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Being human,

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we each view ourselves as a unique and independent individual,

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but we're never alone!

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Millions of microscopic beings inhabit our bodies,

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and no two bodies are the same.

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Each is a different habitat for microbial communities:

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from the arid deserts of our skin,

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to the villages on our lips,

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and the cities in our mouths.

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Even every tooth is its own distinctive neighborhood,

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and our guts are teaming metropolises of interacting microbes.

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And in these bustling streets of our guts,

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we see a constant influx of food,

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and every microbe has a job to do.

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Here's a cellulolytic bacteria, for example.

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Their one job is to break down cellulose,

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a common compound in vegetables, into sugars.

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Those simple sugars then move along to the respirators,

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another set of microbes that snatch up

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these simple sugars and burn them as fuel.

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As food travels through our digestive tract,

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it reaches the fermentors who extract energy from these sugars

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by converting them into chemicals,

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like alcohol and hydrogen gas,

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which they spew out as waste products.

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Deeper in the depths of our gut city,

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

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At each step of this process,

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energy is released,

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and that energy is absorbed

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by the cells of the digestive tract.

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This city we just saw is different in everyone.

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Every person has a unique and diverse community of gut microbes

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that can process food in different ways.

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One person's gut microbes may be capable

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of releasing only a fraction of the calories

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that another person's gut microbes can extract.

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

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Well, things like our genetic makeup

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and the microbes we encounter throughout our lives

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can contribute to our microbial ecosystems.

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The food we eat also influences

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which microbes live in our gut.

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For example, food made of complex molecules,

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like an apple,

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

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But, if a food is made of simple molecules,

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like a lollipop,

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some of these workers are put out of a job.

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Those workers leave the city, never to return.

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What doesn't function well are gut microbial communities

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with only a few different types of workers.

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For example, humans who suffer from diseases

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like diabetes or chronic gut inflamation

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typically have less microbial variety in their guts.

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We don't fully understand the best way

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to manage our individual microbial societies,

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but it is likely that lifestyle changes,

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

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can help revitalize our microbial ecosystems in our gut

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and across the entire landscape of our body.

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So, we are really not alone in our body.

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Our bodies are homes to millions of different microbes,

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

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As we learn more about how our microbes

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interact with each other and with our bodies,

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we will reveal how we can nurture

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this complex, invisible world

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that shapes our personal identity,

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our health,

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and our well-being.