Scientists have discovered an ancient virus that resides in the intestines of half the human population. The virus, crAssphage, is a bacteriophage that inhabits Bacteroides in the digestive tract. Bacteroides and crAssphage are just two of the plethora of microbes that populate the human gut and have a huge impact on human health.
The human body is a microbiome of tiny bacteria, fungi, protists and viruses – most of them performing a specific function, or occupying a particular niche in the body. Some scientists consider these microbiota another organ because their balance indicates health and stability of human life. In fact, microbes outnumber human cells 10 to one, although, their combined weight is between 200 and 1400 grams which illustrates how tiny they are. The human gut is especially rich in microbiota as they help to break down food into particles small enough to be transported through the bloodstream and used by cells.
Interestingly, every person is now thought to have a slightly unique mix of gut bacteria based on genetics, diet and location. Scientists have identified three main enterotypes, or bacterial ecosystems. Enterotypes are characterized by the levels of the genus Bacteroides as well as the mix of other bacteria. Scientists are interested in Bacteroides because studies have shown they may be related to obesity and diabetes as well as other chronic diseases. Large numbers of Bacteroides are found in people whose diet contains plenty of meat and animal products. It seems that low incidences of Bacteroides are associated with obesity. Obese people are more likely to have larger numbers of Firmicutes in their gut. Firmicutes usually aid in digesting carbohydrates. These bacteria have a higher capacity for gaining energy from food. They are more efficient so people absorb more calories from the same amount of food. However, the composition of microbiota in the gut is so varied and the microbes so small and hard to observe that scientists say sweeping generalizations about the correct balance are premature.
In any case, the majority of bacteria and viruses in the human gut are far from harmful. In fact, it seems that the bacteria evolved along with humans to aid in the digestive process and protect humans from pathogenic microbes. If that is so, then it is likely that the bacteriophages found inside the bacteria also evolved as an important aspect of human health.
Bacteriophages are the most common and diverse entities in the world. Viruses are not considered living organisms because they need to take over an actual cell in order to reproduce. Bacteriophages infect bacteria and inject their DNA into the bacteria’s cytoplasm then use the bacteria’s ribosomes and RNA to manufacture more of themselves. The relationship between bacteria and viruses is more complicated than predator/prey or parasite/host. The virus can cause the bacteria to act differently. It may strengthen the bacteria population by eliminating weak members. Different viruses work on different bacteria so they could function to keep the gut microbiome in balance instead of a particular virus overproducing.
Scientists are only beginning to learn about the human gut microbiome. Identifying crAssphage involved meticulous computer analysis of DNA segments in human feces. Robert Edwards, a bioinformatics professor at San Diego State University, first discovered the virus when evaluating the genetic information on a set of 12 fecal samples. The same 97,000 base pairs kept showing up but the strand of DNA had no match on any database. The team isolated the genes and digitally restructured the DNA. The virus is named after the cross assembly software program used to discover it. SDSU virologist John Mokili was able to amplify the DNA in the original fecal samples to prove that it actually did exist.
Most remarkable about crAssphage is the number of people who carry it. Edwards hypothesizes that 50 to 75 percent of the world’s population hosts the virus. Scientists have found it in samples of Americans, Europeans, Japanese and Koreans. Edwards is sure that as the search is expanded they will find it all across the world. The fact that so many humans have the virus evidences that the virus is ancient. It must have been inhabiting humans for most, if not all, of their development. It must be passed between people somehow because it has not been found in the guts of infants with immature gut microbiomes.
The more scientists look into the human body, the more they find. Not only are people made of an infinitely complex working of cells, but they play host to a whole microcosm of other organisms. Scientists have discovered an ancient virus that resides in the intestines of half the human population. If they can grow crAssphage in a laboratory and understand how it affects the number and health of Bacteroides in the gut they may be one step closer to solving health problems such as obesity, diabetes, gastrointestinal ailments and Chron’s disease.
By: Rebecca Savastio