From bacteria to viruses, mites to fungi, our bodies are literally riddled with life. Perhaps to a greater extent than you ever thought imaginable. But are these guests up to mischief; should we throw them out for trashing the house? Or do we welcome them with open arms and beg them not to leave. From assisting your immune system to having sex on your face (wait, what?!), this article is going to explore the organisms that have set up camp on and inside our bodies.

The Human Microbiome

Let’s start with microbes. Microbes are microscopic organisms such as bacteria, archaea, protozoa and fungi, and these microorganisms that reside on or within your body are referred to as the human microbiome. This microbial colonization begins during and shortly after birth. Your body is composed of about 10 trillion human cells, but bacteria outnumber them by at least ten to one. Microbes actually make up around 1 to 3% of your total body mass. These microbes colonize lots of different areas of the body including your skin, gut, mouth, eyes, and genitals.

What's lurking in your gut

You can find anywhere between 500 to 1,000 different species of bacteria in your gut alone, and they actually make up 60% of the dry mass of your feces- yummy. Common bacterial species found includes E. coli, B. bifidum and E. faecalis. These bacteria are far from just being gate crashers; we have developed a symbiotic relationship with them. We provide these bacterial species with a nice home to live and replicate in, whilst they provide us with a myriad of benefits. For example, they assist in the breakdown of food, and they help to prevent harmful species from colonizing which could adversely affect our health. They also assist in the production of hormones and vitamins, namely vitamins B and K. It is also becoming increasingly apparent that the bacteria present in your gut play a role in modulating and training your immune system to be able to recognize friend from foe.

Scanning electron micrograph of E. faecalis​, found in the human colon. Wikimedia commons.

But do these bacteria only do good things? Apparently not. There is an increasing amount of evidence to suggest that certain bacterial species colonizing the gut are actually associated with obesity. These microbes are more commonly found in those living in colder northern areas of the world, compared to warmer southern areas. It seems that certain types of bacteria found in leaner individuals are better at breaking down certain types of food.

Spotty Skin and Smelly Pits

Your skin also has plenty of microbes, especially your hands- it's amazing what you can grow in a petri dish if you stick your finger on one. Bacteria present on your skin can sometimes associate with oil glands, playing a role in the development of acne. Your sweat is also naturally odorless; it’s the bacteria found on the skin that make it smell. Interestingly, your unique body odor correlates with the presence of specific microbes, as they break down non-volatile compounds into volatile compounds with characteristic scents. It was discovered that the higher the density of a group of bacteria called coryneforms, the more the underarms smelt.

The skin microbiome. Wikimedia commons.

Recently the bacterial species living in our navels (belly buttons) was investigated; it was found to be colonized to an extent greater than scientists could have ever predicted. They swabbed 60 bellybuttons and found a whopping 2,368 phylotypes of bacteria, which probably corresponded to an even bigger number of bacterial species. It was thought that 1,458 of these bacterial species could be new to science. Amazingly, one volunteer harbored a species of bacteria that has only ever been isolated in Japanese soil- and he had never been to Japan! Another (who admitted to not washing very much...) had a species of extremophilic (thrives in extreme environments) bacteria that is often found in deep sea hydrothermal vents and ice caps. Intriguing or disgusting, but our bellybuttons are turning out to be a microscopic jungle, and it's exciting microbiologists a great deal. Just imagine what could be in yours! Or maybe don't...

Most of the bacterial species found in our bodies don’t do us any harm, and so are referred to as commensals. But they can become opportunistic pathogens, for example if immunity is lowered or natural barriers become compromised, such as the blood brain barrier. This is where they can go rogue and cause problems.

Organisms That Can Alter Your Behavior?

So, we know that organisms can affect your health, in positive and negative ways. But what about your personality? There has been some very interesting research emerging on a protozoan organism called Toxoplasma gondii which is found in 22-84% of the human population (depending on the country). The most intriguing effect of toxoplasmosis is the apparent “fatal attraction syndrome” which occurs in infected rodents. T. gondii can only productively replicate in cats, but it can infect humans and rodents. Amazingly, it has found a way to alter the brain signaling of the rodents to make them more likely to be consumed by the cat. Infected rodents in studies had slower reaction times and displayed no aversion to the scent of cat urine, which rodents usually have an innate fear of.

