This is the sixth in a series of articles exploring the gut microbiota. This week, our safari is leaving the duodenum and dropping into the jejunum. We wouldn’t really notice it except for a constriction where a muscle attachment is made.
The villi in the jejunum are a little longer and it has more folds, so it is better at absorption than the duodenum, but not much change is apparent. Nevertheless, thanks to the antacids secreted in the duodenum, the jejunum is considerably more inviting to bacteria.
So inviting that this is where we see diseases like small intestinal bacterial overgrowth (SIBO), which often correlates with irritable bowel syndrome (IBS), celiac and Crohn’s disease. It involves an overeager bacterial community that outstays its welcome. This overload of fermenting bacteria creates gas that causes bloating and pain. SIBO and its brethren are all associated with anxiety.
SIBO has many possible causes, and so it probably doesn’t have a single easy solution either. One thing the sufferer can try is to reduce sugar consumption, starving the bacteria back to reasonable numbers, but each case is unique. Here we can see the excess bacteria, but in general, the gut lining is intact, at least visually. IBS is difficult to diagnose for just this reason. However, beyond its healthy pink surface, the stressed tissues can become excessively permeable, allowing toxins or even microbes to breach the barrier and enter the bloodstream. This is so-called “leaky gut syndrome” that can produce low-grade inflammation affecting both mood and cognition.
The jejunum then turns into the ileum, again with little fanfare. There are a few differences that we can see: throughout the ileum, but particularly toward the last sections, we see patches of cells connected to the lymphatic system. They include immune cells whose job is to kill pathogenic bacteria. B cells make antibodies to label the pathogens, while T cells — along with the ever present phagocytes — vacuum up the tagged bacteria. The appropriately named Natural Killer cells stab holes in the bacteria and squirt toxins in, causing them to commit suicide and unceremoniously dissolve.
Considering how much effort is put into killing microbes, it’s a wonder that any of them survive at all. However, the mystery lifts a little when we consider that a select cadre of bacteria have coevolved with humans over millions of years, and have figured out strategies to evade or even to work hand-in-hand with the immune system. Some of these tricks were worked out when we were babies and our microbiota got a once-in-a-lifetime opportunity to educate our immune system. Having introduced themselves early, these beneficial microbes get a free lifetime pass from the immune system.
From the point where we start to eat solid food, we develop a core set of bacteria that stays with us for life. Or at least until we take antibiotics and kill them off. Even then, many microbes in our personalized microbiota will come back, presumably because they can hide out deep in our crypts and other hard-to-reach spots like the appendix. Still, some researchers worry that we might be witnessing an extinction event, losing some gut bacteria forever due to bad diets and antibiotics.
As we reach the end of the ileum, we can see that the amount and variety of bacteria is steadily increasing. With distance from the bile duct and the continuous spurting of mucus, the acidity is falling and the environment is improving for the psychobiotic Lactobacillus species in fermented foods like yogurt. These include L. acidophilus, L. casei, L. delbrueckii and L. bulgaricus, all of which enjoy this neighborhood.
Other Lactobacillus bacteria found in cheese and buttermilk, including L. rhamnosus, L. lactis and L. helveticus, also have psychobiotic properties and make themselves home here. And fermented foods like sauerkraut and pickles contain L. brevis, which is a denizen of these parts as well. Many of these bacteria won’t actually colonize the gut, but in this accommodating environment they produce important metabolites to heal and nourish the gut lining while in transit.
Yeast can also take up residence here. In fact, some of the same yeast used to brew beer, Saccharomyces (sugar-fungus), is found in the small bowel of most people. In rare cases, overgrowth of S. cerevisiae can lead to auto-brewery syndrome, where the person actually ferments beer in their gut and gets inebriated. It’s a cheap drunk, but the hangovers are banging. These people often end up depressed, which may have more to do with the fungal infection than the recurring hangover. These episodes can be exacerbated by antibiotic treatments, since bacteria help to keep the yeast population manageable. Anti-fungals, along with bacterial probiotics may help people with this syndrome.
Candida is another yeast that can live harmoniously in our gut until its population gets out of control. Again, that can happen when antibiotics kill off the bacteria that keep its numbers in check. This can happen in hospitals where it is a major cause of mortality.
The ileum is where we pick up vitamin B12, a large, complex molecule that neither plants, fungi nor animals can produce. Oddly enough, only bacteria can synthesize vitamin B12. It’s an important vitamin: B12 protects against brain atrophy and is essential to maintain a healthy nervous system. Vitamin B12 deficiency can lead to mania and psychosis, meaning that the lactic acid bacteria that produce B12 have psychobiotic properties.
The end of the ileum also has specialized tissue that absorbs the bile acids that were secreted from the gall bladder to digest fats. Where it can, the body likes to recycle.
The chyme has been prepared. It has fiber in the form of complex sugars that couldn’t be broken down by acids or enzymes. The chyme’s microbial entourage has been stripped of pathogens, and the survivors are heading toward their Shangri-La.
We are aimed at a port that will take us to the final part of our alimentary journey: the colon. Next week, we’ll head on down to that bustling microbial metropolis. If you haven’t already, make sure to subscribe. This safari is about to come to an end, but it is still free to jump in! If you have gut or brain issues (or are just curious), the journey could make a big difference in your life.
If you have already subscribed, congratulations on your discerning mind, and make sure to invite your friends to come along for the last leg of this safari.
Good question! You can test for auto-brewery syndrome by checking for blood or breath alcohol after a dose of glucose. And you're right, it is wise to check first: antibiotics are likely to exacerbate the situation since yeast and bacteria compete for resources. In fact, lengthy antibiotic treatments can result in the syndrome!
Most doctors have never heard of it! It's pretty rare, but the people who have it know something is wrong...