What have bacteria ever done for you?

Last week’s blog post was about the connection between the gut and the brain. There is no doubt about the existence of communication lines between the two, and one of those is based on chemical messages from bacteria. This week, I want to add a little more on gut bacteria.


gut bacteria microbiome

When we first became aware of bacteria, their reputation was more than bad: Germs were found to be behind some of the most dangerous and deadly infectious diseases, such as syphilis, typhoid, cholera and the bubonic plague. Any tiny wound could become infected and kill us. It was a scary thought that something so tiny, it was completely invisible, could kill a human – an animal much, much bigger than itself. The worst epidemics in history were of bacterial origin. The bubonic plague, for instance, wiped out 25 million people in Europe in just four years. The discovery of penicillin and other antibiotic drugs later on thankfully put an end to those horrific and dangerous diseases … at least for some time. The liberal use of antibiotics has led to the formation of new, resistant strains of bacteria. Although antibiotics are said to be overprescribed, the use of antibiotics in animal husbandry dwarfs their use in medicine. While it is difficult to tell whether agriculture or medicine are to blame for the increase of resistant bacterial strains, scientists do agree that we are facing a serious problem.

Considering the devastation just a small number of bacteria is capable of, it is hard to imagine that we are walking around with a hundred billion of them inside of us. Together, they amount to 1.5 to 2 kg (3 – 4 lbs) of our body weight. Half of our stool consists of waste, the other half of dead bacteria. Although that may sound gross, we now know that we couldn’t live without them.

Science has identified more than 1,000 different bugs, but that may just be the tip of the iceberg. For a long time, all they could do was to apply stool samples to a petri dish, wait for them to multiply and then see what was in there. However, the majority of our gut bacteria are anaerobic, which means that they cannot survive if exposed to oxygen – such as in a petri dish. Now, however, researchers can sequence the genome of bacteria, which has lead to the discovery of hundreds more species just in the last few years.

People in the ‘civilised’ world provide a home to approx. 150 to 200 different species of microbes. The indigenous population of the Amazon delta, for example, houses at least twice as many species. Part of the reason may be the super-hygienic conditions we live in, part of it is thought to be our limited diet. We know that the more varied the diet, the more varied the microflora of any given individual. But your gut bacteria are a result of much more than just what you eat. They tell the story of your life! They tell how you were born – natural birth or Cesarian? -, whether or not you were breastfed, whether you shared ice cream with your dog or a bag of peanuts with your friends down the stables. It’ll tell about the terrible case of food poisoning you contracted on a trip to the Tropics, show whether or not you are vegetarian, whether you frequently wash your hands or whether you are not that bothered about hygiene. All of this leaves traces in your gut.

When the bacteria in our gut were first discovered, they were thought to have just three functions:

1) digestion
2) defence against other, detrimental microbes
3) defence and fortification of the gut wall and ‘training’ of immune cells

All of this is indeed what they do, but we now know that it doesn’t end there. When I say that bacteria communicate with the brain, then that doesn’t mean that they do that ‘in person’. They wouldn’t get in anyway – the blood-brain barrier sees to that and protects the brain from intruders. Bacteria can, however, affect the brain through chemical communication. Just a few examples:

Gut microbes break down complex carbohydrates into the short-chain fatty acid butyrate, which strengthens the blood-brain barrier. They also actively contribute to the production of neurotransmitters. 90% of the serotonin – the happiness hormone – in humans is made in the gut, with input from the microbiome.

Gut bacteria are involved in our mood and how well we handle stress. In 2016, scientists discovered bacteria that feed on GABA – and refused to eat anything else. GABA is a neurotransmitter we need to remain calm and collected. Another species, Lactobacillus rhamnosus, on the other hand can enhance the effect of GABA.

In studies with mice, scientists succeeded in inducing conditions in mice that they see as equivalent to human anxiety, depression and even autism, by tinkering with the animals’ gut flora. By inoculating them with benign bacteria, they managed to restore their behaviour to normal. Although most research is currently based on experiments with rodents and may not transferable to humans, what it does show is that gut bacteria do have the power to change the chemical environment of the body enough to affect mental health.

A lot of research on the microbiome has been done in the last 5 to 10 years, and so much more is being done as we speak. We can look forward to much more exciting research coming out, so watch this space.

To read more about the bacteria and their influence on mental health, click here for an article from the Scientific American or here for an article from Nature magazine.