We generally associate our digestive tract with nutrition; the part of the body where food is absorbed and from which waste products are excreted. This is of course correct, however there are other equally and in some cases more important functions that our digestive tract is responsible for, and which bacteria including probiotics play a key role in.
If it was removed and laid out, the human digestive tract would resemble a tube around 12 metres in length. Due to the various folds and wrinkles on the inside of this tube, the total surface area is approximately 300 square metres which is roughly the same as a tennis court. This is a vast area of tissue exposed to the outside world; anything we put in our mouths will pass over this surface, similarly, any bacteria which enter our digestive tract have an enormous area to attach to and make their home.
The human immune system is based around a highly complicated and dynamic set of cascades and interactions between chemical substances, complex molecules and specialised cells. Up to 85% of these cells and interactions take place in and around the digestive tract, which is perhaps not surprising for an environment where there are upwards of 100 trillion bacteria living at any one time.
To begin to understand these interactions it’s helpful to start at the beginning. In the womb our digestive tract is sterile and devoid of bacteria; in parallel our immune system is almost completely naive. With the exception of certain viruses and assuming the mother is in good health a baby in the womb is immunologically a blank page. All the tools and equipment are in place to deal with an infection, they just haven’t been used yet.
The first early exposures to bacteria are critical to the development of a healthy immune system. Normally this happens quite naturally during the process of birth, breastfeeding and the first few months of life in a new world full of microorganisms. During this period the baby’s immune system, especially those components in the digestive tract encounter their first dose of bacteria, and natural mechanisms quickly kick in to foster a healthy population of good bacteria, and deal with any pathogens.
The complex processes which direct this continue throughout life, and research in the last 20 years has demonstrated that the interaction between bacteria and the human immune system rather than being a weeding process in which the body kills unwanted bacteria is in fact a two-way street with bacteria calling the shots as well as the immune system.
For example, certain species of bacteria found in the bowel interact with the immune system to trigger a response which has a direct anti-inflammatory effect. This modulating effect gives rise to the term immunomodulation and is becoming one the most important mechanisms of action under investigation for probiotics.
People lacking those species of bacteria may be more susceptible to inflammatory conditions such as Crohn’s disease or ulcerative colitis. Similarly, introducing these types of bacteria can initiate an anti-inflammatory reaction which may help to manage the symptoms of such diseases.
Any live microorganism which confers such a benefit upon the host can be called probiotic; so even without adding anything to our diet by way of supplements a healthy bowel is full of probiotic bacteria.
bacteria. There are thousands of different species and strains of bacteria which can inhabit the human gut, laboratories and hospitals around the world are investigating the most likely candidates to establish which are most beneficial.
The results of this research may one day enable us to tailor specific probiotic products to particular conditions, and potentially not just conditions of the digestive tract, since immunomodulation which begins in the bowel can affect other systems around the body responsible for problems including eczema, asthma and many other auto-immune conditions.