HEALTH, MICROBES AND COEVOLUTION

CCBE — Part 11: You, an ecosystem for microorganisms

How our tiny microbial colonisers contribute to life, health, and disease

Today, we’ll look at ourselves as an ecosystem. Much to our initial surprise, we and other animals are giant colonies of microorganisms (aka microbes, microbiota, or ‘germs’).

We’ll discuss how our microbes contribute to health and disease. As always, this will highlight the yin and yang of cooperation and competition, and how this affects our biology and even psychology.

The ecosystem within

Since Louis Pasteur proved germ theory in the 19th century, we’ve mainly viewed microorganisms like bacteria with suspicion, quite reasonably in many cases. For example, pasteurisation (named after Pasteur) uses heat to kill potentially harmful microbes in food and drinks.

If infection by microbes can cause disease, people inferred that the organs of a healthy person must be sterile. But just because some microbes cause disease, that doesn’t mean they all do.

The gut microbiome

Far from being sterile, our body is a playground for microbes. The gut is a good example, as it’s colonised by a vast network of microorganisms. The gut’s importance is also often underestimated, even though it contains more neurons than the spine.

Gut microbes make crucial contributions to health, like digesting food. In a way that raises questions about free will, gut microbes also seem to influence feelings of hunger, and even taste preferences.

But gut microbes are only one case, and colonies of microorganisms are found in virtually every organ of our bodies. Even the brain, once thought to be totally sterile, is home to a small colony of microorganisms.

Microbes, pregnancy, and the immune system

Pregnancy is another striking example. Once again, it was assumed that the womb must be sterile and have sophisticated anti-microbial defences.

True enough, the womb has many clever immune defences, and it’s mostly sterile, but not entirely. Even in the amniotic sac, the cradle of life itself, harmless microbes can be found during pregnancy. One idea is that they may even help to prime the immune system of the developing fetus.

Immune systems are mostly inherited from loved ones and shared environments, much of which comes directly from mothers in utero. Birth is also important, as the birth canal provides a type of lesson to the baby’s immune system, preparing it for the environment into which it’s about to be born. For this reason, babies born by C-section may miss out on an important immune boost. Without going into detail, there are methods to deal with this issue.

As part of this process, the microbes found in utero may instruct the fetal immune system. And it doesn’t end there, as breast milk also contains microbes. Even further, the evolution of the mammalian placenta can be traced to a viral infection back in the far reaches of biological history. Clearly, our microbial guests have been entwined in our lives from the beginning – although whether viruses count as microbes remains controversial.

In a similar vein, the hygiene hypothesis argues that many conditions found today are due to a lack of exposure to microbes. A common example is the emergence of various allergies in recent centuries. Not long ago, such allergies seem to have been essentially unheard of. Conditions like this are often called diseases of civilisation.

From organ to organ, we’re a giant ecosystem for microbes. Collectively, our microbes are known as the microbiome, just as our collection of genes are the genome, and the sum of connections in our brain is the connectome. Even researchers joke that there’s an ‘ome’ for everything these days.

Do other animals have microbiomes?

You bet. Before we wrap up, let’s look at some interesting examples from a few different species. For instance, in his book Bioverse, William Miller describes how the bobtail squid’s light emitting organ relies on symbiotic microbes (Vibrio fischeri).

Similarly, the late evolutionary biologist and award-winning science writer, Ed Wilson, wrote about other fascinating cases in his book Half Earth. For example, a type of bioluminescent bacteria live on the underside of certain marine prey animals. Ingeniously, they hide the animal’s silhouette from predators swimming below.

Another example concerns a type of marine worm that lives on the sea floor. This strange little worm ‘eats’ bones, like whale skeletons, but has no mouth or intestines. Instead, it relies on symbiotic bacteria kept in bodily extensions that it injects into the bone. The bacteria then metabolise the material, and share the spoils with their worm host.

What about parasites?

Before we leave the microbiome, we have one last stop. Many microbes reside in our bodies, and often contribute to our health in a range of ways. But some are not so cooperative: parasites.

We’ve always coevolved with parasites. They can seem fairly remote in more metropolitan areas, but the situation is different in other parts of the world.

For example, among Hadza hunter-gatherers in Tanzania, basal metabolic rate is higher than in the West. Studies have linked this to increased immune activity, due in part to greater parasitic load.

How Toxoplasma gondii manipulates rodents in order to complete its life cycle inside a cat’s stomach

The list of parasites is alarmingly long, so let’s focus on one that’s really interesting and bizarre: Toxoplasma gondii. Also known as T. gondii, this parasite has a very weird life cycle.

For reasons that are poorly understood, T. gondii reproduces in the stomach of cats. This sets up some curious evolutionary dynamics that define the goals and strategies of T. gondii.

Say we’re T. gondii. We just successfully reproduced in a cat’s stomach – yay us! But then we got pooped out by the cat. This has quickly gone from success to struggle. So what’s a parasite to do?

We need to get back into a cat’s stomach to complete our life cycle, but how? One of T. gondii’s solutions is pure evil genius. It infects rodents and changes the way they respond to cat odours, making them more likely to be eaten by cats.

Normally, no self-respecting rodent wants to go near cat odour. Even in lab animals separated from the wild by hundreds of generations, put them in an environment with cat odour and they immediately show defensive behaviours like freezing and escape.

But once infected by T. gondii, most rodents lose their aversion to cat odour, and some even start to seek out cat odour. In the wild, this would make an easy meal for a hungry cat, and so the cycle continues.

T. gondii also infects people, and it’s extremely prevalent. Shockingly, evidence suggests that about 1/3 of the global human population either is currently, or was previously, infected by T. gondii. How could a pandemic be hiding in plain sight in the post-COVID era? Because the effects of T. gondii are mostly nil or benign.

But there are risks to pregnant women, and evidence that T. gondii might influence decision making. For example, some studies suggest T. gondii increases risk taking, especially in men. Given Australia’s current feral cat problem, T. gondii could also be relevant for efforts to conserve Australian wildlife. But topics like these are understudied.

Key points

We’re a mothership for microbes, and that’s okay. More than okay, it’s essential for good health. This sits on a bedrock of cooperation between microbiome and host, who team up to cooperate and compete with other organisms and their microbiomes.

But not all microbes are allies. To stay healthy, we must be able to tell friend from foe, and know how to interact with each. It speaks to the genius of nature that most of the time we get it right.

We can only understand biology and evolution if our conceptual toolkit includes cooperation and competition. This applies to us as an ecosystem, and as an organism navigating its own ecosystem. Microbiome science is young, but there’s excitement about its potential to improve health.

(If you’d like to know more about microbiome science and its advice for good health, check out my article here. If you’d like to know about the history of microbes, and how we understood life, health, and disease before Louis Pasteur and modern germ theory, see my other article here.)

Next time

This ends our deep dive into microbiology. From here on, we’ll keep zooming out. The series will end when we discuss cooperation and competition at the global level.

This will focus mostly on human civilisation and its impact on the biosphere. I hope you enjoy this broad sweep from bottom to top, and find some of its lessons helpful.

Next time, we’ll look at how organisms cooperate and compete with members of their own and other species. We’ll see how these dynamics play out at the individual and group levels, and flesh out the implications for biology and evolution. Until then!