Question

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Be familiar with the 3 mechanisms for horizontal transfer of genetic material in bacteria. 1. Note general similarities and differences between the mechanisms. 2. What are the 2 lifecycles of bacteriophage lambda? What is occurring in each stage? 3. What are the different outcomes of F+ mating? Hfr?

Answer 1

Horizontal gene transfer, also known as lateral gene transfer, is a process in which an organism transfers genetic material to another organism that is not its offspring. The ability of Bacteria and Archaea to adapt to new environments as a part of bacterial evolution most frequently results from the acquisition of new genes through horizontal gene transfer rather than by the alteration of gene functions through mutations.

There are three mechanisms of horizontal gene transfer in bacteria: transformation, transduction, and conjugation.

	TRANSFORMATION	TRANSDUCTION	CONJUGATION
DEFINATION	Transformation is a form of genetic recombination in which a DNA fragment from a dead, degraded bacterium enters a competent recipient bacterium and is exchanged for a piece of DNA of the recipient.	Transduction involves the transfer of a DNA fragment from one bacterium to another by a bacteriophage.	Genetic recombination in which there is a transfer of DNA from a living donor bacterium to a living recipient bacterium by cell-to-cell contact.
EXAMPLES	Neisseria gonorrhoeae, Neisseria meningitidis, Hemophilus influenzae, Legionella pneomophila, Streptococcus pneumoniae, and Helicobacter pylori	Staphylococcus, Escherichia, Salmonella, and Pseudomonas.	E. coli, Proteus, Klebsiella, Enterobacter, Serratia, Pseudomonas,Salmonella and Shigella

Bacteriophage lambda: It is a bacterial virus that infects bacterial species Escherichia coli (E. coli).

Bacteriophages usually has 2 lifecycles: Lytic cycle and lysogenic cycle.

The Lytic Cycle

During the lytic cycle of virulent phage, the bacteriophage takes over the cell, reproduces new phages, and destroys the cell. T-even phage is a good example of a well-characterized class of virulent phages. There are five stages in the bacteriophage lytic cycle. Attachment is the first stage in the infection process in which the phage interacts with specific bacterial surface receptors (e.g., lipopolysaccharides and OmpC protein on host surfaces). Most phages have a narrow host range and may infect one species of bacteria or one strain within a species. This unique recognition can be exploited for targeted treatment of bacterial infection by phage therapy or for phage typing to identify unique bacterial subspecies or strains. The second stage of infection is entry or penetration. This occurs through contraction of the tail sheath, which acts like a hypodermic needle to inject the viral genome through the cell wall and membrane. The phage head and remaining components remain outside the bacteria.

bacteriophage host bacterial cell phage DNA host DNA capsid bacteriophage IN Attachment The phage attaches to the surface of the host. Penetration The viral DNA enters the host cell. Biosynthesis Phage DNA replicates and phage proteins are made. Maturation New phage particles are assembled. (5) Lysis The cell lyses, releasing the newly made phages. Figure 6.2.1: A virulent phage shows only the lytic cycle pictured here. In the lytic cycle, the phage replicates and lyses the host cell.

The Lysogenic Cycle

In a lysogenic cycle, the phage genome also enters the cell through attachment and penetration. A prime example of a phage with this type of life cycle is the lambda phage. During the lysogenic cycle, instead of killing the host, the phage genome integrates into the bacterial chromosome and becomes part of the host. The integrated phage genome is called a prophage. A bacterial host with a prophage is called a lysogen. The process in which a bacterium is infected by a temperate phage is called lysogeny. It is typical of temperate phages to be latent or inactive within the cell. As the bacterium replicates its chromosome, it also replicates the phage’s DNA and passes it on to new daughter cells during reproduction. The presence of the phage may alter the phenotype of the bacterium, since it can bring in extra genes (e.g., toxin genes that can increase bacterial virulence). This change in the host phenotype is called lysogenic conversion or phage conversion. Some bacteria, such as Vibrio cholerae and Clostridium botulinum, are less virulent in the absence of the prophage. The phages infecting these bacteria carry the toxin genes in their genome and enhance the virulence of the host when the toxin genes are expressed. In the case of V. cholera, phage encoded toxin can cause severe diarrhea; in C. botulinum, the toxin can cause paralysis. During lysogeny, the prophage will persist in the host chromosome until induction, which results in the excision of the viral genome from the host chromosome. After induction has occurred the temperate phage can proceed through a lytic cycle and then undergo lysogeny in a newly infected cell .

The phage infects a cell. The phage DNA becomes incorporated into the host genome. The cell divides, and prophage DNA is passed on to daughter cells. Under stressful conditions, the prophage DNA is excised from the bacterial chromosome and enters the lytic cycle.

The cell lyses, releasing New phage particles Phage DNA replicates and the newly made phages. are assembled. phage proteins are made. Figure 6.2.2: A temperate bacteriophage has both lytic and lysogenic cycles. In the lysogenic cycle, phage DNA is incorporated into the host genome, forming a prophage, which is passed on to subsequent generations of cells. Environmental stressors such as starvation or exposure to toxic chemicals may cause the prophage to be excised and enter the lytic cycle.

Hfr conjugation

Hfr conjugation begins when an F+ plasmid with tra genes coding for mating pair formation inserts or integrates into the chromosome to form an Hfr bacterium. (A plasmid that is able to integrate into the host nucleoid is called an episome.) A nuclease then breaks one strand of the donor's DNA at the origin of transfer (oriT) location of the inserted F+ plasmid and the nicked strand of the donor DNA begins to enter the recipient bacterium. The remaining non-nicked DNA strand remains in the donor and makes a complementary copy of itself.

When an F+ cell conjugates/mates with an F− cell, the result is two F+ cells, both capable of transmitting the plasmid to other F− cells by conjugation. In the case of Hfr transfer, the resulting transconjugates are rarely Hfr. The result of Hfr/F− conjugation is a F− strain with a new genotype. When F-prime plasmids are transferred to a recipient bacterial cell, they carry pieces of the donor's DNA that can become important in recombination.