Microbiology - Chapter 13 Viruses, Viroids, and Prions

Virus

Non-cellular entities which consist mainly of protein and nucleic acid (DNA or RNA) and which can replicate only after entry into specific living cells

Host range

The spectrum of host cells that each type of virus can infect. it is determined by specific host attachment sites and cellular factors

Virion

a complete viral particle

Phage

virus that infects a bacterial cell

General Characteristics of Viruses

Obligatory intracellular parasites

Contain DNA or RNA

No ribosomes

No ATP-generating mechanism

Contain a protein coat

No metabolism and, few or no enzymes of their own.

No intrinsic motility.

Cannot grow on artificial lab media.

Do not respond to physical stimuli in their environment

Some viruses are enclosed by an envelope

Some viruses have spikes

Most viruses infect only specific types of cells in one host

Size of a virus

Viruses range from 20 to 450 nm in diameter and 20 to 14,000 nm in length.

Most viruses are quite smaller than bacteria and can only be visualized by electron microscope.

Some large viruses (such as vaccinia virus) are about the same size as some small bacteria.

The smallest virus (Polio Virus) is about 20 nm in diameter.

General Structural Properties of Viruses

Polyhedral (many sided) Viruses

Helical Viruses

Enveloped Viruses

Complex Viruses

Bacterial viruses

Polyhedral (many sided) Viruses

Many animal, plant, and bacterial viruses are icosahedron (with 20 faces and 12 corners).

Helical Viruses

Resemble long rods that may be rigid or flexible.

Enveloped Viruses

Have a roughly spherical but somewhat variable shape even though their nucleocapsid can either be icosahedral or helical

Complex Viruses

Many bacterial viruses have capsid symmetry that is neither purely icosahedral nor helical. They may possess tails and other structures or have complex, multilayered walls surrounding the nucleic acid

Virion Structure

Virion is an infectious viral particle

Nucleic acid

DNA or RNA Never both

Circular or linear DNA

Or single or double stranded RNA

Capsid

Capsomeres

Envelope

Spikes

DNA viruses

Papillomavirus

Adenovirus

RNA Viruses

Influenza virus

Paramyxoviruses -Mumps

Capsid

(nucleocapsid) is composed of protein subunits called Capsomeres

envelope

consists of combination of lipids, proteins, and CHO, an outer membranous layer surrounding the nucleoapsid

naked virus or nonenveoped virus

virus without envelope

Capsomeres

protein subunits that make capsids

Spikes

A receptor on the surface of certain enveloped viruses that facilitates specific attachment to the host cell

Are CHO and protein complex

Can be used as means of identification.

Cause clumping of RBC called Hemagglutination

Plaques

a visible structure formed within a cell culture, such as bacterial cultures within some nutrient medium (e.g. agar). The bacteriophage viruses replicate and spread, thus generating regions of cell destructions

Oncogenes

A gene that when activated can transform normal cells into cancerous cells

Agents that can activate oncogenes

high-energy radiation

mutagenic chemicals

some viruses

Protooncogenes

Normal cellular genes that regulate cell proliferation and differentiation that can become oncogenes.

Growing Viruses

Viruses must be grown in living cells

Bacteriophages form plaques on a lawn of bacteria

Animal viruses may be grown in living animals or in embryonated eggs or in cell cultures (continuous cell lines)

multiplication of viruses

lytic cycle

lysogenic cycle

Lytic cycle

lysis and death of host cell

Lysogenic cycle

Host cell does not die

Viral DNA incorporates into host cell DNA

Virus remains latent

6 steps - lytic cycle

Attachment

Penetration

Uncoating

Biosynthesis

Maturation

Release

Attachment

phage attaches by tail fibers to host cell

Animal viruses attach with spikes or specific sites

Penetration

Phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell

Animal virus enters by endocytosis or fusion of cell membrane

Uncoating

Removal of viral coat in animal viruses by host or viral enzymes

Biosynthesis:

production of phage or viral DNA and proteins

Maturation

assembly of viral particles

Release

Phage lysozyme breaks cell wall

Animal virus by budding (enveloped viruses) or rupture

Release of Bacteriophages

Eclipse period

Burst size

Burst time

Eclipse period

from the time of attachment to biosynthesis

No complete viruses or no infection particles are detectable in the host cell

Burst size

Number of mature viruses released from a single host cell (250 - 300)

Burst time

the time between attachment and release (20 - 30 minutes).

Lysogenic Cycle of Bacteriophages

DNA incorporated as a Prophage into the DNA of the host cell

Lysogenic cells are immune to reinfection by the same type of phage

Lysogenic cells may exhibit new properties

resistance to antibiotics

production of toxins

specialized transduction

Virus species

Adenovirus

Papillomavirus

Paramyxoviruses

Influenza virus

Bacteriophages

herpesvirus

Adenovirus

Oncogenic DNA viruses

double stranded DNA nonenveloped

Causes various respiratory infections in humans tumors in animals

Papillomavirus

Oncogenic DNA viruses

double stranded DNA nonenveloped

human wart virus

Causes warts and cervical cancer

Paramyxoviruses

- strand, one strand of RNA

Causes parainfluenza, mumps, and newcastle disease in chickens

Influenza virus

- strand, multi strands of RNA

Envelope spikes canagglutinate red blood cells

Causes the flu

Bacteriophages

viruses that infect bacteria

herpesvirus

A virus that causes infections in humans, such as herpes, cytomegalovirus,chicken pox, shingles, mononucleosis, measles, and kaposi's sarcoma

Prions

tiny pieces of protein (infecious protein)

Disease by prions are caused by

Inherited and transmissible by ingestion, transplant, and surgical instruments

Diseases caused by prions

Spongiform encephalopathies: sheep scrapie

Creutzfeldt-Jakob disease

Gerstmann-Sträussler-Scheinker syndrome

Fatal familial insomnia

Mad cow disease

- there is no treatment