Why Dysbiosis, SIBO, Hydrogen Sulfide Intolerance, MCAS, and Long COVID May All Be Downstream of a Single Event — Meet the colonocyte: the cell that decides who is allowed to live in your gut

To understand the model, you have to meet its central character. A colonocyte is one of the epithelial cells that line the wall of your large intestine — the single layer of cells standing between the trillions of microbes in your gut lumen and the rest of your body. The instinct is to imagine these cells as a passive wall, a row of bricks. That instinct is wrong, and the error is the source of a huge amount of confusion in this field. The mature colonocyte is not a brick. It is a metabolically specialized engine, and one of its jobs — arguably its most important ecological job — is to control the oxygen environment of the colon. Here is the setup. Your gut wall is richly supplied with blood, so the tissue side of the colonocyte sits in oxygen. But the lumen — the open channel where your microbes live — is supposed to be almost completely oxygen-free, a condition called physiological hypoxia (literally "normal low-oxygen"). This matters enormously, because most of the beneficial gut bacteria are obligate anaerobes: organisms that are poisoned by oxygen. Faecalibacterium, Roseburia, Eubacterium, Anaerostipes, the Clostridial clusters IV and XIVa — these are the keystone species of a healthy colon, and they can only dominate when the neighborhood stays anaerobic. So who keeps the lumen anaerobic? The colonocyte does. And the way it does it is the heart of this entire model.