Throughout society, we frequently pride ourselves on being physically and mentally unique from each other and the rest of the biological world, we are all our own original person, so we believe. However in reality, you and I are an individual walking rainforest, our own diverse and different habitat harbouring trillions of living organisms and thousands of unique species.
We as humans, along with many other members of the animal kingdom, contain a complex mix of microbes that constitute our microbiota. Ultimately, the microbiota is a whole living biome of microorganism species that have colonized and inhabited our human bodies including bacteria, fungi, viruses and protists.
These invisible organisms are found throughout the body on external surfaces such as our teeth, lips and skin but primarily reside within the gastrointestinal tract, more commonly known as the gut.
From person to person, each individual’s microbiota is distinctively varied, harbouring a specific array of microorganism species. A whole host of factors come into play as to why each of us has a different microbiota composition from one another. Ranging from lifestyle choices such as diet and exercise to how we were born and personal usage of antibiotics, our microbiota can be heavily influenced creating the variation we see in each other.
From person to person microbiota is hugely varied. For instance, C-section operations lead to new-born babies having a different microbiota composition from a naturally born baby. Having gone through its mother’s birth canal, the baby is coated in beneficial microbes whereas C-section babies are first colonised by the bacteria found in the hospital and on the mother’s skin. Also, our gut microbiota can be very different depending on our diet, with meat eaters and vegetarians having noticeable differences in microbe composition.
PHOTOGRAPH BY Jonathan Borba
Interest in why we have this thriving community of microbes in our bodies and what their roles are has driven this sector of science to grow exponentially to figure out our relationship with these mysterious residents. Although research into human microbiota is relatively recent, we already have resolved that the microbes that we support, are imperative for the function of the human body. Our microbiota is responsible for allowing core processes in the body to operate, for instance aiding our digestion, to regulating our immune system and producing vital vitamins.
But what is in it for the microbes? The microorganisms that aid these functions are rewarded by us in a symbiotic relationship. Whilst they carry out these functions for our benefit, we in return provide a suitable pH, an optimum temperature and constant nutrients for their own survival. However, not all microbes within the human body live within this supposed harmony. Some microbes are harmful to us and can act as parasites taking nutrients from our bodies at our expense.
It is estimated that our microbiota contains 100 trillion microbial cells, compared to the 10 trillion human cells that comprise our bodies. Additionally, within these microbial cells, they contain 30 times the genetic information that our human cells harbour and in total contribute two to six pounds of our overall body weight. With such an abundance of microbes residing in our bodies, we are starting to question what actually defines ‘us’ and contemplating how much control these colonizers really have.
Do the members of this two to six-pound microbial mass have the ability to change our behaviour, controlling what we feel and do, at certain times? We need to look further into how these microorganisms impact other living creatures including mammals and insects and begin to learn how these microscopic organisms affect us. What can we learn about how they function, and can we begin to manipulate them back, for our advantage?
This modelled image of the large intestine shows the abundance of bacteria that resides in our guts, aiding our digestion. Microbes are found throughout the world and have become vital for the functioning of millions of species. Their abundance is an extraordinary insight into the magnitude of life our bodies contain. For each gram of human faeces, it contains more individual bacteria than the world's entire human population.
PHOTOGRAPH BY Alpha Tauri
Microbes existed as the first fundamental life forms on earth dating back to their origin, roughly 3 billion years ago. Found throughout the planet, microbes have become truly influential in the evolution of certain species, shaping the vast majority of life we see today. They have become critical for the function of many present-day animals, no such more than us.
Microbes have become entwined even into the fundamental beginning of a human’s life. As we are born, we pass through our mother’s birth canal which in turn coats the baby with a combination of beneficial microorganisms to help them develop. Babies born via C-section, however, miss this initial coating leading to them being unintentionally colonized by microbes found on the mother’s skin and surrounding hospital surfaces. Skipping this initial evolutionary intended stage has led to babies born via C-section missing key microbes, creating a higher risk of conditions such as asthma.
Additionally, our microbiota has become very closely linked to aiding our digestion. We rely upon microbes to provide us with 10% of our calories from the food we consume daily. Naturally, our bodies do not produce all the required enzymes needed in order to break down our diverse diets. Indigestible carbohydrates are converted to short chain fatty acids by the specific enzymes produced by our beneficial microbes, allowing us to access the energy stored within.
