Scientific and technical background

What is the human microbiome?

The human microbiota is the collection of all of the microorganisms that live in and on your body, alongside and amongst your human cells. This includes bacteria, fungi, viruses and other microbes. The human microbiome is the collection of genes from the microbiota, although the term is usually used to refer to the human microbiota as well.

The number of non-human microorganisms in your body is estimated to be approximately 30 trillion, which is believed to be similar to the number of human cells in the body (though these estimates vary by an order of magnitude). However, the cells of these microorganisms have about 30 million genes, compared to only about 20,000 genes for the human cells. By either measure, the human microbiome plays an important role in the body. Many of the microorganisms are beneficial but some—or the lack of some—can be associated with diseases.

What is the relationship between the human microbiome and disease?

The proteins produced by the human microbiome can interfere with normal body processes, for example by interacting with mimicking other proteins in the body.

Most of these proteins have not been explored in detail. However, there appears to be a link between the populations of microorganisms in the human gut and diseases like Type 1 diabetes, Crohn's disease, and ulcerative colitis. Understanding more about the human microbiome should uncover the link to these and other diseases. Once scientists discover which proteins play key roles in these diseases, they can then turn to working on controlling them to develop new treatments.

What are proteins?

Proteins are the fundamental building blocks for many components within living organisms. More specifically, they are a long sequence of subunits called amino acids, of which there are 20 kinds. Each gene specifies the specific order of the amino acids assembled to make a particular kind of protein. When this sequence of amino acids is constructed by the organism, it tangles or folds into a very particular shape. The shape (structure) of the protein determines its function.

Proteins can be small or large, sometimes containing thousands of atoms. When you eat food containing proteins, your digestive system breaks them down into their constituent amino acids so they are available for your cells to make new proteins according to your genetic codes.

Learn more about proteins at

What is protein folding?

Proteins are chemical compounds consisting of a chain of smaller compounds called amino acids. There are 20 different kinds of amino acids. A gene specifies the order in which the kinds of amino acids are to be linked in the chain to form the protein. As the protein chain emerges from the assembly machinery, it starts to fold or tangle up into a very specific shape. Some amino acids have electrical charge patterns on their surface which make some attract to each other and some repel. These patterns of charges could be thought of as weak little bar magnets. This makes various parts of the chain of amino acids prefer to stick to certain others, and thus form a very specific structure. In the cell, there are other proteins which sometimes help guide the folding process so that the proteins do not fold into incorrect shapes. 

Given that some proteins can be so large as to contain thousands of amino acids, it can be very difficult to figure out the final shape of the folded chain of amino acids, and from there to determine the shape or structure of the protein. Knowing the shape is important because this determines the function or role of the protein. One way of finding the shape is to get many of the same protein molecules and try to make them crystalize into a regular shape (as salt might turn into cubic crystals from a brine solution). Then, x-rays are shown through the crystal. As the x-rays pass the atoms in the proteins, they deflect (diffract) in a very particular pattern which can be deciphered using mathematical techniques to finally know the positions of the atoms and the structure of the protein. However, it can be difficult, if not impossible, to get the proteins to crystalize. 

Scientists have turned to computers as an alternate way of discovering the structures of proteins. They use software programs such as Rosetta (originally developed by David Baker's lab at the University of Washington and now with collaborators from other academic institutions including New York University) to simulate the protein folding process. The software attempts to fold the amino acid sequence many different ways, trying to find the lowest energy configuration, which should represent the actual structure of the folded protein. For very large proteins, they use methods which work on portions of the protein at a time and then assemble the portions.

How does understanding protein structure help scientists understand the role of bacteria in human health?

Different proteins can have many different structures (shapes). They can have sticky portions which like to attach to certain chemical compounds, or repel them. They can sometimes be flexible. For example, enzymes are proteins which can enhance certain chemical reactions or even cut proteins or other compounds apart. Their structures and the patterns of electrical charges on their surface determine which other compounds they may interact with and how they may alter the other compounds.

Since cells use proteins for most of their basic life processes, their functions, determined by their structure, are very important. If these functions are disturbed, diseases can result. Since the human microbiome has over 1,000 times more different kinds of proteins than the human body, many of those have the potential for affecting human cells' operations. Some of these proteins can be beneficial or even necessary, while others may be harmful. This is an area which still needs a lot of exploration. One of the early steps in understanding the role of the microbiome is to discover the functions of its proteins. That requires discovering the structure of those proteins.

Where can I learn more?

The Invisible Universe of the Human Microbiome (National Public Radio video):

Rob Knight: How our microbes make us who we are (TED Talk video):

How Many Cells Are in the Human Body—And How Many Microbes? (National Geographic):