I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

there are more bacteria in your gut than there are stars in our galaxy.

“Animals might be evolution’s icing, but bacteria are really the cake.”4 They have always been part of our ecology. We evolved among them. Also, we evolved from them. Animals belong to a group of organisms called eukaryotes, which also includes every plant, fungus and alga. Despite our obvious variety, all eukaryotes are built from cells that share the same basic architecture, which distinguishes them from other forms of life.

It’s our creation story: two great domains of life merging to create a third, in the greatest symbiosis of all time. The archaeon provided the chassis of the eukaryotic cell while the bacterium eventually transformed into the mitochondria.

There are fewer than 100 species of bacteria that cause infectious diseases in humans;

Speaking of palms, your right hand shares just a sixth of its microbial species with your left hand.

Perhaps it is less that I contain multitudes and more that I am multitudes. These concepts

Why can organisms as disparate as humans and bacteria live together and cooperate? Because we share a common ancestor. We store information in DNA using the same coding scheme. We use a molecule called ATP as a currency of energy. The same is true across all life.

metagenomics, the genomics of communities.

These illnesses are caused by communities of microbes, which have shifted into configurations that harm their hosts. None is a pathogen in its own right; instead, the entire community has shifted to a pathogenic state. There’s a word for such a state: dysbiosis.

the team showed that obese people (and mice) have different communities of microbes in their guts.10 The most obvious difference lay in the ratio of the two major groups of gut bacteria: obese people had more Firmicutes and fewer Bacteroidetes than their leaner counterparts.

Akkermansia muciniphila, one of the more common species of gut bacteria, is over 3,000 times more common in normal mice than in those genetically predisposed to obesity.

microbes matter but so do we, their hosts. Our guts, like all ecosystems, aren’t defined just by the species within them but also by the nutrients that flow through them.

The Extended Phenotype, Richard Dawkins introduces the idea that an animal’s genes (its genotype) do more than sculpt its body (its phenotype). They also indirectly shape the animal’s environment.

Mitochondria certainly count: as we’ve seen, these cellular batteries were once free-living bacteria that became permanently enclosed within a larger

realised that every creature lives in communities with many others. In 1991, she coined a word to describe this unity: holobiont, from the Greek for “whole unit of life”.

An animal doesn’t just depend on its own genes but also on those of its microbes, which are often many times more numerous.

hologenome, which “should be considered as the unit of natural selection in evolution”.

mucus-loving bacterium, Akkermansia muciniphila, whose presence correlates with a lower risk of obesity and malnutrition. There’s Bacteroides fragilis, which stokes the anti-inflammatory side of the immune system. There’s Faecalibacterium prausnitzii, another anti-inflammatory bug, which is conspicuously rare in the guts of people with IBD, and whose arrival can reverse the symptoms of that disease in mice.

Substances that selectively nourish beneficial microbes are called prebiotics