Bacteriophage Adsorption, Penetration, and Life Cycle
Adsorption and Penetration
Bacteriophages have an adsorption and penetration mode, best known for its injection mechanism. Proteins form the spines of the basal plate, contacting their receptors on the bacterial wall and anchoring to it. The hydrolytic action of some capsid enzymes breaks the wall, which is then pierced by the tubular shaft. The genome is inserted through the shaft into the bacteria’s cytoplasm, leaving the capsid outside the cell.
Plant viruses move through the cell wall via small cracks or insect bites. Animal viruses have more direct access to the cell membrane. Non-enveloped viruses penetrate completely into the cell by endocytosis. Once inside the vesicle, the virus releases hydrolytic enzymes to break the vesicle and become free in the cytoplasm. Enveloped viruses enter by fusing their envelope with the cell membrane, forming an endocytic vesicle attached to an endosome, which fuses with the viral membrane envelope, allowing the virus to escape digestion inside.
Multiplication, Assembly, and Release
Multiplication: This involves replicating the viral genome and transcribing mRNA for protein synthesis.
Assembly: Spontaneous assembly occurs in simple viruses. In the mosaic virus, RNA units are packaged by proteins to form an infectious particle. Complex viruses do not self-assemble. Sometimes, proteins assemble before being degraded and reorganized by enzymes.
Release: Viruses are released by:
- Lysis of the cell (in bacteriophages)
- Exocytosis (non-enveloped viruses)
- Budding (enveloped viruses)
Bacteriophage Life Cycle
The lambda phage is a double-stranded DNA virus that exhibits both lytic and lysogenic cycles. It infects Escherichia coli via the lysogenic cycle until an injury causes the virus to exit its dormant state and initiate the lytic cycle.
Retrovirus Life Cycle
Retroviruses have a unique propagation method. The influenza virus and HIV are retroviruses that can develop tumors and integrate their genome into the cellular genetic material, altering gene expression. In the specific case of HIV, infected cells are lymphocytes (T4), which have CD4 receptors on their membrane. The virus releases its contents (viral RNA) and reverse transcriptase into the cell. The reverse transcriptase copies the viral RNA into DNA, which is integrated into the host DNA. This provirus may remain dormant until it awakens to synthesize viral RNA and viral proteins.
Prion Infection and Propagation
The mechanism proposed by Prusiner is as follows: PrPSc associates with normal protein, couples to it, and causes a conformational change to an abnormal form. This dimer can bind to two normal proteins, forming a tetramer. This produces a domino effect, damaging the cell. The destroyed cell releases prions that infect other cells, spreading encephalopathic infection.
Transmission between individuals is caused by ingesting infected tissues from cattle (eating the remains of diseased animals), organ transplantation, cannibalism, or some religious practices. Bovine Spongiform Encephalopathy (BSE) prions may have passed from cows to humans through ingesting infected beef organs, although this is not certain in most cases, and other hypotheses are not ruled out.