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References: Meet the Microbiome

  1. Revised Estimates for the Number of Human and Bacteria Cells in the Body - Sender R, Fuchs S, Milo R. PLoS Biology, 2016 - The open-access paper behind the "about 38 trillion bacteria and 30 trillion human cells" estimate. Its clear tables show where the old "10 to 1" figure went wrong and why most human cells are blood cells.

  2. Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans - Sender R, Fuchs S, Milo R. Cell, 2016 - A short companion essay that explains, in a few pages, why the bacteria-to-human cell ratio is much closer to one to one.

  3. Human Microbiome Project (HMP) - NIH Common Fund - Archive page for the US government project (2007 to 2016) that mapped the microbes of healthy people. Good for the project's history and goals. Note: as of October 2026 the page still repeats the outdated "ten to one" ratio, which this chapter corrects.

  4. Recovery of gut microbiota of healthy adults following antibiotic exposure - Palleja A, et al. Nature Microbiology, 2018 - The 12-person antibiotic study analyzed in this chapter's "How to read a gut-brain claim" box. The abstract summarizes the bloom of opportunists, recovery within about 1.5 months, and the 9 species still missing after 180 days.

  5. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation - Dethlefsen L, Relman DA. Proceedings of the National Academy of Sciences, 2011 - A small but detailed study of three adults taking two antibiotic courses over 10 months, showing fast loss of diversity, mostly but not fully complete recovery, and big differences between people.

  6. About C. diff - US Centers for Disease Control and Prevention (CDC) - Plain-language page on C. difficile infection: how common it is, why antibiotics raise the risk, symptoms, and when to contact a doctor. Source for the warning signs in the At the Bedside case.

  7. About Escherichia coli Infection - US Centers for Disease Control and Prevention (CDC) - Explains that most E. coli are harmless residents of a healthy gut while some kinds cause illness, a clear example of why bacterial strains matter.

  8. Microbiota-mediated colonization resistance against intestinal pathogens - Buffie CG, Pamer EG. Nature Reviews Immunology, 2013 - Research review on how commensal bacteria protect against infections such as C. difficile, and how antibiotics weaken that protection. Advanced; the abstract is a good summary.

  9. Human milk oligosaccharides: every baby needs a sugar mama - Bode L. Glycobiology, 2012 - Open-access review of the undigestible sugars in human milk that feed helpful infant bacteria, with honest notes on which benefits still need trials in babies.

  10. International consensus statement on microbiome testing in clinical practice - Porcari S, et al. Lancet Gastroenterology & Hepatology, 2025 (online December 2024) - Dozens of experts explain what microbiome tests can and can't do, and why direct-to-consumer tests have no proven clinical value. Useful for judging home test kits.

  11. Establishing or Exaggerating Causality for the Gut Microbiome: Lessons from Human Microbiota-Associated Rodents - Walter J, Armet AM, Finlay BB, Shanahan F. Cell, 2020 - A critical review showing that 36 of 38 transplant studies in germ-free mice and rats reported disease traits transferring, and arguing that this overstates the microbiome's role in human disease. A good case study in correlation versus causation.

  12. Growing up in a Bubble: Using Germ-Free Animals to Assess the Influence of the Gut Microbiota on Brain and Behavior - Luczynski P, et al. International Journal of Neuropsychopharmacology, 2016 - Explains how germ-free mice are raised and studied, why they are so useful for microbiome-brain research, and why their results don't translate directly to people.