Hello everyone and welcome to my 25th blog post in which I will be giving an overview of what we know about the human microbiome. I will discuss what the microbiome is, how it develops and how microbes can manipulate our thoughts. I will also explain the hygiene hypothesis and how it can cause autoimmune diseases such as asthma. Next week, I will go further and elaborate on its applications, both in medicine and disease control.
The human microbiome has been one of the most discussed areas of science in the last few decades, as evidenced by a large number of companies that have sprung up to exploit it. Indeed, currently, there are now over 120 biotech companies in the UK. Additionally, worldwide there are currently 337 registered trials with ‘microbiome’ in the title, all of which are attempting to advance our knowledge on the subject and help us improve our medical knowledge.
According to Joshua Lederberg (a Nobel prize-winning molecular biologist), a microbiome is ‘a convenient ecological framework in which to examine biocontrol systems is that of the microbiome. This may be defined as a characteristic microbial community occupying a reasonably well-defined habitat which has distinct physicochemical properties.’
The human body contains about 10^14 visible human cells, such as skin cells, liver cells etc. In addition, the average adult human body also contains roughly the same amount of microbes. These microbes make up over 2kg of an adult’s body mass. Although these microbes can’t be seen by the naked eye, they make up an important part of the functioning body. Indeed, without them, life would not be possible.
Microbes are microscopic organisms which are often unicellular but may also be multicellular. Microbes in the human body are predominantly bacteria, but may also be archaea, as well as fungi and viruses.
Much like a fingerprint, the composition of the microbiome is different for every person. The composition of the human microbiome is first defined at birth. When a fetus develops in the uterus it has no microbiome - much like a blank slate. The human microbiome then typically evolves over the course of the lifespan due to various environmental factors and the microbes that the human is exposed to.
Typically, a baby receives its first microbes from the birth canal of its mother, however, the recent rise in the number of babies born by caesarean section has lead to some disturbing results. In these cases, babies typically receive their first microbes from the hospital operating theatres. Indeed, studies have shown that as much as 1/3 of a baby's microbiome (who was born by caesarean section) consisted of hospital microorganisms. These microbes are much more dangerous to babies as these microbes are more likely to be pathogenic, as well as being more likely to have increased resistance to common antibiotics.
One common misconception is that all microbes are harmful. It's more accurate to say that the harmfulness of microbes can be measured on a spectrum. While some microbes are pathogenic, others can actually benefit the human body and indeed are essential for life. For example, while the human genome has approximately 23,000 genes, the microbes in the human body contain around 9 million genes. This provides an important level of genetic complexity which the human body doesn't have, allowing us to do many more things than we otherwise could. For example, humans do not have the ability to break down a large number of sugars found in human breast milk. Nonetheless, babies guts' contain the bacterium Bifidobacterium which is able to breakdown these complex sugars into products which we can use.
The gut bacteria, which are stimulated by fibre, have a very important job in the human body. Not only do they breakdown certain complex molecules and help synthesise vitamins such as vitamin K but they also help protect the epithelial cells lining the gut. In addition, the gut microbiome also has an important role in the maintenance of our strong immune system. I will discuss this point in more depth in part two of this blog post next week.
Microbes also have a huge number of other functions in the human body, including supporting the immune system. In addition, the beneficial microbes of our microbiome also reduce the likelihood of more harmful microbes colonising our body. This is simply due to the fact that they take up space. The human body is full niches with ideal conditions for microbes to grow. One of the benefits of having our microbiome is that these beneficial, or at worst neutral, microbes occupy these spaces and prevent more harmful microbes.
These 'space-occupying' feature of our microbes highlights one of the main problems with taking antibiotics prophylactically or when not necessary. According to many microbiologists, antibiotics are like atom bombs to the human microbiome, in that they destroy all bacteria, including the ones that make up the resident human microbiome. Not only does it mean that humans can no longer do the things that the bacteria did, such as digest certain proteins, but it also means that harmful bacteria and other microbes are able to grow and take their place in our body. As such, this is yet another reason why antibiotics should only be taken when absolutely necessary, not as a precaution.
While we can control our own microbiome by manipulating our internal environment, our microbiome can also control us. Indeed, scientists suggest that these microbes are able to interfere with the nerves carrying signals to the brain and change our own decisions. These ideas are based on what we have seen in other animals.
In the 1990s, researchers discovered that the protozoan Toxoplasma gondii is able to manipulate rodents in order the replicate. This is because the protozoan needs to infect both rodents and cats, in different stages of its replication cycle. As such, T. gondii causes these rodents to overcome their innate fear of cats and run towards then, before being eaten, thus allowing the pathogen to spread from one species to the other. The proposition that our microbiome could be secretly influencing our thoughts is a very sinister idea. It underlines the fact that our microbiome is not our friend, but rather something which we need to keep in check at all times
Research into this human microbiome has led to the hygiene hypothesis. This hypothesis states that because of better hygiene practices in recent times, leading to a decrease in the number of infectious diseases in the western world, the occurrences of autoimmune diseases have increased. The hypothesis states that this is because, since many parents believe that their children must be kept in a super-clean environment, they are not exposed to a large diversity of bacteria.
Ordinarily, the human immune system is constantly exposed to a variety of different pathogens, especially during the early years of life. This means that their immune system does not develop properly. As such, when an infection occurs, instead of reacting normally, the body's internal inflammatory response system overcompensates, leading to excessive inflammation and diseases such as asthma.
Thank you for reading this week's article. Make sure to subscribe so that you get notified on next week's article about how our knowledge about the microbiome can be used in medicine.
Sources:
I Contain Multitudes by Ed Yong
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