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The hidden viral universe inside our digestive system
The human gut harbors trillions of viruses called bacteriophages, or phages, which infect and interact with gut bacteria in ways scientists are only beginning to understand. These microscopic entities form what researchers now call the phageome-a complex viral ecosystem that may hold keys to improving human health.
A staggering diversity yet to be fully mapped
Bacteriophages, meaning "bacteria eaters" in Greek, exist in unimaginable numbers within the human digestive tract. Estimates suggest there may be one phage for every bacterial cell, though the exact count remains uncertain. Many phages remain unidentified, leading scientists to refer to them as the "dark matter" of the phageome.
Researchers at the University of Colorado Anschutz School of Medicine note that phage science has surged in recent years, yet the field is still grappling with the sheer variety of these viruses. The Gut Phage Database currently lists over 140,000 phages, but this is likely just a fraction of what exists. Colin Hill, a microbiologist at University College Cork, describes their diversity as "extraordinary."
Key players in the gut's viral landscape
Scientists identify phages by analyzing genetic sequences from human fecal samples. Among the most common is crAssphage, named after the cross-assembly technique used to detect its genes. Recent research by Hill and his team revealed that crAssphages have a distinctive light-bulb shape, featuring a 20-sided body and a stalk used to inject DNA into host bacteria.
CrAssphages infect Bacteroides, one of the most prevalent groups of gut bacteria. While their direct impact on human health is unclear, Hill suspects they play a role. Other notable phages include Gubaphage (gut bacteroidales phage) and LoVEphage (lots of viral genetic elements), both of which also target Bacteroides.
Phages and bacteria: A complex partnership
Phageomes vary widely among individuals and are influenced by factors such as age, sex, diet, and lifestyle, according to a 2023 review in the Annual Review of Microbiology. While phages can kill bacteria, their relationship is far more nuanced than simple predation.
"We used to think that phage and bacteria are fighting, but now we know that they're actually dancing; they're partners."
Colin Hill, University College Cork
Phages can benefit bacteria by transferring new genes. During replication inside a bacterial host, a phage may accidentally package bacterial genes along with its own DNA. When it infects a new host, these genes can provide advantages, such as antibiotic resistance or the ability to digest new substances.
Phages also help maintain bacterial diversity. For example, Bacteroides bacteria can produce up to a dozen different sugary coats on their surfaces, each offering unique advantages, such as evading the immune system or colonizing different parts of the gut. However, crAssphages force Bacteroides to constantly switch coats to avoid infection, ensuring a mix of coat types and enabling the bacterial population to adapt to various challenges.
Balancing the gut ecosystem
The gut functions like an ecosystem, with phages acting as predators that keep bacterial populations in check. This balance is crucial for health. Disruptions in the phageome have been linked to conditions like inflammatory bowel syndrome (IBS), irritable bowel disease, and colorectal cancer. For instance, individuals with IBS often exhibit low viral diversity in their gut.
Current efforts to rebalance the gut microbiome include dietary changes and, in severe cases, fecal transplants. However, targeting specific phages could offer a more precise approach. Researchers are exploring therapeutic uses of phages, such as infecting bacteria responsible for stomach ulcers.
The future of phage research
Paul Bollyky, an infectious disease physician at Stanford Medicine, suggests that some phages may be beneficial while others could be harmful. However, much remains unknown about their roles in human health. Breck Duerkop, a bacteriologist at the University of Colorado, emphasizes the potential of harnessing phages to improve health outcomes.
Without phages, Hill warns, a few bacterial species might dominate the gut, potentially leading to digestive issues like an inability to process certain foods or increased gas and bloating. The phageome, with its intricate dance between viruses and bacteria, is proving to be a vital partner in maintaining human health.
