Bacterial Transduction- Definition, Principle, Steps, Examples

Transduction is a process of genetic recombination in bacteria in which genes from a donor cell are incorporated into the genome of a recipient cell via a virus particle. The virus particle that infects bacteria is called a bacteriophage or phage, and the phages used for the transfer of DNA are called transducing phages. Not all phages are transducing phages.

Transduction is one of the three mechanisms of horizontal gene transfer in bacteria, along with transformation and conjugation. Horizontal gene transfer allows bacteria to acquire new genetic traits from other bacteria or the environment, which can be beneficial for survival. For example, transduction can transfer antibiotic resistance genes or virulence factors between bacteria.

Transduction has an advantage over conjugation in that it does not require physical contact between the donor and recipient cells, and it is resistant to DNase enzymes that degrade free DNA in the environment. Transduction also has an advantage over transformation in that it can transfer larger fragments of DNA, up to hundreds of kilobases.

Transduction is also a useful tool for molecular biologists to introduce foreign genes into bacterial cells or study gene function and regulation. Transduction can be used for mapping genes, creating mutants, transferring plasmids and transposons, and isolating genes of interest.

The principle of bacterial transduction is based on the mechanism of infection of the bacteriophage. A bacteriophage is a virus that infects bacteria and uses their cellular machinery to replicate its own DNA and produce new virus particles. Depending on the type of bacteriophage, it can either enter a lytic cycle or a lysogenic cycle in the bacterial host cell.

In the lytic cycle, the bacteriophage injects its DNA into the bacterial cytoplasm and takes over the bacterial metabolism to produce more phage DNA and proteins. The phage DNA and proteins assemble into new virus particles, which then lyse the bacterial cell and release hundreds of progeny phages. During this process, some of the bacterial DNA may be degraded into fragments and accidentally packaged into some of the phage capsids instead of the phage DNA. These phages are called generalized transducing phages, and they can transfer any part of the bacterial genome to another bacterium.

In the lysogenic cycle, the bacteriophage integrates its DNA into the bacterial chromosome and becomes a prophage. The prophage remains dormant and replicates along with the bacterial DNA during cell division. The prophage can be induced by environmental factors such as UV light or chemicals to exit the bacterial chromosome and enter the lytic cycle. During this process, some of the adjacent bacterial genes may be excised along with the prophage DNA due to imprecise recombination. These phages are called specialized transducing phages, and they can transfer specific parts of the bacterial genome to another bacterium.

In both cases, the transducing phages can infect a new bacterial host cell and inject their DNA containing the donor bacterial genes into the cytoplasm. The donor DNA can then recombine with the recipient bacterial DNA through homologous recombination or site-specific recombination, depending on the type of transduction. This results in the transfer of genetic information from one bacterium to another, which can alter their phenotypic traits or confer new abilities such as antibiotic resistance or toxin production.

Depending on how the bacterial DNA is incorporated into the phage particle, there are two types of transduction: generalized and specialized.

Generalized Transduction

Generalized transduction occurs when a phage mistakenly packages a fragment of bacterial DNA instead of its own DNA during the assembly of new phage particles. This results in a defective phage that contains only bacterial DNA and no phage DNA. The defective phage can still infect another bacterium, but it cannot replicate or produce more phages. Instead, it injects the bacterial DNA into the new host cell, where it may recombine with the homologous DNA of the recipient bacterium. This process can transfer any part of the bacterial genome from one cell to another, as long as it fits into the phage head.

Specialized Transduction

Specialized transduction occurs when a phage that has integrated its DNA into the bacterial chromosome (a prophage) excises itself from the host genome and carries along a piece of adjacent bacterial DNA. This results in a hybrid phage that contains both phage and bacterial genes. The hybrid phage can infect another bacterium and inject both types of DNA into the new host cell. The phage DNA may integrate into the recipient genome at a specific site, bringing along the bacterial genes as well. This process can transfer only specific parts of the bacterial genome that are adjacent to the prophage integration site.

Generalized transduction is a type of bacterial transduction in which any part of the bacterial genome can be transferred from a donor to a recipient bacterium via a phage particle. The phage particle that carries out generalized transduction is called a generalized transducing phage.

Generalized transduction occurs when the phage makes a mistake during the assembly of new virus particles in the lytic cycle. Instead of packaging its own DNA into the phage head, the phage accidentally packages a fragment of the bacterial DNA. This results in a defective phage that contains bacterial DNA but no phage DNA. The defective phage can still infect another bacterium, but it cannot replicate or produce more phages. However, it can inject the bacterial DNA into the new host cell, where it can recombine with the recipient`s chromosome by homologous recombination. This process requires a host enzyme called recA.

Generalized transduction can transfer any gene from the donor to the recipient, as long as the size of the gene fits into the phage head. The frequency of generalized transduction depends on the efficiency of packaging and recombination. Generalized transduction can be used for gene mapping, mutagenesis, plasmid transfer, and gene cloning.

Specialized transduction is a type of transduction where the phage transfers only specific regions of the bacterial genome that are adjacent to the phage integration site. This occurs when the phage DNA is excised from the bacterial chromosome in an imprecise manner, resulting in the inclusion of some bacterial genes along with the phage genes. The phage particles that contain both phage and bacterial DNA are called specialized transducing phages.

Specialized transduction can only occur with temperate phages that undergo lysogeny, which is the integration of the phage DNA into the bacterial chromosome. The phage DNA remains dormant in the bacterial genome until it is induced to enter the lytic cycle, which is the production of new phage particles and lysis of the bacterial cell.

