chapter 5 microbiology PowerPoint

The Position of Viruses in the Biological Spectrum

Viruses infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals.

Seawater can contain 10 million viruses per milliliter.

For many years, the cause of viral infections was unknown:

• Louis Pasteur hypothesized that rabies was caused by a
  "living thing" smaller than bacteria.

• He also proposed the term virus, which is Latin for
  "poison"

Discovery of Viruses

Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus.

Loeffler and Frosch discovered an animal virus that causes foot-and-mouth disease in cattle.

Filterable virus:

• These early researchers found that when fluids from host organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious.

• This proved that an infection could be caused by a fluid containing agents smaller than bacteria.

The Viral Debate

Two sides of the debate:

• Since viruses are unable to multiply independently from the host cell, they are not living things and should be called infectious molecules.

• Even though viruses do not exhibit most of the life processes of cells, they can direct them, and thus are certainly more than inert and lifeless molecules.

Viruses are better described as active or inactive rather than alive or dead

The Vital Role of Viruses in Evolution

Infect cells and influence their genetic makeup.

Shape the way cells, tissues, bacteria, plants, and animals have evolved.

8% of the human genome consists of sequences that come from viruses.

10 to 20% of bacterial DNA contains viral sequences.

Obligate intracellular parasites:

• Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses

Properties of Viruses

• Are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals.

• Estimated 1031 virus particles on earth, approximately 10 times the number of bacteria and archaea combined.

• Are ubiquitous in nature and have had major impact on development of biological life.

• Are ultramicroscopic in size, ranging from 20 nm up to 1,000 nm (diameter).

• Are not cells; structure is very compact and economical.

• Do not independently fulfill the characteristics of life.

• Basic structure consists of protein shell (capsid) surrounding nucleic acid core

Properties of Viruses

• Nucleic acid can be either DNA or RNA, but not both.

• Nucleic acids can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA.

• Molecules on virus surfaces give them high specificity for attachment to host cell.

• Multiply by taking control of host cell's genetic material and regulating the synthesis and assembly of new viruses.

• Lack enzymes for most metabolic processes.

• Lack machinery for synthesizing proteins

How Viruses Are Classified and Named

For many years, animal viruses were classified on the basis of their hosts and the diseases they caused. Newer classification systems emphasize the following:

• Hosts and diseases they cause.

• Structure.

• Chemical composition.

• Similarities in genetic makeup.

International Committee on the Taxonomy of Viruses:

• 8 orders and 38 families (another 84 families not yet assigned to any order)

Virus Size Range

1. Smallest infectious agents.

2. Smallest viruses: parvoviruses around 20 nm in diameter.

3. Largest viruses: herpes simplex virus around 150 nm in length.

4. Some cylindrical viruses can be relatively long (800 nm) but are so narrow in diameter (15 nm) that their visibility is limited without an electron microscope

Viral Components

Viruses bear no resemblance to cells and lack any of the protein-synthesizing machinery found in cells.

Viral structure is composed of regular, repeating subunits that give rise to their crystalline appearance.

The structure contains only those parts needed to invade and control a host cell:

• External coating.

• Core containing one or more nucleic acid strains of DNA or RNA.

• Sometimes one or two enzymes

Viral Components

Capsid: protein shell that surrounds the nucleic acid:

• Nucleocapsid: the capsid together with the nucleic acid.

• Naked viruses consist only of a nucleocapsid.

Envelope: external covering of a capsid, usually a modified piece of the host's cell membrane.

Spikes can be found on naked or enveloped viruses:

• Project from the nucleocapsid or the envelope.

• Allow viruses to dock with host cells.

Virion: a fully formed virus that is able to establish an infection in a host cell

Viral Capsid

Capsid:

• Most prominent feature of viruses.

• Constructed from identical protein subunits called capsomeres.

• Capsomeres spontaneously self-assemble into the finished capsid.

Two different types:

• Helical.

• Icosahedral

Viral Envelope

Enveloped viruses:

• Take a bit of the cell membrane when they are released from a host cell.

Enveloped viruses can bud from:

• Cell membrane.

• Nuclear envelope.

