What is a Virus: Lytic Bacteriophage Replication

What is a Virus

• Viruses are not cells.
• They lack a cell membrane, nucleus, organelles, and cytoplasm.
• Viruses are obligate intracellular parasites and can replicate only within a living cell.
• The viral structure contains either DNA or RNA enclosed by a protein capsid.
• Some viruses are enveloped, while others are naked.
• Viruses are too small to be seen using a light microscope, ranging in size from 20-30 nm.
• Their ability to infect specific cells makes them very specific.

Replication Steps of a Lytic Bacteriophage

• Adsorption involves the phage latching onto a bacterial cell wall.
• Penetration includes the phage entering the bacterial cell wall and cell membrane, leading to the injection of phage DNA.
• Bacterial DNA is then disrupted.
• Synthesis involves the virus making new copies to be released.
• Phage DNA directs the host cell's metabolism to produce viral components, including proteins and copies of phage DNA.
• The cell synthesizes empty phage heads and pieces of phage DNA.
• Maturation is when collars, sheaths, and base plates attach to the heads, with tail fibers added last.
• Release involves the bacterial cell lysing, which results in the mature phages being released.

Bacteriophage as Drugs

• Bacteriophages can kill bacteria.
• They have been used to treat patients with antibiotic-resistant infections.
• Bacteriophages are a common treatment in Russia.
• Advantages of bacteriophages:
  • They are cheap.
  • Have few side effects, though still need more testing.
  • They are very specific to phages.

The Lysogenic Cycle

• Not all viruses kill their host cells.
• Lysogeny occurs when a virus does not lyse the cell and becomes part of the host cell genome.
• The virus is called a prophage or provirus.
• Proviruses copy themselves instead of killing their host.

Adsorption/Recognition in Animal Viruses

• Viruses recognize proteins on the membrane of the rhinovirus capsid.
• This recognition is very successful with HIV.
• No specific drug exists for rhinovirus.
• It targets an area for antiviral agents.

Penetration of an Animal Cell

• Viruses sometimes enter cells or are allowed in.
• Some can go all the way into the nucleus to be copied.
• Every virus is different, causing problems with the treatment of affections.
• Penetration can occur through endocytosis.

Maturation of Animal Viruses

• "Growth" of a virus includes viral replication.
• The viral replication depends on the type of virus.
• This occurs either in the nucleus or the cytoplasm of host cells, where it uses the host cell's machinery.

Release of New Virus

• Release may occur through budding, also known as "shedding."
• This may or may not kill the host cell.
• The process may lyse the host cell, which can cause clinical symptoms
  • Including the destruction of skin cells, such as herpes and pox
  • Destruction of nerve cells, such as polio

Why We Care About Viruses

• Understanding all stages of a viral life cycle helps identify targets for antiviral agents.

How We Study Viruses

• Animal viruses are cultured.
  • Culturing used to require growing viruses in whole animals.
  • A chicken embryo (egg) is now used for influenza.
  • Cell culture can also be used, in which cells are grown in the lab.

Viruses and Teratogenesis

• Teratogens are agents that cause defects during embryonic developments.
• Damage is more extensive during earlier developments.
• Many viruses are teratogens:
  • Cytomegalovirus (CMV)
  • Rubella (German measles)
  • Chickenpox
  • Erythrovirus (Fifth disease)
  • Herpes
  • Zika

Viruses and Cancer

• Cancer is the uncontrolled growth of cells but is not one disease.
• About 15% of cancers are strongly linked to viruses.

Virus-like Agents

• This includes viroids and prions.

Viroids

• Viroids consist of a single strand of RNA only, with 300-400 nucleotides.
• They are smaller than viruses, contain no protein, and are infectious agents in plants that can cause disease.
• They have not been found in animals.

Prions - Infectious Protein Particles

• Prions are only protein but can be passed among animals.
• They do not contain nucleic acid and are a mutant form of a normal brain protein.
• The mechanisms of how they cause disease is unknown.
• Prions cause a stack of protein to build up in the cell, eventually killing it and build holes.
• This mostly occurs in brain cells because they do not reproduce.
• Diseases associated with prions make the brain look like a sponge.
• Prion-related diseases:
  • Mad Cow disease (Bovine spongiform encephalopathy)
  • Creutzfeldt-Jakob disease (Humans)
  • Scrapie (Sheep)
  • Chronic wasting disease (Elk, deer)
  • Kuru (Cannibals)

Other Microbes

• Protozoa are single-celled eukaryotes.
• Fungi can be single-celled eukaryotes.
• Helminths may be parasitic.

Symbiosis

• Parasitism is when one organism lives at the expense of another (the host).
• Commensalism is living in or on another organism without harming it.
• Mutualism is when both organisms benefit: E. coli lives in our gut and produces vitamins.
• Ecto- lives on the outside.
• Endo- lives on the inside.
• Obligate organisms can only live in one particular way.
• Facultative organisms can act as a parasite or not, and can tolerate either condition.
• A vector-transmitter transmits something.
  • A biological vector will develop a parasite.
  • A mechanical vector is only a carrier.

Why Study Parasitology

• About 15 million people die each year from parasitic infections.
• These infections are often seen in places with poverty.
• More than one million people die from malaria alone (most are children).
• There are 350-500 million infections.
• About 3 billion people worldwide, including in the US, harbor parasitic worms.
• 30 million are severely ill, and 50% are children.

Protozoa

• Protozoa are unicellular eukaryotes.
• They are free-living, commensals, or parasites.
• Some are motile, but Plasmodium (malaria) is not.
• Parasitic protozoa are medically important.

Fungi

• Fungi are eukaryotes.
• Most are saprophytes, which digest dead organic matter.
• When infecting a cell, they kill it first and then digest it.
• Some are facultative parasites.
• Some are beneficial as decomposers and provide nutrients.
• Some produce antibiotics.
• The study of fungi is mycology.

Worms

• Helminthiasis is the most widespread parasitic infection.
• 1/3 of the world's population- including those in the U.S., are affected.
• Many people have multiple infestations.

Helminths (Worms)

• Helminths are animal-multicellular eukaryotes.
• They can be free-living or parasites, parasitizing humans.
• Flatworms include tapeworms.
• Roundworms.
• Animals can get infected by some worms by eating microscopic cysts.
• Sometimes, an insect can transmit worms.