from other microbes is a perfect example of this tendency.
Biology is complex because the world is full of novelty, and much of it would like to eat you. The only way to keep up is to acknowledge it and go with the flow, not against it. There are no guarantees in life, but a diet designed to keep your gut microbes happy is a healthy start.
Do you have a friend or loved one with gut issues, anxiety, or depression? Psychobiotic has actionable science-based advice. Share it with them. They will thank you.
References
Walker, A.W., Hoyles, L. Human microbiome myths and misconceptions. Nat Microbiol 8, 1392–1396 (2023).
Patil, Abhinandan, and Neha Singh. “Microbiota Modulation: Examining the Effects on Pathogen Colonization and Infection.” Preprints, July 17, 2023.
Schneider, Tamar. “The Microbiome Function in a Host Organism: A Medical Puzzle or an Essential Ecological Environment?” Biological Theory, March 27, 2023.