Bacteriophages: Lytic and Lysogenic Cycles

The Nature of Viruses

• Viruses are best described as active or inactive, rather than alive or dead.
• Most viruses cannot multiply independently from a host cell, and are therefore considered non-living.
• Viruses are able to direct certain life processes of cells, making them more than inert and lifeless molecules.
• Viruses are obligate intracellular parasites require invading a specific host cell.
• Parasites use the host cell's genetic and metabolic machinery to create new viruses.
• Viruses can sometimes influence a host's genetic makeup.
• Viruses have shaped the way cells, tissues, bacteria, plants, and animals have evolved. -Between 40% and 80% of the human genome may be remnants of ancient viral infections.
• Viruses are part of the normal microbiome.
• Viruses can infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals.

Unique Properties of Viruses

• Viruses are not cells.
• They are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals.
• Viruses do not independently fulfill the characteristics of life.
• They are inactive macromolecules outside a host cell.
• Viruses are ultramicroscopic in size ranging from 20 nm to 450 nm
• Viruses possess either DNA or RNA.
• They can have double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA.
• Viruses carry molecules on their surface, which determines specificity for attachment to a host cell.
• Viruses multiply by taking control of a host cell's genetic material and regulating the synthesis and assembly of new viruses.
• Viruses lack enzymes for most metabolic processes.
• They also do not have the machinery for synthesizing proteins.

Viral Size Range

• Viruses are ultramicroscopic in size and are smaller than the average bacterium.
• Electron microscopes are needed to detect viruses.

Viral Components

• A virus particle consists of a covering and a central core.
• The covering includes the Capsid and the Envelope
• The central core includes Nucleic acid molecule(s) (DNA or RNA), Matrix proteins, Enzymes
• The capsid is a protein shell that surrounds the nucleic acid.
• A nucleocapsid consists of the capsid and the nucleic acid together.
• Capsids made up of subunits called capsomeres.
• The envelope is a modified piece of the host cell membrane, and is not seen in all viruses.
• Naked viruses consist only of a nucleocapsid.

Spikes and Virion

• Spikes are found on both naked and enveloped viruses.
• Spikes project from either the nucleocapsid or envelope.
• Spikes allow viruses to dock with their host cells.
• A virion is a fully formed virus that is able to establish infection in a host.

Nucleic Acids

• The genome is the full complement of DNA and RNA carried by a cell.
• Viruses only contain either DNA or RNA, but not both.
• The number of viral genes is small compared to that of a cell.
• Viruses possess only the genes necessary to invade host cells and redirect their activity.

Multiplication Cycles in Animal Viruses

• General phases include:
  • Adsorption
  • Penetration
  • Uncoating
  • Synthesis
  • Assembly
  • Release from the host cell

Adsorption

• The invasion begins when the virus encounters a susceptible host.
• It adsorbs (attaches) specifically to receptor sites on the cell membrane.
• Host range: A virus can attach to and invade its host cell only through an exact fit with a specific host molecule, or receptor and through specificity.
  • Restricted host range: Hepatitis B only infects liver cells of humans.
  • Intermediate host range: Poliovirus infects intestinal and nerve cells of primates.
  • Broad host range: Rabies virus infects various cells of all mammals.

Penetration and Uncoating of Animal Viruses

• Endocytosis (a): The entire virus is engulfed by the cell and enclosed in a vacuole or vesicle.
• Fusion (b): The outer membrane of the virus fuses with the cell membrane of the host.
• Uncoating: Enzymes in the vacuole dissolve the envelope and capsid, releasing the virus into the cytoplasm.

Replication of DNA, Synthesis of Proteins and Assembly

• Viral nucleic acid takes control over the host's synthetic and metabolic machinery to synthesize nucleic acid and proteins.
• The mechanism varies depending on whether the virus is a DNA or RNA virus.
  • RNA viruses replicate in the cytoplasm.
  • DNA viruses replicate in the nucleus.
• Materials that are produced are then assembled.

