4.4 The Gut Flora – The Missing Link in Immunity

The gut flora or microbiome truly is a wonderful thing. There are more bacterial cells in our body than actual human cells. We harbour an estimated 100 trillion bacterial cells. That number is almost impossible to comprehend. Billions of bacteria live in our digestive system from the mouth all the way down to the bowel, in our, nose and urinary and genital tracts. We live symbiotically with bacteria. The commonly used terms, ‘good or beneficial’ gut bacteria and ‘bad’’ gut bacteria is just a simplistic way to explain the fact that some bacteria are beneficial to our health and others are pathological. It is the beneficial bacteria that live symbiotically with us – in other words, we provide them with somewhere to live and they provide us with certain health benefits – the relationship is mutually beneficial!

The gut microbiome plays an important role in regulating multiple aspects of digestive health, from regulating peristalsis, through to housekeeping and nutrient synthesis such as Vitamin B12 synthesis but also our microflora are part of our innate and adaptive immune system. 

Beneficial bacteria reduce the ability of pathogenic bacteria to colonise by competing with pathogenic bacteria for available resources and crowding them out. The gut microbiome also aids in the maintenance of the gut epithelial barrier. Studies show the importance of diversity of gut flora to ensure a resilient immune response. A varied microbiome results in a flexible immune response to act on threats as appropriate. A lack of diversity of gut flora leads to an insufficient immune response and development of disease and inflammation. 

An area of research into gut flora that has really started to accelerate in recent years is the role that gut flora plays in systemic immunity. It truly is the missing link in immunity. The link between gut flora and immunity was initially observed when mice that were raised in completely sterile, germ-free environments (i.e. where they were not exposed to any bacteria at all so their own bacterial colony was nonexistent) displayed significant immunological deficiencies. One of the most noticeable was very poorly developed lymphoid tissues and lower numbers of immunological cells in key areas of their bodies. This led to a further discovery that the health of the gut flora was directly associated with the health of the immune system and that specific bacterial strains activated specific immune cell lines such as regulatory T cells, etc. 

But how can a bacterial colony that lives in our gut affect what an immune cell does in our big toe? The truth is, at this stage, we simply don’t know for sure. It seems almost certain that it involves an interaction between gut flora and gut-associated lymphoid tissue (GALT). The GALT is made up of several different types of tissue throughout the entire digestive tract. Of most interest though, is a group of tissue patches called Peyer’s patches. 

The Peyer’s patches are clusters of lymphoid nodules that can essentially be seen as surveillance stations. Our digestive tract is an obvious route into our body for opportunistic pathogens, so as a region it must be tightly monitored. Peyer’s patches relay information from within the gut to the rest of the immune system via chemical messengers. The surveillance stations are able to differentiate between different bacteria to stimulate the correct immune response It is most likely that this is the level at which gut flora communicates with the rest of the immune system. This interaction with Peyer’s patches sets off a series of systemic chemical messages that can activate, stimulate, reduce or switch off.

The integrity of the intestinal wall is of paramount importance in offering protection against pathogenic bacteria. Given the right environment (prebiotic fibres) gut bacteria produce short-chain fatty acids, butyrate, propionic acid and acetate. These fats support the integrity of the intestinal wall. Secretory immunoglobulin A (SigA) is found within the mucosal layers and that includes intestinal mucosal layers. It is like an antiseptic paint on the gastrointestinal mucosa. SigA neutralises invaders before they can cross the mucosal barrier and reach circulation. In other words, it’s the line of defence to combat pathogens without activating an inflammatory response. It also provides a layer of defence by neutralising toxins. Stress can lead to lower levels of SigA hence stress will affect our immunity and physical health. If stress compromises SigA, then pathogens will reach circulation triggering an inflammatory response. Lower levels of SigA have also been associated with leaky gut and increased uptake of food antigens.

Supporting gut flora through ensuring the intake of both prebiotic and probiotic foods will always be a major part of any program to enhance immunity. Whilst many studies in relation to gut flora and their role in immunity are performed with probiotic supplements we firmly take the view that food sources of probiotics and prebiotics should be the first course of action in colonising and maintaining the gut flora. Probiotic supplements contain just a handful of probiotic strains and yet we live with trillions of bacterial cells. There is evidence supporting the linkage between specific probiotic strains and health conditions. However, to maintain good health and good gut flora it is important to have a constant supply of pro and prebiotic foods as probiotics have not been shown to permanently alter intestinal gut flora.