---
title: "The Host Microbiome: Composition, Causation and Intervention"
description: "Sequencing a community gives composition, and composition on its own cannot say what causes what. This stop moves from a survey to a causal claim using gnotobiotic transfer, controlled intervention an"
canonical: https://lightmysky.com/learn/science/the-host-microbiome-composition-causation-and-intervention-mt_-F8lNccQCU
source: https://lightmysky.com/learn/science/the-host-microbiome-composition-causation-and-intervention-mt_-F8lNccQCU.md
retrieved: 2026-09-12
---

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# The Host Microbiome: Composition, Causation and Intervention

Sequencing a community gives composition, and composition on its own cannot say what causes what. This stop moves from a survey to a causal claim using gnotobiotic transfer, controlled intervention and a stated mechanism.

Subject: Science · Area: Microbiology · Ages 23 to 24
Page: https://lightmysky.com/learn/science/the-host-microbiome-composition-causation-and-intervention-mt_-F8lNccQCU

## Ready when they can

- Explain why compositional data need different treatment from ordinary abundance counts
- Design a test that separates a microbiome cause from a microbiome consequence
- Assess a probiotic or transplant claim against the evidence it would need to be true

## Lesson: From microbiome snapshots to causal proof

A microbiome names the whole community living at one body site, not a single species. Sequencing reports relative shares, so every sample sums to a whole. Treat the numbers as compositional: ask what could have moved the whole mix, not just one slice.

Residents guard their niche by occupying attachment sites and nutrients, which leaves little room for invaders. When disease changes the local habitat, the community changes with it. A shifted profile may therefore record the illness instead of causing it, and only a direct test tells which way the arrow points.

**Example.** Design the test by changing the microbes while holding the rest steady, ideally by randomizing the intervention. Transfer work in germ-free animals, defined strains with proof they reached the site alive, a pre-chosen endpoint, and a comparison group turn a snapshot into a causal claim.

**Tip.** Grade every probiotic or transplant claim against that chain. Demand the strain name, engraftment proof, the endpoint picked in advance, and the control group. If improvement tracks attention or other care instead of engraftment, the microbes were likely passengers.

**Recap.** Composition describes, only controlled transfer convinces, so demand the full causal chain.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Measuring Diversity: Richness, Evenness and Turnover Between Sites](https://lightmysky.com/learn/science/measuring-diversity-richness-evenness-and-turnover-between-sites-mt_d_iIiIArWZ)
- [Applied Microbiology: Fermentation, Bioremediation and Host Microbiomes](https://lightmysky.com/learn/science/applied-microbiology-fermentation-bioremediation-and-host-microbiomes-mt_WcfGvV_dD9)