A bacteriophage, or phage, is not a separate kind of germ. It is a virus whose only host is bacteria.1Frontiers in Microbiology. Phage family classification under Caudoviricetes: A review of current tools using the latest ICTV classification framework Phages are also the most numerous biological entities on Earth, outnumbering every other organism combined.2Frontiers in Microbiology. Phage family classification under Caudoviricetes: A review of current tools using the latest ICTV classification framework Viruses on opposite sides of the tree of life have arrived at strikingly similar ways to break into cells and to hide inside them.
What Phages and Other Viruses Have in Common
Every virus is a set of genes inside a protective coat, and none of them can grow, eat, or copy itself alone. Outside a cell, a virus is an inert particle, which is why scientists still debate whether viruses count as alive.
Borrowing a cell’s machinery has a specific meaning. A cell has ribosomes, which read genetic instructions and assemble proteins, and a virus has none of its own. Its genes act like a forged work order: the cell reads them as if they were its own and starts producing viral parts. The cell also supplies the raw building blocks and the energy to make new viral genomes and shells. Because the virus runs on the host’s own machinery, it is not touched by antibiotics, which target parts that bacteria have and viruses lack.
The coat is built from many copies of a few protein pieces that snap together in a repeating pattern. It protects the genes outside the cell and helps the particle find and enter a host. Many viruses that infect people also wear an outer fatty envelope taken from a host cell, while phages usually lack one.
The genetic material varies within both groups. Some human viruses, like flu and coronaviruses, carry RNA, while others, like herpes, carry DNA. Phages lean heavily toward DNA: about 99% of identified phages have DNA genomes.3WashU Medicine. Study finds vast diversity among viruses that infect bacteria An RNA phage is a rarity, while RNA viruses are common among those that make people sick.
Where viruses came from is still open. One idea says they arose very early, before or alongside the first cells. Another says they descend from cells that shrank into parasites, or from cell genes that escaped.4PubMed Central. Introduction to Virus Origins and Their Role in Biological Evolution
How Phages and Human Viruses Get Inside and Multiply
Many phages look like tiny lunar landers, with a head and a tail that latch onto a bacterium. A phage works like a syringe: the shell stays outside and only the DNA goes in. That DNA is packed so tightly that it pushes outward with pressure of tens of atmospheres.5Elsevier / Current Opinion in Microbiology (PubMed Central). Is phage DNA ‘injected’ into cells–biologists and physicists can agree Pressure alone cannot push all of it in, because the bacterium’s interior pushes back. One explanation is that water rushing into the cell drags the rest of the DNA in.6Elsevier / Current Opinion in Microbiology (PubMed Central). Is phage DNA ‘injected’ into cells–biologists and physicists can agree
Viruses that infect people are often taken into the cell whole, either engulfed by it or fused with its membrane, and release their genes inside. The difference reflects the host’s structure: bacteria have a tough cell wall outside the membrane, which a phage must get through first. One phage’s tail carries a loop that punches through the cell membrane much as some animal viruses do.7PubMed. The bacteriophage Ï•29 tail possesses a pore-forming loop for cell membrane penetration
A phage infection runs in four steps. The tail fibers bump into and stick to a bacterium’s surface. The tail anchors and the DNA is pushed through, leaving the empty protein head outside. Inside, the bacterium’s machinery is redirected to copy the phage genome and build new heads and tails. Finally the new particles are assembled and released, and each can go on to find another bacterium.
That grip on the surface is also why phages cannot infect human cells. The tail fibers hold onto specific molecules on a bacterium’s surface, and human cells lack those molecules, so phages have nothing to attach to. The same targeting usually limits a phage to one species or strain of bacteria.
