If you’re new to the microbiome and would like to dig into the academic papers that drive the field, here’s the selection that I consider required reading.
Microbiome science is in its infancy, but its enormous potential makes it an environment rich in highly speculative research, often with results that are overturned rapidly with new discoveries. So before you read anything else, I encourage a peek at this 2014 Nature article by Harvard epidemiology professor William P. Hanage: Microbiome science needs a healthy dose of skepticism
You will find several themes repeated regularly in the popular microbiome press
The Firmicutes/Bacteroidetes ratio
The most recent, well-respected review (Walters, Xu, Knight 2014) says flat-out:
the ratio changes between normal and obese individuals are not statistically significant overall and therefore should not be considered a general feature distinguishing normal and obese human gut microbiota across populations.
Another study (Finucane 2014) goes into deeper statistical detail to conclude the same thing.
Walters William A.,Xu Zech and Knight Rob(2014), Meta-analyses of human gut microbes associated with obesity and IBD, FEBS Letters, 588, doi: 10.1016/j.febslet.2014.09.039 Finucane MM, Sharpton TJ, Laurent TJ, Pollard KS (2014) A Taxonomic Signature of Obesity in the Microbiome? Getting to the Guts of the Matter. PLoS ONE 9(1): e84689. doi: 10.1371/journal.pone.0084689
Obesity and the microbiome
Although it’s exciting to think that an obesity cure might be found in the microbiome, the most recent review shows that it’s more difficult than originally thought.
Sze, Marc A., and Patrick D. Schloss. “Looking for a Signal in the Noise: Revisiting Obesity and the Microbiome.” mBio 7, no. 4 (September 7, 2016): e01018-16. doi:10.1128/mBio.01018-16.
“We are only 10% human”
It’s a number based on a guess dating from 1977, but finally updated in 2016:
Our analysis updates the widely-cited 10:1 ratio, showing that the number of bacteria in our bodies is actually of the same order as the number of human cells. Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria.
Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. bioRxiv. Cold Spring Harbor Labs Journals. Retrieved from http://biorxiv.org/content/early/2016/01/06/036103.abstract
Cure/cause obesity by FMT
Several studies in mice hint that an obese microbiome can be transferred to a skinny one and vice versa:
Turnbaugh, P. J., Ley, R. E., Mahowald, M. A., Magrini, V., Mardis, E. R., & Gordon, J. I. (2006). An obesity-associated gut microbiome with increased capacity for energy harvest. Nature, 444(7122), 1027–31. http://doi.org/10.1038/nature05414
Here, we present the largest human microbiota time series analysis to date, covering two individuals at four body sites over 396 timepoints. Don’t miss the 30-second Youtube summary.
Caporaso, J. G., Lauber, C. L., Costello, E. K., Berg-Lyons, D., Gonzalez, A., Stombaugh, J., … Kelley, S. (2011). Moving pictures of the human microbiome. Genome Biology, 12(5), R50. http://doi.org/10.1186/gb-2011-12-5-r50
When you’re ready to go to the original sources, be careful: there are tens of thousands of studies, many of them contradictory and quickly out of date. Here are the ones I think deserve to be read first.
- Historic papers (HMG)
- Population studies (enterotype, population studies)
- Specific microbes (Akkermansia, Bifidobacterium, etc.)
A detailed technical review of how scientists study the microbiome, with an emphasis on how to judge the quality of results. This is a good overview for a smart person who wants an introduction to how we know what we know.
Tyler, Andrea D, Michelle I Smith, and Mark S Silverberg. “Analyzing the Human Microbiome: A ‘How To’ Guide for Physicians.” The American Journal of Gastroenterology 109, no. 7 (July 2014): 983–93. doi:10.1038/ajg.2014.73.
The final paper describing conclusions of the Human Microbiome Project:
Human, T., Project, M., & Figures, S. (2012). Structure, function and diversity of the healthy human microbiome. Nature, 486(7402), 207–14. http://doi.org/10.1038/nature11234
Track as much as you can about two people for an entire year: their diet, physical activity, and microbiome; look for correlations. Conclusion: the microbiome is remarkably stable and quickly recovers to its baseline. The “Methods” section is especially interesting because it goes into detail on how to find interesting statistical results with such complicated data.
