we each view ourselves as a unique and independent individual,

but we're never alone!

Millions of microscopic beings inhabit our bodies,

and no two bodies are the same.

Each is a different habitat for microbial communities:

from the arid deserts of our skin,

to the villages on our lips,

and the cities in our mouths.

Even every tooth is its own distinctive neighborhood,

and our guts are teaming metropolises of interacting microbes.

And in these bustling streets of our guts,

we see a constant influx of food,

and every microbe has a job to do.

Here's a cellulolytic bacteria, for example.

Their one job is to break down cellulose,

a common compound in vegetables, into sugars.

Those simple sugars then move along to the respirators,

another set of microbes that snatch up

these simple sugars and burn them as fuel.

As food travels through our digestive tract,

it reaches the fermentors who extract energy from these sugars

by converting them into chemicals,

like alcohol and hydrogen gas,

which they spew out as waste products.

Deeper in the depths of our gut city,

the syntrophs eke out a living off the fermenters' trash.

At each step of this process,

energy is released,

and that energy is absorbed

by the cells of the digestive tract.

This city we just saw is different in everyone.

Every person has a unique and diverse community of gut microbes

that can process food in different ways.

One person's gut microbes may be capable

of releasing only a fraction of the calories

that another person's gut microbes can extract.

So, what determines the membership of our gut microbial community?

Well, things like our genetic makeup

and the microbes we encounter throughout our lives

can contribute to our microbial ecosystems.

The food we eat also influences

which microbes live in our gut.

For example, food made of complex molecules,

like an apple,

requires a lot of different microbial workers to break it down.

But, if a food is made of simple molecules,

like a lollipop,

some of these workers are put out of a job.

Those workers leave the city, never to return.

What doesn't function well are gut microbial communities

with only a few different types of workers.

For example, humans who suffer from diseases

like diabetes or chronic gut inflamation

typically have less microbial variety in their guts.

We don't fully understand the best way

to manage our individual microbial societies,

but it is likely that lifestyle changes,

such as eating a varied diet of complex, plant-based foods,

can help revitalize our microbial ecosystems in our gut

and across the entire landscape of our body.

So, we are really not alone in our body.

Our bodies are homes to millions of different microbes,

and we need them just as much as they need us.

As we learn more about how our microbes

interact with each other and with our bodies,

we will reveal how we can nurture

this complex, invisible world

that shapes our personal identity,

our health,

and our well-being.