Almost exactly 3 years ago, I wrote a post about your blood type affecting your microbiome. From that post: “Like me, I’m sure most of you know that the best known way of categorizing blood type is the ABO system. That is, each of us has A, B, AB or O type blood…These actually refer to the antigens (carbohydrates, also referred to as glycans) on the outside of the red blood cells. (The word antigen means any substance that evokes an immune response by the body (like the production of antibodies, for example).) Which antigen you express (or in the case of O blood, no antigen) is genetically determined.”
If you remember, those antigens are presented not just on blood cells, but also on many kinds of immune cells as well as many bodily excretions (saliva, tears, breast milk, mucus, etc.) as well as in the mucus that coats the lining of the gut. About 80% of the human population are known as “secretors,” meaning that they are genetically prone to secreting these antigens in their bodily fluids. The two papers I described to you in that previous post demonstrate that secretors tend to have more stable microbiomes as compared to non-secretors because the mucus of the intestines serves as a food source for specific kinds of bacteria, including many probiotic species. What wasn’t known yet is exactly how this works: why are certain bacteria more likely to colonize a person with, let’s say, A type blood versus B?
Today’s paper answers that question.[i] Researchers at the Quadram Institute in the UK have been studying the gut’s mucus lining, trying to sort out how it encourages certain bacteria to colonize – and to colonize the correct part of the intestine. Remember that mucus is made of proteins called mucins, which are covered in sugars (carbohydrates). Mucin proteins, which present those blood group antigens, are “capped” with different molecules that prevent bacteria from getting to those sugars…unless, it turns out, the bacteria produce a specific enzyme that can break down that particular antigen-presenting cap. It is this specificity of the cap and the matching bacterial enzyme that promotes the growth of specific bacteria in certain intestinal sites.
These scientists focused on Ruminococcus gnavus bacteria because it is so common, found in about 90% of humans; and it is also one of the first bacteria to colonize infants. R. gnavus is also positively and negatively associated with certain diseases, including neurological disorders and inflammatory bowel diseases. They found that these bacteria produce a specific enzyme for breaking down the “caps” on the proteins that present only blood group A antigens.[ii] This means, of course, that we now know the mechanism by which certain blood types would lead to the colonization of certain bacteria – a really important step forward. That said, there are many unanswered questions still: what is the actual significance of this finding in humans? Does this mean that people with certain blood types have similar microbiome profiles? (So far, the results on that are mixed and provide no concrete evidence either way.) Can we use this to manipulate the gut bacteria to improve health?
The hope is that the study of these enzymes may eventually lead to a means of combating infections, since we know now that the blood antigens encourage the growth of specific bacteria: “Blood group antigens can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses.” For example, we know with certainty that people with blood types O and A are more susceptible to norovirus, while those with B or non-secretors are far less susceptible. Some studies done over the last couple of years suggest that those with O-negative blood type are less at risk of getting severe COVID, while A type blood is more susceptible. (These findings are still being researched and debated, however.)
I find this subject particularly interesting so stay tuned for further developments! Nice to see progress being made!
[i] Wu, H., et al. (2021) The human gut symbiont Ruminococcus gnavus shows specificity to blood group A antigen during mucin glycan foraging: Implication for niche colonisation in the gastrointestinal tract. PLOS Biology. doi.org/10.1371/journal.pbio.3001498.