Bacteriophage: Definition, Structure, Examples

May 16, 2023, 16:45 IST

Bacteriophages, also known as phages, are viruses that infect and replicate within bacteria. They are the most common type of virus and have a variety of effects on their hosts. This blog post will explore what a bacteriophage is, how they work, the two different types of cycles they have, and the advantages and disadvantages of using them. By the end of this post, you will have a greater understanding of this fascinating organism and its role in biology.

What is a Bacteriophage?

Bacteriophages, also known as phages, are viruses that infect and replicate within bacteria. They are the most abundant organisms on the planet, and they play an important role in the cycling of matter and energy within microbial communities. Phages have been studied for their role in the prevention and treatment of bacterial infections and in the development of biotechnology. They are found everywhere on earth. They have genetic material which can be DNA or RNA.

The discovery of bacteriophages is often attributed to British bacteriologist Frederick Twort in 1915 and French-Canadian microbiologist Félix d'Herelle in 1917. 

FrederickTwort observed the destruction of bacteria in culture by a mysterious entity.

Félix d'Herelle discovered similar entities while studying the bacteriology of dysentery in India.

Both Twort and d'Herelle independently described the entities as bacteriophages and recognized their potential as a treatment for bacterial infections.

The discovery of bacteriophages marked the beginning of phage therapy and led to a deeper understanding of the role of viruses in biology and medicine. A bacteriophage, also known as a phage, is a virus that infects and replicates within bacteria. Bacteriophages are composed of a nucleic acid molecule, usually DNA or RNA, surrounded by a protein capsid and sometimes an outer lipid membrane. They are the most abundant biological entity on the planet and are found in almost every environment on Earth. Phages can be naturally occurring, or they can be created in a laboratory setting. Bacteriophages are used as a natural form of control for certain bacterial infections, as well as in research to better understand bacterial biology.

How Do Bacteriophages Work?

Bacteriophages, or phages, are viruses that infect bacterial cells. They are composed of a head and tail, with the head containing the genome of the virus and the tail containing the components that allow it to attach to the host cell. When a phage attaches to a bacterial cell, it injects its genetic material into the cell, hijacking its reproductive machinery to create copies of itself. This process continues until the host cell is destroyed and new phage particles are released, continuing the cycle. Phages are highly specific, meaning they can only infect certain species of bacteria, making them useful for treating bacterial infections.

Structure Of Bacteriophage

The structure of bacteriophages can vary greatly, but most bacteriophages have a few basic components:

• Capsid: The outer shell that encloses the genetic material of the phage. 
• Genome: The genetic material of the phage  can be either DNA or RNA. The genome encodes the instructions for the phage's replication and host-cell infection.
• Tail: Many bacteriophages have a tail structure that helps the phage attach to the surface of its host cell and inject its genome into the cell.
• Base plate: The base plate is located at the bottom of the tail and helps the phage anchor itself to the host cell. Some bacteriophages also have specialized appendages, such as spikes, that help with attachment to the host cell.
• Enzymes: Bacteriophages often carry enzymes that help with replication and the lysis of host cells.

This is a general structure of bacteriophages and some phages may have additional components or unique structures that enable them to infect specific types of bacteria.

Types Of Bacteriophages

Bacteriophages are viruses that infect bacteria. There are  types of bacteriophages, including:

• Lytic Bacteriophages: These bacteriophages infect a bacterial cell and multiply within it, eventually causing the cell to lyse (burst open) and release the newly formed bacteriophages. T4 is a common lytic bacteriophage.
• Lysogenic Bacteriophages: These bacteriophages integrate their DNA into the bacterial chromosome and can remain latent for long periods. In some cases, the bacteriophage DNA can be passed on to daughter cells during replication, leading to stable infection. Lambda phage is a well-known lysogenic bacteriophage.
• Tailed Bacteriophages: These bacteriophages have a distinctive tail structure that helps them to attach to the surface of the bacterium of  the host cell. The T-even phages (T2, T4, T6) are examples of tailed bacteriophages.
• Filamentous Bacteriophages: These bacteriophages have a long and thin filamentous structure that helps them to infect bacterial cells. They typically infect gram-negative bacteria and do not have a tail. M13 is a well-known filamentous bacteriophage.
• Siphoviridae: These bacteriophages have a long, non-contractile tail, and they infect gram-positive bacteria. An example of a bacteriophage belonging to the Siphoviridae family is lambda phage.
• Podoviridae: These bacteriophages have a short, contractile tail and they infect gram-negative bacteria. An example of a bacteriophage belonging to the Podoviridae family is T7.

 

Life Cycle Of Bacteriophage

Lytic Life Cycle:-

The lytic life cycle of bacteriophages refers to the process by which these viruses infect and replicate within a bacterial cell, leading to the lysis (bursting) of the cell and the release of new bacteriophages. 

The lytic life cycle of bacteriophages can be divided into the following stages:

• Attachment: The bacteriophage attaches to specific receptors on the surface of the bacterium.
• Penetration: The bacteriophage uses its tail to inject its DNA into the bacterium.
• Replication: The bacteriophage DNA takes over the host cell's machinery, directing the synthesis of new bacteriophage particles and enzymes needed for replication.
• Assembly: The new bacteriophage particles self-assemble and are packaged into the bacterium.
• Lysis: The bacteriophage DNA triggers the lysis of the bacterium, releasing the new bacteriophage particles, which can then infect other bacteria.

