Although the role of "good" viruses in human health is still relatively mysterious, we are slowly unraveling the importance of our viral visitors. In this special feature, we introduce a neglected section of the microbiome — the virome.
The role of bacteria and our microbiome in health and disease is at the forefront of medical research.
We are a long way from answering the many questions posed by recent findings, but it is now firmly established that without our personal fleet of "friendly" microorganisms — our microbiome — we would not thrive.
Medical science, however, does not sit on its haunches; its eyes are always fixed on the horizon, straining to describe the shape of things hidden in the far distance.
As we struggle to unpick the almost unbearably complex interactions between bacteria and health, the next challenge is already waiting in the wings: the role of the virome.
What is the virome?
When we hear the word "microbiome," we immediately think of bacteria, but technically, the microbiome is the sum of all microorganisms in a particular environment. Some scientists use the term to refer to the sum of the genetic material of these microorganisms.
So, aside from bacteria, the microbiome also includes viruses (the virome) and fungi (the mycobiome), among other visitors. To date, scientists have paid comparatively little attention to the virome or mycobiome.
Viruses have made themselves at home in a range of ecological niches in the human body, especially on mucosal surfaces, such as the insides of the nose and mouth and the lining of the gut.
In this feature, we will concentrate on the gut virome because it hosts the greatest number of viral occupants and has been investigated the most.
Of course, viruses are most famous for causing diseases, such as smallpox, hepatitis, HIV, and rabies. Because of the urgency associated with viral disease, this aspect has taken up the lion's share of researchers' time. However, many viruses do not have the slightest interest in human cells.
Introducing the bacteriophage
Scientists consider the virome to be "the largest, the most diverse, and the most dynamic part of [the] microbiome," and the majority of the viruses in our guts are bacteriophages. Wherever there are bacteria, there are bacteriophages in abundance.
As other researchers explain: "Phages are the most abundant life forms on Earth, being virtually omnipresent. [...] Some freshwater sources may contain up to 10 billion per [milliliter]."
Bacteriophages infect bacteria, commandeer their cell machinery, and use it to replicate their genetic material.
It is now abundantly clear that gut bacteria influence health and disease, so it is no surprise that viruses that infect gut bacteria may have a significant influence, too.
Phage therapy
From the 1920s to the 1950s, scientists investigated whether bacteriophages could be used to treat bacterial infections. After all, these viruses are adept at destroying human pathogens.
Scientists found that phage therapy was both effective and, importantly, free from side effects.
When antibiotics were discovered, phage therapy faded into the background. Antibiotics could be manufactured with relative ease, and they killed a broad spectrum of bacterial species.
However, with today's hi-tech capabilities and the fearsome backdrop of antibiotic resistance, interest in phage therapy may enjoy a resurgence.
One factor that makes phage therapy attractive is its specificity. Often, antibiotics wipe out a wide spectrum of bacterial species. Now that we know that "good" bacteria live in the gut, however, it is clear that this is not ideal.
Bacteriophages, meanwhile, only target a narrow range of strains within the same bacterial species.
Plus, they only replicate if their target bacteria are in the local area. Taken together, this means that they only attack the desired bacterium, and they continue to replicate until they have wiped out the infection.
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