Our bodies are made up of nearly equal numbers of human and microbial cells. These tiny passengers are vital for our health but are dwindling in number.
The human gut is home to bacteria, viruses, fungi, and small worms in some parts of the world.
While the average person’s body contains 30 to 40 trillion human cells, bacteria add another 38 million to this number.
Our microbiome – which is the total collection of all the genes that microbes contribute to our bodies – and our immune system are intricately linked.
Without our microbial passengers, our immune systems would not develop, which is what happens to mice that grow up in germ-free environments.
Modern life affects the microbes that live in our gut. This poses a threat to our health.
With the advent of modern molecular biology techniques, scientists have started to shed light on how our microbiome develops and which factors are detrimental to our health.
What is in the human gut?
The level of variation in the gut microbiome between different people and locations across the globe is astonishing.
While over 1,000 different bacterial species have been found to exist in the human gut, each individual is thought to harbor only around 160 of these.
For years, scientists tried to identify a list of bacterial species that were associated with health.
Today, some believe that a core set of metabolic and molecular processes are at the heart of a healthy gut. Which species support these processes is less relevant than how well they are working together.
The hallmark of this functional core of gut bacteria is a set of genes that promote long-term residence in the gut and active contribution to human metabolic function.
Specifically, these microbes aid digestion, as well as the production of vitamins, hormones, and essential amino acids.
Diet is a major player in determining which microbes take up residence in our guts long-term. A Western diet, high in fat and refined sugars but low in fiber, is thought to reduce microbial diversity.
This can have detrimental effects on health.
A study published in the journal Nature investigated the mechanism.
When mice were fed a low-fiber diet for 4 weeks, the levels of 60 percent of microbial species decreased significantly. About half of these returned to normal levels when the mice were switched back to a high-fiber diet.
But even a short burst of such an unhealthful diet left long-lasting effects, or “scars,” on the microbial diversity, as the researchers pointed out.
More importantly, the loss of diversity became permanently established within four generations when the mice continued to consume a low-fiber diet.
What effect does a loss of microbial diversity have on our health? And how do we come by our tiny passengers in the first place?
The origins of the human microbiome
For many years, scientists assumed that gut of the developing fetus was sterile, meaning it does not contain any microbes.
However, there is evidence from research using mouse models that the amniotic fluid that surrounds the growing fetus contains some microbial species. These can be detected in the newborns first poop, the meconium. On the list are Firmicutes, particularly lactic acid bacteria.