David, L. A., Materna, A. C., Friedman, J., Campos-Baptista, M. I., Blackburn, M. C., Perrotta, A., … Alm, E. J. (2014). Host lifestyle affects human microbiota on daily timescales. Genome Biology, 15(7), R89. http://doi.org/10.1186/gb-2014-15-7-r89
Looking for a good overview of studies that link various microbes to diet?
Portune, Kevin J., Alfonso Benítez-Páez, Eva Maria Gomez Del Pulgar, Victor Cerrudo, and Yolanda Sanz. “Gut Microbiota, Diet and Obesity-Related Disorders - the Good, the Bad and the Future Challenges.” Molecular Nutrition & Food Research, June 2016. doi:10.1002/mnfr.201600252.
Here’s another one; see the supplements for details about which foods affect which bacteria.
David, Lawrence A., Corinne F. Maurice, Rachel N. Carmody, David B. Gootenberg, Julie E. Button, Benjamin E. Wolfe, Alisha V. Ling, et al. “Diet Rapidly and Reproducibly Alters the Human Gut Microbiome.” Nature 505, no. 7484 (December 11, 2013): 559–63. doi:10.1038/nature12820.
The intriguing idea that there may be identifiable patterns in our microbiomes, called enterotypes, was proposed in this highly-cited paper, which includes a detailed methods supplement to show you how to compute it yourself:
Arumugam, Manimozhiyan, Jeroen Raes, Eric Pelletier, Denis Le Paslier, Takuji Yamada, Daniel R. Mende, Gabriel R. Fernandes, et al. “Enterotypes of the Human Gut Microbiome.” Nature 473, no. 7346 (May 12, 2011): 174–80. doi:10.1038/nature09944.
The idea that identifiable enterotypes may exist has been viewed skeptically in follow-up work.
Large population summaries
Twin studies help tease out the different effects of human and microbial DNA. This is a recent update to a study of 1,126 twin pairs:
Goodrich, Julia K., Emily R. Davenport, Michelle Beaumont, Matthew A. Jackson, Rob Knight, Carole Ober, Tim D. Spector, Jordana T. Bell, Andrew G. Clark, and Ruth E. Ley. “Genetic Determinants of the Gut Microbiome in UK Twins.” Cell Host & Microbe 19, no. 5 (May 2016): 731–43. doi:10.1016/j.chom.2016.04.017.
Two excellent papers present a detailed analysis of the microbiomes and associated phenotypic information from several thousand healthy people in the Belgian Flemish Gut Flora Project (N = 1106) and the Dutch LifeLines-DEEP study (N = 1135).
Falony, G., M. Joossens, S. Vieira-Silva, J. Wang, Y. Darzi, K. Faust, A. Kurilshikov, et al. “Population-Level Analysis of Gut Microbiome Variation.” Science 352, no. 6285 (April 29, 2016): 560–64. doi:10.1126/science.aad3503. Zhernakova, A., A. Kurilshikov, M. J. Bonder, E. F. Tigchelaar, M. Schirmer, T. Vatanen, Z. Mujagic, et al. “Population-Based Metagenomics Analysis Reveals Markers for Gut Microbiome Composition and Diversity.” Science 352, no. 6285 (April 29, 2016): 565–69. doi:10.1126/science.aad3369.
Be sure to study the supplemental materials, especially Supplement Table 11, which includes details of the specific microbes.
A good overview of the current state of how microbiome analysis is performed, from the sample collection processing, to the data pipeline and final bioinformatics summaries. It includes references to the top platforms (e.g QIME, Mothur, PICRUSt) along with the various tradeoffs of each:
Amato, Katherine R. “[An Introduction to Microbiome Analysis for Human Biology Applications](http://onlinelibrary.wiley.com/doi/10.1002/ajhb.22931/full): Amato.” American Journal of Human Biology, October 2016. doi:10.1002/ajhb.22931.
Elizabeth Bik keeps an excellent Microbiome Papers Collection of a few dozen classic academic papers.