But what’s perhaps more interesting is what they have found in humans. It was discovered that people with toxoplasmosis had slower reaction times and actually enjoyed the smell of cat urine! No other types of urine (they tested a few), just cat urine, whereas uninfected individuals did not like the smell. Is the organism trying to alter the behavior of the human to make it more likely to be eaten by cats?! It goes on; they studied people involved in road traffic accidents in Prague. They found that those involved in accidents, whether they were a pedestrian or driver, were more likely to be infected when compared with residents not involved in accidents. This suggested that those individuals were perhaps sometimes more reckless. This research has certainly been met with raised eyebrows, and confounding factors are always a possibility. For example, it may not be that toxoplasmosis affects your personality, but perhaps people with a particular personality are more susceptible to toxoplasmosis.

Wikimedia commons.

Your Face As A Breeding Ground

So what other weird things live on your body? There are some very interesting creepy crawlies alongside these microbes. Mites, or more specifically Demodex, live in places such as your eyelash and eyebrow follicles. They're the most common ectoparasite (parasite that lives on the surface of the host) on the human skin. Usually these critters will do you no harm, although in some instances they have been found to cause skin disease. They can leave the follicles and walk around the skin at night, meeting others to then mate at the follicle opening. Once they’ve done the dirty deed, they go back to the inside of the hair follicle to lay eggs. You give these mites a comfy bed and what do they do? Have sex on your face. They're worse than teenagers. Perhaps even more disgustingly, Demodex don't actually have an anus. This means they have no way of getting rid of their feces, so when they die they explode all of this feces into your pores. Some scientists may believe that this is sufficient to stimulate an immune reaction, and may be the cause of the acne-like skin condition rosacea.

Nobody Loves Me... Think I'll Go And Eat Worms.

Although worms are not found in everyone, they're surprisingly common in the human population. There are 3 main types found in humans; flatworms such as tapeworms, flukes such as blood flukes, and roundworms such as threadworms, pinworms or hookworms.

The largest of the intestinal worms found in humans is the roundworm Ascaris lumbricoides, which can reach a whopping 35 centimeters. After ingesting the eggs they hatch and bury into the intestinal wall, entering the blood stream. From here, they travel to the lungs where they're coughed up and swallowed again, returning to the gut once more.

​Wikimedia commons, Ascaris lumbricoides.

Tapeworms are transmitted via infected food, and once inside the body it attaches to the intestine via hooks found on its head. Here, it can survive for up to 25 years! If you're unlucky enough to become infected with hookworms (Necator americanus), these intestinal parasites actually drink your blood and can cause a type of anemia.

And finally my favorite, the pinworm. These little guys are only a few millimeters in length, and once again live in the intestines. They mate by traumatic insemination- the male stabs the female with his penis, and then dies. At least they die happy...

Let's Not Forget Viruses!

It might surprise you to learn just how many viruses there are in our bodies. It recently emerged that like the human microbiome, humans also have a virome. Our human viromes have been suggested to be even more diverse than our microbiomes, and they also vary considerably between people. In one study investigating twins and their mother, they found between 52-2773 different viruses. We also have a surprising amount of viruses in our bodies that infect bacteria- these are called bacteriophages. For every bacterium in your body, it is estimated that there is a whopping 100 bacteriophages. Going back to poo- there is approximately 10 billion in each gram . Wow.

Wikimedia commons.

Just like how it has been suggested that the bacteria in your gut can contribute to obesity, there has been some interesting, if very controversial, research to suggest that perhaps viruses can also play a role. A small number of researchers have been investigating a particular type of adenovirus, and they found that infection correlated nicely with obesity. This was triggered by observations in animal models where this virus was found to dramatically increase the amount of fat found in the body. Not everyone is convinced by this virus, but some people are beginning to take it more seriously. The idea that obesity could be an infectious disease, possibly spread by a simple cough or sneeze, has met some raised eyebrows.

Just like bacteria, viruses can live harmlessly in the body, but can then go on to cause problems when the person becomes immune suppressed.

I could go on, but simply there’s just too many things living inside you to talk about them all. You are literally riddled with life, but these organisms play a big part in your life, and most of them are quiet do-gooders that we wouldn’t even know were there. You can thank some of these organisms for helping to shape who you are.