Both groups also have two strategies once inside: burst or hide. Many phages copy themselves and then burst the cell open.8ASM. Marine Viruses: Submerged Players of Climate Change Others can instead tuck their DNA into the bacterium and stay quiet.9PubMed. Common Strategies Governing Temperate Phage and α-Herpesvirus Persistence Herpes viruses do the same in human nerve cells, which is why cold sores come back.10PubMed. Common Strategies Governing Temperate Phage and α-Herpesvirus Persistence In both cases similar cues, such as damaged DNA, poor nutrition, or signals between cells, flip the virus back to active.11PubMed. Common Strategies Governing Temperate Phage and α-Herpesvirus Persistence
Phages Can Hand Bacteria New Abilities
Phages do more than kill. They can carry pieces of bacterial DNA from one cell to another.12PubMed. Embracing the enemy: the diversification of microbial gene repertoires by phage-mediated horizontal gene transfer This happens by accident: when a phage assembles new particles inside a bacterium, it can occasionally package a piece of the host’s DNA. When that particle infects another bacterium, it delivers the DNA into the new host instead of a normal phage genome. The new host may keep the gene, so one bacterium effectively passes a trait to another without dividing.
A dormant phage can also add genes that give its host new traits, and this is widespread.13PubMed. Embracing the enemy: the diversification of microbial gene repertoires by phage-mediated horizontal gene transfer These added traits often help bacteria compete or interact with other organisms.14PubMed. Embracing the enemy: the diversification of microbial gene repertoires by phage-mediated horizontal gene transfer Viral genes found in coral communities, for example, include ones linked to bacterial virulence and metabolism.15Scientia. Technology, Science and Society. Exploring the Multifaceted Role of Viruses in Sustaining Biodiversity: Mechanisms, Implications, and Contributions to Ecosystems Stability
This matters for illness. Some harmful traits in disease-causing bacteria arrived via phage genes, and a harmless strain can become more dangerous by picking up such genes. That is one reason phages chosen for therapy are screened to avoid gene carriers.
Some human viruses also leave genes behind in our DNA, but over evolutionary time. Phage gene swapping among bacteria happens fast and constantly, and it is one of the main ways bacteria evolve new abilities.
How Bacteria and Humans Defend Against Viruses
Bacteria use at least two systems that chop up incoming phage DNA.16PubMed. CRISPR-Cas and restriction-modification systems are compatible and increase phage resistance One cuts foreign DNA at specific spots but gives only brief protection, which phages quickly overcome.17PubMed Central. Cleavage of viral DNA by restriction endonucleases stimulates the type II CRISPR-Cas immune response That first cut helps trigger a second system, CRISPR, which stores a snippet of the phage to recognize it next time.18PubMed Central. Cleavage of viral DNA by restriction endonucleases stimulates the type II CRISPR-Cas immune response The cutting leaves fragments that help CRISPR learn what to remember. Within 30 minutes of infection, a few cells start building that memory.19PubMed Central. Cleavage of viral DNA by restriction endonucleases stimulates the type II CRISPR-Cas immune response Together the two systems give stronger protection than either alone.20PubMed. CRISPR-Cas and restriction-modification systems are compatible and increase phage resistance
The bacterium stores a short piece of the invader’s DNA in a special region of its own genome. If that phage returns, the stored piece is copied into a guide that leads cutting proteins to matching DNA. Because the memory sits in the genome, it is passed to the bacterium’s offspring, unlike human immune memory. Scientists turned this system into the gene-editing tool known as CRISPR.
Humans rely on alarms and specialized cells. Infected human cells release interferons, alarm signals that switch on antiviral defences in nearby cells.21PubMed. Role of early cytokines, including alpha and beta interferons (IFN-alpha/beta), in innate and adaptive immune responses to viral infections These signals activate natural killer cells that destroy infected cells.22PubMed. Role of early cytokines, including alpha and beta interferons (IFN-alpha/beta), in innate and adaptive immune responses to viral infections Lasting protection comes partly from T cells that multiply and target the virus.23PubMed. Role of early cytokines, including alpha and beta interferons (IFN-alpha/beta), in innate and adaptive immune responses to viral infections
The contest never ends. Phages evolve around defences, and hosts evolve new ones. Resistance often costs the host slower growth, which keeps a mix of types alive.