• Endoplasmic reticulum.

More flexible than the capsid so enveloped viruses are pleomorphic

Nucleic Acids: At the Core of a Virus

Genome: the sum total of the genetic information carried by an organism.

Viruses contain DNA or RNA, but not both.

The number of viral genes is quite small compared with that of a cell:

• Four genes in hepatitis B virus.

• Hundreds of genes in some herpesviruses.

• Possess only the genes needed to invade host cells and redirect their activity

Variety in Viral Nucleic Acid

DNA viruses: Single-stranded or double-stranded ( linear or circular).

RNA viruses: can be double-stranded, but more often single-stranded:

• Positive-sense RNA: ready for immediate translation.

• Negative-sense RNA: must be converted before translation can occur.

• Segmented: individual genes exist on separate pieces of RNA.

• Retroviruses: carry their own enzymes to create DNA out of their RNA

Other Substances in the Virus Particle

Enzymes for specific operations within their host cell:

• Polymerases that synthesize DNA and RNA.

• Replicases that copy RNA.

• Reverse transcriptase synthesizes DNA from RNA.

The vast majority of viruses completely lack the genes for synthesis of metabolic enzymes.

Some viruses carry away substances from their host cell:

• Arenaviruses pack along host ribosomes.

• Retroviruses borrow the host's tRNA molecules

COVID-19

SARS-CoV-2, like all coronaviruses, is a single-stranded (+) sense RNA virus.

How Viruses Multiply

Viruses are minute parasites that seize control of the synthetic and genetic machinery of cells.

The way this cycle works dictates:

• The way the virus is transmitted.

• What it does to the host.

• Responses of immune defenses.

• Human measures to control viral infection

Multiplication Cycles in Animal Viruses

General phases of the animal viral replication cycle:

• Adsorption.

• Penetration.

• Uncoating.

• Synthesis.

• Assembly.

• Release.

The length of the replication cycle varies from 8 hours in polioviruses to 36 hours in herpesviruses

Adsorption

A virus can invade its host cell only through making an exact fit with a specific host molecule.

Host range: the limited range of cells that a virus can infect:

• Hepatitis B: liver cells of humans.

• Poliovirus: intestinal and nerve cells of primates.

• Rabies: various cells of all mammals.

Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus.

Tropisms: specificities of viruses for certain tissues

Penetration and Uncoating

The flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid.

Penetration through endocytosis happens when an entire virus is engulfed by the cell and enclosed in a vacuole or vesicle.

Direct fusion of the viral envelope with the host cell membrane:

• Envelope merges directly with the cell membrane, liberating the nucleocapsid into the cell's interior.

Synthesis: Replication and Protein Production

DNA viruses:

• Enter the host cell's nucleus and are replicated and assembled there.

RNA viruses:

• Replicated and assembled in the cytoplasm.

Retroviruses turn their RNA genomes into DNA

Life Cycle of dsDNA Viruses

Early phase:

• Viral DNA enters the nucleus, where genes are transcribed into a messenger RNA.

• RNA transcript moves into the cytoplasm to be translated into viral proteins (enzymes) needed to replicate the viral DNA.

• The host cell's DNA polymerase is involved in this phase.

Late phase:

• Parts of the viral genome are transcribed and translated into proteins required to form the capsid and other structures.

• New viral genomes and capsids are assembled.

• Mature viruses are released by budding or cell disintegration

Assembly and Release

Assembly: virus is put together using "parts" manufactured during the synthesis process.

Release: the number of viruses released by infected cells is variable, controlled by:

• Size of the virus.

• Health of the host cell.

Poxvirus-infected cell: 3,000 to 4,000 virions.

Poliovirus-infected cell: 100,000 virions.

Immense potential for rapid viral proliferation.

Life Cycle of Animal Viruses

1. Adsorption.

• The virus encounters a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane.

• The membrane receptors that viruses attach to are usually proteins that the cell requires for its normal function.

• Glycoprotein spikes on the envelope (or on the capsid of naked viruses) bind to the cell membrane receptors.

