A review on Bacteriophages

Viruses capable of infecting bacteria are known as bacteriophages. These viruses infect the bacterial cells and multiply their genome. They infect various strains of Escherichia coli. The viral properties stop the bacterial replication process. They use the bacterial genetic machinery for their survival and replication. Different institutes worked on bacteriophages and used these viruses as model organisms. It is possible to map the genes with the help of phage genome studies. The bacteriophages follow two types of reproductive cycles such as lytic and lysogenic cycle. Different types of phages exist in the world of microbes.

Bacteriophage consists of a protein coat and a genome. The bacteriophage genetic material consists of DNA or RNA. The bacteriophage structure is made up of capsid head and a filamentous tail. Phage capsid involves proteinaceous coat. The icosahedral capsid is common among bacteriophage. The genome resides in the phage head. During the phage particle assembly, the genetic material gets packaged into the phage head.

There are various classes of bacteriophages. Some of the bacteriophages have a non-enveloped contractile tail whereas some others have an enveloped and rod-shaped tail. Some of them have isometric, ovoid, bottle-shaped, and lemon-shaped structures. It involves variation in the genomes. Some of the bacteriophages have a linear and double-stranded DNA. Examples include T4, Mu, PBSX, and P2 phages. M13 phages have a circular single-stranded DNA. Some other bacteriophages have a circular double-stranded DNA. Linear and segmented RNAs exist in bacteriophages.

Image: Bacteriophage

1.     Bacteriophages as model organisms:

Bacteriophages help in genetic analysis. They serve as vectors in DNA cloning and genetic engineering. It is possible to study genetic recombination using bacteriophages. Bacteriophages infecting bacterial cells produce plaques upon lysis. Hence, it is easy to culture them. Bacteriophages serve as cloning vectors since they exhibit DNA transferring properties.

2. Lytic and lysogenic cycles:

There are two cycles of bacteriophage replication such as lytic and lysogenic cycles. The lytic cycle involves complete lysis of the bacterial cells. The first step involves the infection of bacterial cells. The phage particle lands on the bacterial cell and gets attached to it. Then it interacts with the cell surface receptor and injects its genome into the bacterial cells. Once the phage DNA gets injected into the cell, bacterial replication stops and phage replication process gets initiated. Bacteriophage uses bacterial enzymes for replication. It takes 22 minutes for completing the phage cycle. After completing the replication process, phage particles get assembled and release through the cell. The lysogenic cycle involves the genomic integration into the bacterial chromosome. Depending on the environmental conditions the phage switches over to lytic cycle.

3. Transducing phages:

These phages possess transducing properties. The transducing phages are known as defective phages. During phage genome replication, the defective bacteriophage takes up the bacterial genome. It transfers it to the other bacterium. Hence, the genome gets transferred from the donor bacterium into the recipient bacterium. Two types of transducing phages exist. Generalized transducing phages undergo generalized transduction. Specialized transducing phages undergo specialized transduction. Generalized transduction does not follow any special pattern of transduction.  Any part of the bacterial genome gets transduced. Specialized transduction involves transduction of specific gene or segment of a bacterial chromosome. The defect in the phage becomes an advantage for the bacteria to transfer genes. Transduction does not involve sex pilli of the bacterial cells. It is a phage-mediated process. Bacterial conjugation involves the transfer of F plasmid through sex pill. However, the transduction process directly involves the role of the defective phages.

4. Bacteriophage gene mapping studies:

Studies involving the mutant and wild-type bacteriophage strains helped in mapping the genes. Fine structure and deletion mapping contributed majorly to the field of genome mapping. The idea of overlapping genes came from the bacteriophage gene mapping. Separate mapping of intragenic and intergenic recombinants is possible.

5. Advantages of Bacteriophages:

The bacteriophages are ubiquitous, meaning they exist everywhere. They are highly specific in their action. It is easy to study them using bacterial cultures. Hence, they serve as tools for detecting the pathogenic bacteria. For example, bacteria resistance to specific antibiotics get detected using phage infection. Bacteriophages play a role in therapeutics for tuberculosis. It is possible to improve the TB-vaccine using genetic engineering and bacteriophages.

6. Applications of phages in recombinant DNA technology:

·    Cosmids: A cosmid vector consists of a lambda cos site. Apart from being vectors cosmids serve as probes. They play an important role in FISH and chromosome painting, where cosmid probes get involved. Genomic library preparation using cosmids helps in mapping the genes.

·        For the construction of a cosmid library, we require the phage packaging extract. Here comes the direct role of bacteriophages. The phage packaging extract consists of phage proteins for multiplication, phage lysate, empty phage heads, and unattached phage tails.

·    Phasmids: They are the vectors based on the bacteriophages. A molecular biology technique known as phage display involves the role of these special types of vectors.

·        It is used to study protein-protein interactions and DNA-protein interactions. Phage display involves M13 and filamentous phage.

·   Phage Therapy: Bacteriophages act as anti-bacterial agents since they lyse the bacterial cells. The antibiotic discovery and phages as therapeutic agents still require research and clinical trials, though scientists are working on the same.

·  The role of bacteriophages in the food industry: USFDA approved various bacteriophage products. For example, LMP-102 Intralytix treated ready to eat meat and poultry products. FDA approved LISTEX. It employed bacteriophages for killing the Listeria monocytogenes on the cheese.

·   In vitro diagnostics: The MRSA/MSSA blood test employed bacteriophages for detecting the S aureus and antibiotic-resistant cultures.

·   Contribution in sanitation: Bacteriophages help in sanitation and disinfection of contact surfaces. 
References:
[1] Microbial Genetics, Keya Chaudhari
[2] Molecular Genetics of Bacteria, Jeremy W. Dale, Simon F. Park
[3] Genetics, G. Ivor Hickey

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