Using Phages and Viruses as Medicine
Phage therapy uses phages to kill the bacteria causing an infection.24PubMed. Phage therapy: A targeted approach to overcoming antibiotic resistance Phages strike specific bacteria, can penetrate slimy bacterial films, and resistance develops less readily than with antibiotics.25PubMed. Phage therapy: A targeted approach to overcoming antibiotic resistance Researchers therefore see them as an alternative or add-on to antibiotics for multidrug-resistant infections.26PubMed Central. Phage therapy: An alternative to antibiotics in the age of multi-drug resistance In one group of ten patients with chronic resistant infections, treatment was safe and most felt at least temporary relief.27PubMed Central. Can Bacteriophages Replace Antibiotics? That evidence is small and has no comparison group, so phage therapy is still mostly experimental and larger controlled trials are needed.
For a patient, it starts with a doctor growing the bacteria from the infection and testing which phages kill it. Because phages attach to specific surface molecules, one that works in one patient may not work in another, which makes therapy more tailored than a standard antibiotic. Sometimes a mixture of phages is used, so that if bacteria resist one, another still works. The chosen phages are given by a route suited to the infection, such as by mouth, inhalation, on a wound, or by injection, and doctors monitor whether the bacteria develop resistance.
It is usually offered through clinical trials or special-access programs, often alongside antibiotics rather than instead of them. If you are considering it, ask a specialist whether a trial fits your infection.
Viruses that infect human cells serve medicine differently. Adeno-associated virus can be stripped of its own genes and filled with a helpful gene.28PubMed Central. Adeno-Associated Virus (AAV) as a Vector for Gene Therapy It rarely causes disease and keeps a delivered gene working for a long time.29PubMed Central. Adeno-associated virus as a delivery vector for gene therapy of human diseases It can also enter both dividing and non-dividing cells,30PubMed Central. Gene Therapy Using Adeno-Associated Virus Vectors and it is now a central tool in clinical gene therapy.31PubMed Central. Adeno-associated virus as a delivery vector for gene therapy of human diseases
Phages in Your Gut and Across the Planet
Your gut holds a large community of phages alongside its trillions of bacteria, and they constantly infect and kill some of them. Gut phages kill more of whichever bacteria grow most crowded,32PubMed Central. Gut Phageome—An Insight into the Role and Impact of Gut Microbiome and Their Correlation with Mammal Health and Diseases since more cells make them easier for phages to find. That keeps the bacterial community diverse and stable, stopping one species from taking over and helping others persist.33PubMed Central. Gut Phageome—An Insight into the Role and Impact of Gut Microbiome and Their Correlation with Mammal Health and Diseases
The balance of gut bacteria affects digestion and how well the gut resists harmful microbes. Phages influence your health indirectly, by shaping which bacteria are there, not by infecting you. Researchers are still learning how changes in gut phages relate to disease.
In the ocean, when phages burst bacteria, the contents spill into the water. Other bacteria absorb these nutrients and use them to grow, so the nutrients circulate among microbes instead of passing up the food chain to plankton and fish.34ASM. Marine Viruses: Submerged Players of Climate Change This keeps carbon in the sea rather than in the air, about 3 billion tonnes a year.35ASM. Marine Viruses: Submerged Players of Climate Change Viruses can also speed up the sinking of carbon to the deep sea.36PubMed. Marine viruses and climate change: Virioplankton, the carbon cycle, and our future ocean Viruses act as an engine of the ocean’s nutrient and carbon cycles.37PubMed. Marine Viruses and Their Role in Marine Ecosystems and Carbon Cycling They are often a major cause of death for algae, though some coexist with their hosts.38PubMed. The ecology of viruses that infect eukaryotic algae