Unexpectedly, our microbiota also plays a surprisingly considerable role in the development and regulation of our immune systems. Our immune system and microbiota are closely linked in an ongoing paired interaction. As the immune system promotes the growth and stability of beneficial microbes, the microbiota in return aids to fine tune immune responses that our bodies require. However, it is not just us who benefit from a relationship with microbes.
Underwater hydrothermal vents are believed to be the sight at which microbes arose, the first known life forms. Microbes have become a powerful factor in driving evolution.
PHOTOGRAPH BY NOAA
The animal world is littered with microbe interactions which in turn have become essential for helping both the animal and microorganisms survive. Ruminant animals, such as cows, sheep and goats, rely on a microbiome found within their special type of stomach called a rumen. Home to billions of microbes, the rumen is the site in which ruminant animals’ cellulose-heavy diet is broken down to digestible sugars. The ruminant’s reliance on their microbiota is so essential, that without these microbes, the majority of livestock around the world would simply starve to death.
Microorganisms are responsible for aiding the creation of the most diverse and intricate underwater environment: the coral reef. To some people’s surprise, corals are animals and have a microbiota just like us. The most famous of their microbial interactions is their relationship with a variety of species of dinoflagellates called zooxanthellae, which are single-celled algae. By producing glucose via photosynthesis, the algae provide the coral with vital sugars allowing them to grow. In return, the coral lends the algae a protected environment to live in and provides them with the nutrients to carry out photosynthesis in the first place. Once again, another genius mutualistic relationship, but what happens when microbes in the animal kingdom have evolved for more sinister roles?
The coral microbiome includes dinoflagellates, viruses, fungi, archaea and bacteria. Just like in humans, the complexity of coral microbiota composition is staggering, the roles of each species and how they interact with one another are still far from being fully understood.
PHOTOGRAPH BY Octo-Focus
Microorganisms have evolved various diversified methods to ensure their own survival, by manipulating and using a ‘more advanced’ multicellular creatures’ body. Plasmodium is a protozoan which is famously known for causing malaria in humans, after completing the initial first stage of its life cycle in a mosquito. However, the way plasmodium gets into humans is not by mere chance, according to numerous studies.
As the parasite progresses through its life cycle, it alters the behaviour of the mosquito. Initially, as the plasmodium grows and develops, the mosquito is non-infectious. Throughout this pre-infectious period, mosquitoes are seen to be less attracted to hosts, less persistent in trying to feed and seen to feed for shorter periods of time. Why risk being squashed by an annoyed human’s hand if you are not developed and cannot move to the next stage of your life cycle?
Once the plasmodium has matured and the mosquito becomes infectious, the behaviour drastically switches. Despite the risk, the mosquito is seen to seek out more hosts being increasingly persistent in feeding attempts driven by the plasmodium’s desire to move on to the next host, us. Is it possible for parasitic microbes to change the behaviour in larger more complex animals, such as mammals?
Cordyceps is a genus of parasitic fungus found commonly in tropical environments infecting insects. They have the ability to kill ants, using its body to then grow and help them reproduce. The fungus controls the ant’s behaviour leading it to a favourable position for fungal growth. Once the conditions are at an optimum for the fungus, it causes the ant to sink its jaws into a plant indefinitely whilst the fungus sprouts out of its body releasing spores to infect its next victim.
PHOTOGRAPH BY Kevin Wells
A different protozoan, called toxoplasma gondii, infects a vast number of mammal species and has the capability to drastically alter the relationship between cats and mice. This certain single-celled protozoan can only sexually reproduce in a cat’s gut, so therefore needs a mechanism to increase its chances of ending up there. Once it finds itself in a mouse, the parasite travels to the brain where it manipulates the host's behaviour by varying the body's biochemistry. Surprisingly, infected mice suddenly become heightened explorers and seemingly lose their innate fear for cats, becoming even actually attracted to the smell of cat urine. Once an infected mouse is eaten by a cat, the parasite can reproduce.
Although it is not completely proven how it yields these behaviours from mice, it is baffling to witness how a single-celled creature with no body, nervous system or apparent consciousness, has the capability to manipulate a mammal’s actions.
Various studies have shown how mice are no longer deterred from the odour of cat urine when infected with the protozoan parasite Toxoplasma gondii. Additionally, they become more exploratory and have decreased reflexes both increasing their chances of ending up in a cat’s belly, just where the parasite needs to be.
PHOTOGRAPH BY Jozef Sowa
Just like mice we are mammals, we have the same basic structures, chemicals and cells that comprise our bodies and brains. Does that leave us open to manipulation?
