The name given to a virus that attacks a bacterium is a bacteriophage, often shortened to simply phage. This term literally means “bacteria eater.” Bacteriophages are the most abundant biological entities on Earth, existing wherever bacteria live, including in soil, water, and the human gut. They play a fundamental role in regulating bacterial populations and influencing global microbial ecosystems.
Physical Anatomy of the Bacteriophage
A typical bacteriophage, such as the T4 phage, possesses a structure designed for host infection. The primary components include a protein head, or capsid, which is often icosahedral and contains the genetic material—either DNA or RNA. This capsid serves as a protective shell for the phage’s genome.
Attached to the head is a tail structure used to attach to and penetrate the bacterial cell wall. In many phages, this tail includes a contractile sheath that shortens during infection to drive the inner core tube through the bacterial membrane. At the end of the tail is a baseplate with tail fibers that act as recognition elements, which specifically bind to receptors on the host bacterium’s surface.
The Two Cycles of Viral Reproduction
Bacteriophages employ two life cycles: the lytic cycle and the lysogenic cycle. The lytic cycle is the destructive pathway, leading to the rapid hijacking and death of the host cell. This cycle begins when the phage attaches to the bacterium and injects its genetic material into the cytoplasm.
The viral genome takes control of the bacterial machinery, forcing the cell to produce new phage components. These parts are assembled into numerous new viral particles, often producing hundreds of progeny phages within minutes. Finally, the phage releases enzymes that weaken the bacterial cell wall, causing the cell to burst, or lyse, and release the new viruses. Phages that only use this method are called virulent phages and are primarily used in therapeutic applications.
The lysogenic cycle allows the host cell to survive and reproduce. After injecting its DNA, the phage genome integrates into the host bacterium’s chromosome, where it is called a prophage. The prophage remains dormant, and every time the bacterium divides, it copies the viral DNA along with its own, passing the phage genome to all daughter cells.
The phage can remain in this state of latency for many generations. However, environmental stressors, such as toxic chemicals or starvation, can trigger the prophage to excise itself from the bacterial chromosome. Once excised, the phage immediately enters the destructive lytic cycle, leading to the death of the host cell and the release of new virions.
Using Phages to Fight Bacterial Infections
The ability of bacteriophages to destroy bacteria is the basis of phage therapy, a medical approach experiencing a resurgence. This therapy uses lytic phages to treat pathogenic bacterial infections, especially those resistant to traditional antibiotics. Phages offer a potential alternative because they exhibit high specificity, targeting only the disease-causing bacteria.
This precision means phages cause minimal disruption to the host’s beneficial gut or skin microbiota, unlike broad-spectrum antibiotics. Furthermore, a single dose of phages can replicate at the site of infection, increasing their numbers as they eliminate the target bacteria. Phage therapy has shown promise against infections caused by multidrug-resistant bacteria, such as Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA). Phages have demonstrated high rates of clinical improvement, particularly against infections involving bacterial biofilms where antibiotics have failed.
Phages in Research and Diagnostics
Beyond their therapeutic use, bacteriophages are invaluable tools in molecular biology, diagnostics, and biotechnology. Their ability to specifically recognize and bind to a bacterial cell makes them ideal for diagnostic applications, such as phage typing which identifies and distinguishes between different bacterial strains based on which phages can infect them.
Reporter Phages
In advanced diagnostics, phages can be engineered as reporter phages by modifying their genome to carry genes for fluorescence or bioluminescence. When these modified phages infect a target bacterium, the expression of the reporter gene illuminates the presence of the pathogen, allowing for rapid and precise detection in clinical or food safety samples.
Phage Display
Another technique, called phage display, uses phages to display foreign peptides or proteins on their surface. This method is a standard tool for identifying diagnostic biomarkers and developing new drug candidates or antibodies.