The lytic life cycle of bacteriophages is particularly useful in controlling bacterial infections, as it results in the destruction of the infected bacterium. Bacteriophages are also used in biotechnology and medicine, as they can be used to produce enzymes, antibiotics, and vaccines. Thus, the lytic life cycle of bacteriophages represents a promising alternative to antibiotics in the fight against bacterial diseases.

Lysogenic Cycle

The lysogenic cycle of bacteriophages is a distinct mode of replication that results in the integration of the bacteriophage DNA into the bacterial chromosome, where it can remain latent for long periods of time. 

The lysogenic cycle can be divided into the following stages:

• Attachment: The bacteriophage attaches to specific receptors on the surface of the bacterium.
• Penetration: The bacteriophage uses its tail to inject its DNA into the bacterium.
• Integration: The bacteriophage DNA integrates into the bacterial chromosome and can remain latent for long periods of time, being passed on to daughter cells during replication.
• Expression: In some cases, the bacteriophage DNA may be expressed, producing new bacteriophage particles.
• Lysis: In certain conditions, the bacteriophage DNA may trigger the lysis of the bacterium, resulting in the release of new bacteriophages.

The lysogenic cycle of bacteriophages is an important mechanism of horizontal gene transfer, allowing the transfer of genetic information between bacteria. This cycle can also result in the creation of bacteriophage-resistant bacteria, as the integrated bacteriophage DNA provides a source of resistance to subsequent infections. However, the lysogenic cycle can also have positive effects, as it can lead to the acquisition of new genetic traits by the bacterium, including antibiotic-resistance genes. Overall, the lysogenic cycle of bacteriophages is a complex and dynamic process..

The Role Of Bacteriophages In Biology And Medicine

Biological role:-

They play a significant role in biology as they are one of the most abundant biological entities on Earth and are found in almost every environment where bacteria exist. In the natural environment, phages control the population of bacteria, preventing them from overgrowing and becoming harmful to other organisms. They also participate in horizontal gene transfer, which is the transfer of genetic material between different bacterial strains.

Medicinal role:-

In medicine, bacteriophages have potential applications in the treatment of bacterial infections, as well as in biotechnology and agriculture. Phages can be used to target specific bacterial pathogens and have been proposed as alternative treatments for antibiotic-resistant bacterial infections. Overall, bacteriophages play a crucial role in shaping bacterial populations in various environments and their study is important for understanding bacterial ecology, evolution, and the interactions between viruses and bacteria.

Advantages and Disadvantages of Bacteriophages

Bacteriophages, also known as “phages”, are viruses that infect and replicate within bacteria. They have long been used in the study and treatment of bacterial infections and are gaining renewed attention as a potential alternative to antibiotics. While bacteriophages offer many potential benefits, there are also some disadvantages associated with their use.

The primary benefit of bacteriophages is their ability to target and kill specific bacteria. Unlike antibiotics, which can have a broad range of effects, phages are only effective against the bacteria they are designed to target. This targeted approach can reduce the risk of overuse and antibiotic resistance, which can lead to increased drug efficacy and reduced side effects. Additionally, phage therapy is less likely to cause collateral damage to the patient’s health.

However, there are some disadvantages associated with phage therapy. Bacteriophages have been known to cause lysogeny, in which the viruses become incorporated into the bacterial genome and replicate with it, leading to the production of new viruses. Additionally, the use of phage therapy has been limited by a lack of scientific understanding, as well as the complex process involved in obtaining and administering the phages. Finally, there is a risk of phage resistance, in which bacteria can become resistant to the viruses over time.

In summary, bacteriophages offer a potential alternative to traditional antibiotics, with the benefit of being able to target specific bacteria without causing collateral damage. However, there are some potential drawbacks to their use, including lysogeny, limited scientific understanding, and the risk of phage resistance.

Bacteriophage Related FAQ’s

Q1. What is a bacteriophage? 

Ans. A bacteriophage is a type of virus that infects bacteria. 

Q2. How do bacteriophages work? 

Ans. Bacteriophages attach to the surface of bacterial cells and inject their genetic material into the cell. This material then takes over the cell, using its resources to produce more copies of the virus. 

Q3. How do bacteriophages infect bacteria?

Ans.T he phage genome takes over the host's metabolic machinery and uses it to produce new phages. Eventually, the host cell is lysed (ruptured) to release the new phages, which can then infect other bacteria.

Q4. What is the significance of bacteriophages in biology?

Ans. Bacteriophages play a crucial role in controlling the populations of bacteria in various environments, preventing them from overgrowing and becoming harmful to other organisms.

Q4. How are bacteriophages used in medicine?

Ans. Phages can be used to target specific bacterial pathogens and have been proposed as alternative treatments for antibiotic-resistant infections. They can be applied topically or taken orally, and in some cases, they have been shown to be effective in reducing the number of bacteria in infections.

Q5. Are bacteriophages safe for human consumption?

Ans. Phages have been used for over 90 years to treat bacterial infections and have been found to be safe for human consumption in most cases. 

Q6. Are bacteriophages a better alternative to antibiotics?

Ans. Phages may be a useful alternative to antibiotics in certain cases, particularly when the bacteria responsible for the infection are antibiotic-resistant.

Q7. What are the benefits of using bacteriophages? 

Ans Bacteriophages can be used to combat bacterial infections, as they are highly specific and can target a particular type of bacteria. This is often used as an alternative to antibiotics, as it is more targeted and less likely to have adverse effects on the patient. 

Q8. Are there any risks associated with using bacteriophages? 

Ans. One potential risk is the development of bacterial resistance, as the virus can mutate and become resistant to the bacteriophage. It is important to use bacteriophages responsibly, and to monitor for any signs of resistance.