The Microbiome: A Beginner's Guide to the Bacteria in Your Gut

Your body is not just yours. It's a community. Right now, inside your gut, there are 100 trillion bacteria. They outnumber your human cells by 10 to 1.¹ They're not invaders. They're meant to be there. And their health determines your health.

Your microbiome is as much you as your brain or your heart. It's responsible for a large portion of your immune function. It produces neurotransmitters that influence your mood. It determines how much energy you extract from food. It influences whether you stay lean or gain weight.

Understanding your microbiome is understanding yourself.

You are a colony

The bacteria in your gut are not a monolith. There are thousands of different species. Different families. Different functions.

Some of these bacteria are so ancient and so integrated with human health that we've co-evolved with them. They do things for us. We do things for them. It's mutualism.

Other bacteria are neutral. They occupy space and eat what they can, but they don't help or harm. They're commensals.

A third category is genuinely pathogenic. These bacteria produce toxins. They damage the gut lining. They trigger inflammation. In a healthy microbiome, they're kept in check. In a dysbiotic microbiome (one that's become imbalanced), they proliferate and cause problems.

Your gut isn't trying to be sterile. It's trying to be balanced. A healthy microbiome contains all three types, but in ratios where the beneficial bacteria dominate.

The organisms that dominate vary by region of the gut. The small intestine is more sparsely populated, dominated by different species than the colon. The colon is where the bulk of the bacteria live, in extraordinary density and diversity.

Commensal bacteria: your allies

The bacteria most people should care about are the ones that actually help you. Bacteroides, Faecalibacterium, Bifidobacterium, Roseburia, and others.

These organisms have several functions:

• Short-chain fatty acid production: They ferment fibre into butyrate, propionate, and acetate. These fatty acids feed the cells of your colon, reduce inflammation, and support the blood-brain barrier.² Butyrate especially is profoundly important for health.
• Vitamin synthesis: Some bacteria produce B vitamins and vitamin K2. You're not wholly dependent on food for these. Your microbiome manufactures them.
• Immune training: Your beneficial bacteria teach your immune system what's a threat and what's safe. Without them, your immune system becomes dysregulated.
• Pathogen suppression: They produce compounds that suppress pathogenic bacteria, essentially out-competing them for resources and space.
• Neurotransmitter production: They produce GABA, serotonin precursors, and other compounds that influence your mood and nervous system function.³

These organisms are called "commensal" when the relationship is mutual benefit. But if you're being strict, the relationship is more mutualistic. They benefit you. You feed them. Everyone wins.

Pathogenic bacteria: the problem organisms

Every gut contains some pathogenic bacteria. That's normal. The issue is balance. In a healthy microbiome, pathogenic organisms are outnumbered and suppressed.

When the balance shifts, when pathogenic organisms proliferate, problems emerge. They damage the gut lining. They produce lipopolysaccharide (LPS), an endotoxin that crosses into the bloodstream and triggers systemic inflammation.⁴ They produce other toxins. They increase intestinal permeability.

Common culprits include certain strains of E. coli, Clostridium species, and Salmonella. Most people carry these, but in numbers that don't matter. When dysbiosis occurs, they explode in abundance.

Dysbiosis isn't an infection. It's an imbalance where the organisms that should be suppressed have taken over because the ones that suppress them have vanished.

The mechanism is usually clear: you took antibiotics (which kill both beneficial and pathogenic bacteria indiscriminately), then you ate a diet that favours pathogenic organisms (processed food, refined carbs, no fibre). The beneficial bacteria couldn't recover. The pathogenic organisms filled the void.

The balance that matters

Your microbiome isn't about one species or one family. It's about the overall balance and diversity.

A healthy microbiome contains hundreds of different species. That diversity is protective.⁵ If one becomes pathogenic, the others suppress it. If one is depleted, others fill the ecological niche.

A dysbiotic microbiome loses that diversity. You get dominated by two or three species. Often pathogenic or inflammatory. The community that protected you is gone.

The marker of a healthy microbiome is high diversity. The marker of dysbiosis is low diversity, often dominated by Firmicutes at the expense of Bacteroides, or vice versa.

This is why probiotic supplements are often disappointing. You're adding a single strain of bacteria to a system that needs ecosystem diversity. It's like restocking a rainforest with a single species of bird. Nice, but insufficient.

Fibre as microbial food

Your beneficial bacteria eat fibre. Not the fibre you eat directly. But the fibre your digestive system can't break down.

Soluble fibre (from vegetables, fruits, legumes, oats) is particularly valuable. Your bacteria ferment it into short-chain fatty acids, which feed your colon and reduce inflammation.

Insoluble fibre (from vegetables, whole grains, seeds) supports motility and feeds different bacterial species.

Most modern diets are fibre-depleted. People are eating refined carbs, processed food, and very few vegetables. The bacteria that feed on fibre starve. They die off. The bacteria that feed on refined sugar and processed food explosively proliferate.

If you want a healthy microbiome, you have to feed the bacteria you want. That means fibre. A lot of fibre.

Soluble fibre targets: oats, barley, vegetables, legumes, fruit (particularly berries and apples with the skin on). Work up gradually. Sudden massive fibre increases cause digestive distress. Your bacteria adapt to processing fibre, and the adaptation takes days or weeks.