Our guts are littered not just with microbes but over 100 million neurons which have a clear part to play in communicating with the brain. The gut and the brain are very closely linked to the point where scientists coined the new term, the gut-brain axis. It is not exactly sure how the two organs communicate, but it is suggested that it is through either the vagus nerve, immune system or endocrine system. At the heart of this relationship, is the newly discovered effects that certain microbes play on this vital communicational system.
Recent studies have investigated how our gut bacteria can impact our behaviour, especially regarding our mental health. Microbes in the gut are heavily involved in the production of neurotransmitters, with 90% of serotonin being produced in the gut, not the brain. Serotonin is an important neurotransmitter in the body controlling multiple biological processes. As serotonin levels vary up and down, it is linked to increased anxiety, schizophrenia and aggressive behaviour. The pieces of the puzzle are not quite fit together yet, but the finding that the majority of serotonin production is controlled by the gut microbiota lends to the fact that microbes can control, even if it is indirectly, our behaviour.
As we begin to understand more in this new frontier of biology, we still must look at other species. Scientists have undergone faecal microbiota transplants (transferring microbes) between mice to study how their behaviour varies. What they found is that switching gut microbiota can cause the behavioural traits of the recipient to become more like those of the donor. Although this research is young, this could become something we can use and study in humans to see to what extent microbes influence our behaviour.
Scientists have described the gut as the second brain due to its abundance of neurones. There are more neurones in the gut than in the spinal cord and we are just grasping how our vast quantities of microbes interact with them. Evidence suggests that certain bacterial species in the gut are able to alleviate anxiety and depressive-like symptoms.
PHOTOGRAPH BY Shutterstock
Due to microbes having the potential to alter the behaviour in mammals such as mice, scientists have had to look at the same possibility in humans. The protozoan which I mentioned earlier, toxoplasma gondii, which reproduces in cats and manipulates mice behaviour, also has the ability to infect us. In fact, it is estimated that 1 in 3 people worldwide are infected with this brain parasite.
Spread via close contact with cats, soil and eating undercooked meat, toxoplasma gondii has been suggested to vary the behaviour of those infected and even possibly help shape certain cultures around the world according to some scientists. Although not entirely conclusive, fascinating links have been found between toxo-infected people and their behaviour. Studies have found that infected people are twice as likely to be in a car crash than non-infected people, possibly being linked to how the parasite reduces reaction times and initiates loss of fear in mice. The same could happen to us. Additionally, the parasite has been seen to disrupt the levels of a neurotransmitter called GABA. Variations in the levels of GABA are closely linked to schizophrenia suggesting that those infected are more likely to have the mental health condition.
To fully support any of these theories is controversial but it would be hard to believe that we would be the only animal not manipulated in some way.
Toxoplasma Gondii is estimated to infect one-third of humans, but infection rates vary in certain places. The UK only has an estimated 7% of its population infected with the parasite, compared to 67% of Brazilians and 84% of people from Madagascar. There is evidence that this parasite can help influence certain cultures seen around the world by promoting types of behaviour and personality traits.
PHOTOGRAPH BY AJ Cann
As we start to understand how vast the role of microbiota is on us and the animal kingdom, we are beginning to recognize how we can utilise microbes to our own tailored advantage.
Wolbachia is a strain of bacteria that has infiltrated the most successful group of animals on earth, the insects. Estimations predict that roughly 40% of insect species are infected with this bacterium which can either be very beneficial to some species or parasitic in others, impacting the insect’s survival.
Wolbachia is a new weapon in aiding the reduction of deaths from deadly viruses such as zika, dengue and yellow fever. Wolbachia does not normally infect tiger mosquitoes which are responsible for transmitting these viruses to humans when they feed. Using artificial lab techniques, Wolbachia can be transmitted to these mosquitoes. Once infected, these mosquitoes are no longer able to transmit these viruses: Wolbachia outcompetes them. Now these mosquitoes can be released and spread throughout vulnerable populations in Southeast Asia where there are high transmission rates, curbing the prevalence of these diseases.
The tiger mosquito, Aedes albopictus, is found across tropical and subtropical regions of Southeast Asia spreading viruses such as Zika and dengue.
PHOTOGRAPH BY Oliver Spiteri
The fact that our bodies alone support such an astounding variety and quantity of microorganisms, reveals to us all how this world has more life than we could have imagined. Across the planet we find these supposedly insignificant life forms thriving in deep water oceanic vents to our own guts. We can now reflect about how these tiny microbes control a lot more on this planet than we initially thought, including us.
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