2. Penetration and Uncoating.

• In this example, the entire virus is engulfed (endocytosed) by the cell and enclosed in a vacuole or vesicle.

• When enzymes in the vacuole dissolve the envelope and capsid, the virus is said to be uncoated, a process that releases the viral nucleic acid into the cytoplasm

Life Cycle of Animal Viruses

3. Synthesis: Replication and Protein Production.

• Viral nucleic acid begins to synthesize the building blocks for new viruses.

• First, the + ssRNA, which is ready to serve as mRNA, starts being translated into viral proteins, especially those useful for further viral replication.

• The + strand is then replicated into -ssRNA becoming the template for the creation of many new + ssRNAs, used as the viral genomes for new viruses.

• Additional + ssRNAs are synthesized and used for late-stage mRNAs.

• Some viruses come equipped with the necessary enzymes for synthesis of viral components; others utilize those of the host.

• Proteins for the capsid, spikes, and viral enzymes are synthesized on the host's ribosomes using its amino acids

Life Cycle of Animal Viruses

4. Assembly.

• Mature virus particles are constructed from the growing pool of parts.

• Capsid is first laid down as an empty shell that will serve as a receptacle for the nucleic acid strand.

• Viral spikes are inserted into the host's cell membrane so they can be picked up as the virus buds off with its envelope.

5. Release.

• Assembled viruses leave their host in one of two ways:

• Nonenveloped and complex viruses that reach maturation in the cell nucleus or cytoplasm are released when the cell lyses or rupture.

• Enveloped viruses are liberated by budding from the membranes of the cytoplasm, nucleus, endoplasmic reticulum, or vesicles.

• During this process, the nucleocapsid binds to the membrane, which curves completely around it and forms a small pouch.

• Pinching off the pouch releases the virus with its envelope

Damage to the Host Cell

Cytopathic effects (CPEs): virus-induced damage to the cell that alters its microscopic appearance.

Types of CPEs include:

• Gross changes in shape and size.

• Development of intracellular changes.

• Inclusion bodies: compacted masses of viruses or damaged cell organelles in the nucleus and cytoplasm.

• Syncytia: fusion of multiple damaged host cells into single large cells containing multiple nuclei (giant cells).

Accumulated damage from a virus infection kills most host cells

Persistent Infections

Some cells maintain a carrier relationship: cell harbors the virus and is not immediately lysed:

• Can last from a few weeks to the remainder of the host's life.

• Can remain latent in the cytoplasm.

Provirus:

• Viral DNA incorporated into the DNA of the host.

• Measles virus.

Chronic latent state:

• Periodically become activated under the influence of various stimuli.

• Herpes simplex and herpes zoster viruses.

Viruses and Cancer

Experts estimate that 13% of cancers are caused by viruses Transformation: the effect of oncogenic, or cancer-causing viruses:

• Some viruses carry genes that directly cause cancer.

• Other viruses produce proteins that induce a loss of growth regulation, leading to cancer

Viruses and Cancer

Transformed cells:

• Increased rate of growth.

• Changes in their chromosomes.

• Changes in cell's surface molecules.

• Capacity to divide indefinitely.

Oncoviruses: mammalian viruses capable of initiating tumors:

• Papillomaviruses.

• Herpesviruses.

• Hepatitis B virus.

• HTLV-I.

Viruses That Infect Bacteria

Bacteriophage: "bacteria eating":

• Most contain double-stranded DNA, but some RNA types exist as well.

• Every bacterial species is parasitized by various specific bacteriophages.

• The bacteria they infect are often more pathogenic for humans

T-Even Bacteriophage

Infect E. coli.

Structure:

• Icosahedral capsid containing DNA.

• Central tube surrounded by a sheath Collar.

• Base plate.

• Tail pins.

• Fibers

Lysogeny: The Silent Virus Infection

Temperate phages:

• Undergo adsorption and penetration.

• Do not undergo replication or release immediately.

Viral DNA enters an inactive prophage state:

• Inserted into bacterial chromosome.

• Copied during normal bacterial cell division.

• Lysogeny: a condition in which the host chromosome carries bacteriophage DNA.