Release of Mature Viruses

• Viral budding or exocytosis is the mechanism of release for enveloped viruses.
  • Nucleocapsid binds to the membrane.
  • A small pouch is formed.
  • The pouch releases the virus with its envelope.
  • Viruses are shed gradually.
• For naked viruses, the mechanism of Viral release is by cell lysis.

Damage to the Host Cell: Cytopathic Effects (CPEs)

• Virus-induced damage to the cell alters its microscopic appearance.
• Cells can become:
  • disoriented
  • undergo major changes in shape or size
  • develop intracellular damage
• Inclusion bodies are compacted masses of viruses, or damaged cell organelles, in the nucleus or cytoplasm.
• Syncytia (singular, syncytium) is the fusion of multiple host cells into single large cells containing multiple nuclei.
• Persistent Infections: A carrier relationship develops in some cells.
  • The cell harbors the virus, and is not immediately lysed.
• Provirus: Viral DNA gets incorporated into the DNA of the host.
• Chronic latent state: Periodic activation occurs after a period of viral inactivity.

Viruses and Cancer

• Oncogenic viruses: Experts estimate that up to 20% of human cancers are caused by viruses and bacteria.
• Transformation:
  • Virus carries genes that directly cause cancer.
  • Virus produces proteins that induce a loss of growth regulation in the cell.

Viruses that Infect Bacteria

• Bacteriophage was discovered in 1915 by Frederick Twort and Felix d'Herelle.
• This virus parasitizes every known bacterial species.
• This virus often makes the bacteria they infect more pathogenic for humans.
• “T-even” bacteriophages infect E. coli.
• It is the most widely studied bacteriophage.
• It goes through similar stages as animal viruses:
• Attach, penetrate, synthesis, assembly, release.
• However, the difference between animal and bacterial viruses is: The whole bacteriophage does not eat the cell and instead, the nucleic acid is injected.

Bacteriophage Life Cycle

• there are 2 choices:
  • Lytic phase/cycle: Life cycle of bacteriophage that ends in destruction of the bacterial cell.
  • Lysogenic cycle: Bacteriophage becomes incorporated into the host cell DNA.

Temperate Phages

• viruses, have the ability to undergo adsorption and penetration.
  • they do not immediately undergo replication or release.
• Prophage: Phage DNA in the inactive state.
• Induction: Activation of a prophage in a lysogenic cell to undergo the lytic cycle.

Lysogeny

• Phage genes occasionally cause the production of toxins or enzymes that cause pathology in the human.
• Lysogenic conversion: The acquisition of a new trait from a temperate phage.
  • Responsible for the diphtheria toxin, cholera toxin, and botulism toxin.

Techniques in Cultivating and Identifying Animal Viruses

• In vivo methods: Viral cultivation in lab animals (mice, rats, hamsters) or embryonic bird tissues.
• In vitro methods: Viral cultivation in cell or tissue culture.
• Primary Purposes of Viral Cultivation
  • Isolate and identify viruses in clinical specimens.
  • Prepare viruses for vaccines.
  • Do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells.

Viruses and Human Health

• Most common cause of acute infections that do not result in hospitalization: colds, chickenpox, influenza, herpes, warts.
• Viral infections that only occur in certain regions: Dengue fever, Rift Valley fever, Yellow fever.
• Some have high mortality rates: Rabies, Ebola.
• Other viral infections lead to long-term disability: Polio, neonatal rubella.
• Viruses mutate at a rapid rate. It is difficult to design therapies against viruses.
• Interferon (IFN): A naturally occurring human cell product used with some success in preventing and treating viral infections.

Explore the lytic and lysogenic cycles of bacteriophages, including prophage activation and the impact of lysogeny on bacterial pathogenicity. Understand how bacteriophages infect bacteria, focusing on nucleic acid introduction and host cell invasion. Investigate the unique aspects of bacteriophage replication compared to animal viruses.