Bacteriophages are ubiquitous viruses that specifically target and infect bacteria. Often called phages, they play a significant role in microbial ecosystems. Their historical discovery laid the groundwork for understanding viral biology, and today, there is growing interest in their potential applications, including addressing the challenge of antibiotic resistance.
What Are Bacteriophages?
A bacteriophage is a virus that infects and replicates exclusively within bacteria. The term, coined by Félix d’Hérelle, means “bacteria-eater.” Phages are composed of proteins encasing their genetic material, which can be DNA or RNA, and often feature a polyhedral head and a helical tail.
Phages vary in size (24 to 200 nanometers), morphology, and genomic organization. They are found in nearly every environment where bacteria exist, from soil to oceans. They are the most numerous biological entities on the planet, with estimates suggesting over 10^31 globally, outnumbering all other organisms combined. They were independently discovered by Frederick W. Twort and Félix d’Hérelle in the early 20th century.
How Phages Attack Bacteria
Bacteriophages employ two primary life cycles: lytic and lysogenic. Some phages only undergo the lytic cycle, while temperate phages can switch between both.
In the lytic cycle, the phage takes over the bacterial cell’s machinery to produce new phage particles. The process begins with adsorption, where the phage attaches to specific bacterial surface receptors. It then injects its genetic material into the cytoplasm, leaving its protein coat outside.
The phage hijacks the cell’s resources, degrading host DNA and directing the cell to synthesize viral components and replicate the phage genome. These components assemble into new phage particles. Finally, the bacterial cell lyses, releasing many new phages. This cycle typically occurs within minutes of infection.
The lysogenic cycle allows the phage to replicate its genetic material without killing the host bacterium. After injecting its DNA, the phage’s genetic material integrates into the host chromosome, forming a prophage.
In this dormant state, the prophage replicates with the bacterial DNA each time the bacterium divides, passing to daughter cells. The host bacterium continues to live and reproduce normally, carrying the phage genome. However, under certain environmental stressors, such as UV radiation, the prophage can excise itself from the chromosome. Once excised, it enters the lytic cycle, leading to new phage production and lysis of the host cell.
Bacteriophages in Medicine
Bacteriophages are drawing attention in medicine, particularly through “phage therapy,” using these viruses to treat bacterial infections. This approach is gaining renewed interest as a potential alternative or complement to conventional antibiotics, especially given increasing global antibiotic resistance. Historically, phage therapy was explored in the early 20th century but declined in Western medicine with the advent of antibiotics, continuing in countries like Georgia and Poland.
A primary advantage of phage therapy is its specificity; phages typically target specific bacterial strains or species. This targeted action means beneficial bacteria in the human microbiome are largely spared, unlike broad-spectrum antibiotics. Phages also self-replicate at the infection site, potentially reducing the need for repeated dosing as their numbers increase with bacterial proliferation. This self-amplifying nature can lead to high efficacy in killing bacteria, even those resistant to multiple antibiotics.
Phage therapy offers a distinct mechanism of action that can overcome bacterial resistance. For example, phages can penetrate bacterial biofilms, communities of bacteria highly resistant to many antibiotics. Combining phages with antibiotics may create a synergistic effect, enhancing treatment efficacy and potentially slowing resistance development. Renewed interest in phage therapy is driven by the need for new strategies against drug-resistant infections, making it a promising area of research and clinical development.
Other Uses of Bacteriophages
Beyond medical applications, bacteriophages demonstrate versatility across several fields. Their ability to specifically target and eliminate bacteria makes them valuable tools.
In food safety, phages detect and control bacterial contamination. They can inhibit foodborne pathogens like Salmonella, E. coli, and Listeria monocytogenes on food surfaces or in processing environments, helping prevent spoilage and foodborne illnesses.
Bacteriophages find application in agriculture, protecting crops and livestock from bacterial diseases. By targeting specific plant or animal pathogens, phages offer an environmentally friendly alternative to chemical pesticides or antibiotics in animal farming.
Their specificity makes phages useful in diagnostics, enabling rapid identification of bacterial pathogens. Genetically engineered phages can produce detectable signals upon infecting a target bacterium, allowing for quick and accurate diagnosis.
Phages contribute to environmental biotechnology, including wastewater treatment and bioremediation. Their natural capacity to regulate bacterial populations can be harnessed to manage microbial communities.