Induction: prophage in a lysogenic cell becomes activated and progresses directly into viral replication and the lytic cycle

The Role of Lysogeny in Human Disease

Occasionally, phage genes in the bacterial chromosome cause the production of toxins or enzymes that the bacterium would not otherwise have.

Lysogenic conversion: when a bacterium acquires a new trait from its temperate phage:

• Corynebacterium diphtheriae - diphtheria toxin.

• Vibrio cholerae - cholera toxin.

• Clostridium botulinum - botulinum toxin.

Techniques in Cultivating and Identifying Animal Viruses

Viruses require living cells as their "medium":

• In vivo: laboratory-bred animals and embryonic bird tissues.

• In vitro: cell or tissue culture methods.

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

Using Live Animal Inoculation

• Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits are the usual choices for viral cultivation.

• Occasionally, invertebrates such as insects or nonhuman primates are used.

• Because viruses exhibit host specificity, certain animals can allow a given virus to grow more readily than others

Using Bird Embryos

Bird eggs containing embryos:

• Intact and self-supporting unit.

• Sterile environment.

• Contain their own nourishment.

Chicken, duck, and turkey eggs are the most common choices for inoculation.

Viruses are injected through the eggshell by drilling a small hole or making a small window

Using Cell (Tissue) Culture Techniques

Isolated animal cells are grown in vitro in cell or tissue culture rather than in an animal or egg.

Cell culture, or tissue culture:

• Grown in sterile chambers with special media that contain the correct nutrients for cells to survive.

• Cells form a monolayer, or single, confluent sheet of cells that supports viral multiplication.

• Allows for the close inspection of culture for signs of infection

Detection of Viral Growth in Culture

Observation of degeneration and lysis of infected cells.

Plaques: areas where virus-infected cells have been destroyed show up as clear, well-defined patches in the cell sheet:

• Visible manifestation of cytopathic effects (CPEs)

Detection of Bacteriophages

This same technique is used to detect and count bacteriophages:

• Plaques develop when the viruses released by an infected host cell radiate out to adjacent host cells.

• New cells become infected, die and release more viruses, and the process continues.

• Plaque manifests as a macroscopic, round, clear space that corresponds to areas of dead cells

Prions

Composed primarily of protein (no nucleic acid).

Exact mode of infection is still being investigated.

Deposited as long protein fibrils in the brain tissue of humans and animals:

• Creutzfeldt-Jakob disease: afflicts the central nervous system and causes degeneration and death.

• Bovine spongiform encephalopathy ("mad cow disease").

• Shy-Drager syndrome or multiple system atrophy resembles Parkinson's disease

Satellite Viruses

Dependent on other viruses for replication.

Adeno-associated virus (AAV):

• Originally thought that it could only replicate in cells infected with the adenovirus.

• Can also infect cells that are infected with other viruses.

Delta agent:

• Naked circle of RNA.

• Expressed only in the presence of the hepatitis B virus.

• Worsens the severity of liver damage

Viroids

• Virus-like agents that parasitize plants.

• About one-tenth the size of an average virus.

• Composed of naked strands of RNA, lacking a capsid or any other type of coating.

• Significant pathogens in economically important plants: tomatoes, potatoes, cucumbers, citrus trees, chrysanthemums

Viruses and Human Health

Common causes of acute infections:

• Colds, hepatitis, chickenpox, influenza, herpes, warts.

Prominent viral infections worldwide:

• Dengue fever, Rift Valley fever, yellow fever.

Infections with high mortality rates:

• Rabies, AIDS, Ebola.

Infections that cause long-term disability:

• Polio, neonatal rubella.

Connection to chronic infections:

• Type 1 diabetes, MS, various cancers, Alzheimer's, obesity

Treatment of Animal Viral Infections

Antibiotics designed to treat bacterial infections have no effect on viruses.

Difficult to find drugs that will affect viruses without damaging host cells.

Almost all antiviral drugs licensed so far have been designed to target one of the steps in the viral life cycle:

• Integrase inhibitor class of HIV drugs interrupts the ability of HIV genetic information to incorporate into the host cell.

DNA, Easier to develop vaccines to prevent viral diseases