The latest cult favorite in the sphere of human genetics is the microbiome, the genes of the bacterial species that live inside and upon the human body. Because bacterial cells outnumber human cells in an adult by approximately ten to one, and tens of thousands of different species make up the human ecosystem, studying this world will be even more of a challenge than the Human Genome Project, which only had to concern itself with a single species: us. But as the microbiome is increasingly discovered to play a role in obesity, diabetes, infant diseases, and hospital-acquired infections, the number of researchers pondering a bacterial angle for their own disease of interest is exploding.
So the microbiome was the ideal topic for the first lecture of the Institute for Translational Medicine seminar series on Advanced Tools, a monthly meeting designed for University of Chicago researchers to share methodological know-how. Leading the discussion was a veteran of the young microbiome scene – Eugene Chang, professor of medicine and an expert on gastroenterology. For several years, Chang has applied the tools of microbiology to the bacterial populations of the human gut, looking for mechanisms involved in digestive diseases. As the techniques for studying the microbiome have evolved, Chang said he has seen the pros and cons of the field’s growth.
“This is an area that is really hot,” Chang said. “It isn’t coincidental that this interest has coincided with emerging technologies, because the emerging technologies over the last decade have allowed us to look at the microbiome in many different ways….but this is a field where you can be easily consumed by the technology.”
Those techniques have changed alongside the trends of the broader field of microbiology, Chang said. Scientists interested in bacteria were once limited to studying what they could both find and grow in a lab dish, which left the vast majority of species unexplored. But new genetic techniques have brought those hidden worlds into the light, allowing scientists to take a more complete census of the bacteria present in a given sample from the Earth’s environment, or the special environments within the human body. With this added power has come a whole new menu of choices for scientists, from low-cost methods (i.e. T-RFLP) that can take a surface-level snapshot of the most common members of a microbial community to deeper sequencing that can identify rare microbes that may turn out to be relevant to disease (i.e. pyrosequencing).
“We have a number of techniques that have advantages and limitations,” Chang said. “What you use is dependent on what your question is and how deep you need to go.”
In Chang’s laboratory, the questions relate to the origins of inflammatory bowel diseases such as ulcerative colitis. A recent study looked at the microbial diversity within the colon, comparing the bacterial populations present in the mucosa of the proximal colon (near the small intestine) to the distal colon (near the anus). A T-RFLP analysis, which looks at fragments of ribosomal DNA in the mucosal samples, found that the microbes present in the two regions were distinct, with higher “richness” (the number of species present) observed in the proximal versus distal colon.
But to determine the role of the microbes in disease, just taking a census isn’t enough. The newest wave of microbiome research is focused on function, using techniques that find out what those billions of bacteria are actually doing inside our bodies or out in the world. With metagenomics, scientists can analyze all the genes from a given sample of soil, skin, or mucus, then group those genes by their functional role (metabolism, transport, etc.) using a technique developed by Argonne called MG-RAST. Many groups, including Chang’s team, are also interested in measuring host-microbe relationships – how the bacterial population affects the biology of their home organism.
“Sure we can say who’s there, but how do we actually know what’s important?,” Chang said.
One way to test the role of the microbiome in host biology is to manipulate it, which can be done using “gnotobiotic technology” – raising an animal from birth in germ-free conditions to control their bacterial communities. Another is to test the affect of microbes on biological measures such as gene expression or protein activity. In last year’s PLoS ONE paper, Chang’s group combined these techniques and found different expression of proteins called Toll-like receptors (TLRs) in proximal vs. distal colon, a difference that disappeared in mice with “blank microbiomes,” raised in germ-free conditions.
As the Human Microbiome Project prepares for its second phase, Chang predicted the focus would shift from developing new techniques to applying their power to questions with medical implications. While directly studying the microbiome of humans has proved challenging – for reasons of consent, sample collection, and computational limits – the ability to “implant” the human microbiome into a germ-free mouse may be a temporary workaround. Such experiments have the potential to launch the microbiome from being the cult favorite of geneticists and microbiologists to a wider audience of physicians and patients.
“I think there will be a call for more proposals that are biologically driven and translational,” Chang said. “If you’re thinking about getting into studies of the microbiome, do it now, it’s really a good time.”
Wang Y, Devkota S, Musch MW, Jabri B, Nagler C, Antonopoulos DA, Chervonsky A, & Chang EB (2010). Regional mucosa-associated microbiota determine physiological expression of TLR2 and TLR4 in murine colon. PloS one, 5 (10) PMID: 21042588