During the induction of the lytic cycle, the phage DNA is excised from the bacterial chromosome by a site-specific recombination enzyme called integrase. However, sometimes the integrase makes a mistake and cuts at a wrong site, resulting in the removal of some bacterial genes along with the phage DNA. These bacterial genes are usually located near the attachment site of the phage DNA, which is where the phage DNA integrates into the bacterial chromosome.

The excised DNA is then packaged into a phage capsid and released from the bacterial cell. The specialized transducing phage can then infect another bacterium and inject its DNA into the cytoplasm. The injected DNA can either integrate into the bacterial chromosome at a specific site or replicate as a plasmid in the cytoplasm. In either case, the recipient bacterium acquires new genes from the donor bacterium and expresses them.

Specialized transduction can be used to transfer genes that are normally not transferred by generalized transduction, such as genes that are located near the origin of replication or genes that are essential for bacterial survival. Specialized transduction can also transfer genes that are involved in bacterial pathogenesis, such as toxin genes or antibiotic resistance genes.

Generalized transduction can be summarized in the following steps:

1. A lytic phage infects a donor bacterium and replicates its DNA and proteins inside the bacterial cell.
2. The phage DNA and the bacterial DNA are both fragmented by endonucleases into smaller pieces.
3. The phage assembles new viral particles using its own proteins and randomly packaged DNA fragments. Some of these particles contain bacterial DNA instead of phage DNA. These are called transducing particles.
4. The phage lyses the donor bacterium and releases the viral particles, including the transducing ones, into the environment.
5. A transducing particle attaches to a recipient bacterium and injects the bacterial DNA from the donor cell into the cytoplasm of the recipient cell.
6. The donor DNA may recombine with the homologous region of the recipient chromosome, replacing the original sequence. This requires the host recombinase enzyme recA and results in a stable transductant with a new genetic trait.
7. Alternatively, the donor DNA may remain as an extrachromosomal element in the recipient cell and be degraded or lost during cell division.

• After the infection of the donor bacterium with the bacteriophage, the phage DNA is integrated into the bacterial chromosome during the lysogenic cycle. This creates a prophage, which is a dormant form of the phage that can be passed on to the bacterial progeny.
• Due to various factors, such as UV radiation or chemical mutagens, the prophage can be induced to enter the lytic cycle. During this process, the phage DNA is excised from the bacterial chromosome by a phage enzyme called integrase.
• Due to the imprecise cutting of the phage DNA, some part of the bacterial chromosome adjacent to the phage integration site is also excised. This results in a hybrid phage DNA that contains both phage and bacterial genes. The hybrid phage DNA is then packaged into a viral capsid and released from the bacterium by lysis.
• The hybrid phage containing some part of the bacterial chromosome then infects a new host (recipient bacterium), and injects its DNA into the cytoplasm. The hybrid phage DNA can either integrate into the recipient chromosome at a specific site (if it is a temperate phage) or replicate as a plasmid (if it is a virulent phage).
• The recipient bacterium then expresses the newly acquired genetic trait from the donor bacterium, along with the phage genes. This can result in phenotypic changes, such as antibiotic resistance, toxin production, or metabolic capabilities.

Bacterial transduction has been observed in many different bacterial species and phages. Here are some examples of transduction and its applications in bacterial genetics and biotechnology:

• Transduction of antibiotic resistance genes: Some phages can transfer genes that confer resistance to antibiotics from one bacterium to another. For example, phage P22 can transduce the chloramphenicol resistance gene from Salmonella typhimurium to other Salmonella strains. This can have important implications for the spread of antibiotic resistance among pathogenic bacteria.
• Transduction of virulence factors: Some phages can also transfer genes that enhance the virulence or pathogenicity of bacteria. For example, phage lambda can transduce the Shiga toxin gene from Shigella dysenteriae to Escherichia coli, resulting in a strain of E. coli that can cause severe diarrhea and hemorrhagic colitis. Similarly, phage Mu can transduce the cholera toxin gene from Vibrio cholerae to other Vibrio species.
• Transduction of metabolic genes: Some phages can transfer genes that enable bacteria to utilize new substrates or produce new metabolites. For example, phage P1 can transduce the lactose operon from E. coli to other Gram-negative bacteria, allowing them to ferment lactose. Phage T4 can transduce the tryptophan operon from E. coli to other enteric bacteria, enabling them to synthesize tryptophan.
• Transduction of regulatory genes: Some phages can transfer genes that affect the expression of other genes in bacteria. For example, phage lambda can transduce the cI gene, which encodes the lambda repressor protein that regulates the switch between the lytic and lysogenic cycles of lambda. Phage P1 can transduce the crp gene, which encodes the cAMP receptor protein that regulates the expression of many genes involved in carbon metabolism in E. coli.
• Transduction for gene mapping: Transduction can be used as a tool for mapping the relative positions of genes on the bacterial chromosome by measuring the frequency of co-transduction of different markers. For example, by using phage P1 to transduce E. coli strains with different mutations, it was possible to determine that the order of genes on the E. coli chromosome is thr-leu-pro-his-gal-bio.
• Transduction for gene cloning: Transduction can be used as a method for cloning or isolating specific genes from bacteria by using phages as vectors. For example, by using phage lambda as a vector, it was possible to clone and express the human insulin gene in E. coli.

These are some examples of bacterial transduction and its applications in bacterial genetics and biotechnology. Transduction is a fascinating phenomenon that demonstrates the diversity and adaptability of bacteria and their interactions with viruses.

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