NON-CELLULAR MICROBES: Viruses, Viroids, and Prions

Microbiology and ParasitologyLiceo de Cagayan University, College of Arts and SciencesNatural Sciences Department, B.S. Biology

Learning Objectives:• Differentiate a virus from a bacterium• Describe the chemical and physical structure of both an

enveloped and a nonenveloped virus.• Define viral species and give an example of a family, genus,

and common name for a virus.• Describe how bacteriophages are cultured.• Describe how animal viruses are cultured.• List three techniques used to identify viruses.• Compare and contrast the multiplication cycle of DNA- and RNA-

containing animal viruses.• Define oncogene and transformed cell.• Discuss the relationship between DNA- and RNA- containing

viruses and cancer.• Provide an example of a latent viral infection.• Discuss how protein can be infectious.• Differentiate virus, viroid, and prion.

GENERAL CHARACTERISTICS OF VIRUSES

The question of whether viruses are living organism has an ambiguous answer.

Viruses are inert outside living host cells & generally not considered living organisms.

Once viruses enter a host cell, the viral nucleic acids become active and viral multiplication results & become alive.

Obligatory intracellular parasites

Viruses Entitle that:

Contain a single type of nucleic acid, either DNA or RNA.

Contain a protein coat (sometimes itself enclosed by an envelope of lipids, proteins, and carbohydrates) that surrounds the nucleic acid.

Multiply inside living cells by using the synthesizing machinery of the cell.

Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells.

FIGURE: Viruses and Bacteria Compared

Host Range The host range of a virus is the

spectrum of host cells the virus can infect.

Viruses than infect bacteria are called bacteriophages, or phages.

For the virus to infect the host cell, the outer surface of the virus must chemically interact with specific receptor sites on the surface of the cell.

The use of bacteriophage to treat bacterial infections is called phage therapy.

Tumor destroying or oncolytic viruses may selectively infect and kill tumor cells.

Viral Size

Viral sizes are determined with the aid of electron microscopy. Different viruses vary considerably in size.

Although most are quite a bit smaller than bacteria, some of the larger viruses (such as the vaccine virus) are about the same size as some very small bacteria (such as the mycomplasmas, rickettsias, and chlamydias).

Viruses range from 20 to 1000 nm in length.

FIGURE: Virus sizes. The sizes of several viruses (teal blue) and bacteria (pink) are compared with a human red blood cell, shown to the right of the microbes. Dimensions are given in nanometers (nm) and are either diameters or length by width.

Viral Structure

A virion is a complex, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat that protects it from the environment and is a vehicle of transmission from one host cell to another.

Viruses are classified by differences in the structures of these coats.

FIGURE: Morphology of a nonenveloped polyhedral virus. (a) A diagram of a polyhedral (icosahedral) virus. (b) A micrograph of the odenovirus Mastadenovirus. Individual capsomeres in the protein coat are visible.

Nucleic Acid

A virus can have either DNA or RNA, but never both.

The nucleic acid of a virus can be single-stranded or double-stranded. Thus, there are viruses which the familiar double-stranded DNA, with single-stranded RNA.

Depending on the virus, the nucleic acid can be linear or circular. In some viruses (such as the influenza virus), the nucleic acid is in several separate segments.

Capsid and Envelope

The nucleic acid of a virus is surrounded by a protein coat called the capsid.

Each capsid is composed of protein subunits called capsomers.

In some viruses, the capsid is covered by an envelope.

Envelops may or may not be covered by spikes.

Viruses whose capsids are not covered by an envelope are known as nonenveloped viruses.

FIGURE: Morphology of an enveloped helical virus. (a) A diagram of an enveloped helical virus. (b) A micrograph of influenzavirus A2. Notice the halo of spikes projecting from the outer surface of each envelope.

FIGURE: Morphology of a helical virus. (a) A diagram of a portion of a helical virus. Several rows of capsomeres have been removed to reveal the nucleic acid. (b) A micrograph of Ebola virus, a flovirus showing helical rods.

General Morphology

Viruses may be classified into several different morphology types on the basis of their capsid architecture.

The structure of these capsids has been revealed by electron microscopy and a technique called X-ray crystallography.

Helical Viruses

Helical viruses resemble long rods that may be rigid or flexible.

The viral nucleic acid is found within a hollow, cylindrical capsid that has a helical structure.

The viruses that care rabies and Ebola hemorrhagic fever are helical viruses.

Polyhedral Viruses

Many animal, plant, and bacterial viruses is covered by an envelope.

Enveloped viruses are roughly spherical. When helical and polyhedral viruses are enclosed by envelopes, they are called enveloped helical or enveloped polyhedral viruses.

An example of an enveloped helical virus is the Influenzavirus. An example of an enveloped polyhedral (icosahedral) virus is the herpes simplex virus (genus Simplexvirus).

Complex Viruses

Some viruses, particularly bacterial viruses, have complicated structures and are called complex viruses.

One example of a complex virus is a bacteriophage.

Some bacteriophages have capsids to which additional structures are attached.

FIGURE: Morphology of complex viruses. (a) A diagram and micrograph of a T-even bacteriophage (b) A Orthopoxvirus, which causes smallpox.

Taxonomy of Viruses

The oldest classification of viruses is based on symptomatology.

Virologists began addressing the problem or viral taxonomy in 1966 with International Committee on the Taxonomy of Viruses (ICTV).

ICTV has been grouping viruses into families based on:

  1. Nucleic acid type 2. Strategy for replication

3. Morphology

Taxonomy of Viruses

The suffix – virus is used for genus names; family names end in –viridae; and order names end in – ales.

In formal usage, the family and genus names are used in the following manner: Family Herpesviridae, genus Simplexvirus, human herpes virus 2.

Taxonomy of Viruses

A viral species is a group of viruses sharing the same genetic information and ecological niche (host range).

Specific epithets for viruses are not used. Thus, viral species are designated by descriptive common names, such as human immunodeficiency virus (HIV), with subspecies (if any) designated by a number (HIV – 1).

Isolation, Cultivation, and Identification of Viruses

Viruses must be provided with living cells instead of a fairly simple chemical medium.

Viruses that use bacterial cells as a host (bacteriophages) are rather easily grown on bacterial cultures.

FIGURE: Families of viruses that affect humans

FIGURE: Families of viruses that affect humans

Growing Bacteriophages in the Laboratory

Bacteriophages can be grown either in suspension of bacteria in liquid media or in bacterial cultures on solid media.

Solid media makes possible the plaque method for detecting and counting viruses.

A sample of bacteriophage is mixed with host bacteria and melted agar.

The agar containing the bacteriophages and host bacteria is then poured into a Petri plate containing a hardened layer of agar growth medium.

Growing Bacteriophages in the Laboratory

The virus-bacteria mixture solidifies into a thin top layer that contains a layer of bacteria approximately one cell thick.

Each virus infects a bacterium, multiplies, and releases several hundred new viruses.

The newly produced viruses infect other bacteria in the immediate vicinity, and more new viruses are produced.

This produces a number of clearings, or plaques, visible against a lawn of bacterial growth on the surface of the agar.

Growing Bacteriophages in the Laboratory

The virus-bacteria mixture solidifies into a thin top layer that contains a layer of bacteria approximately one cell thick.

Each virus infects a bacterium, multiplies, and releases several hundred new viruses.

The newly produced viruses infect other bacteria in the immediate vicinity, and more new viruses are produced.

This produces a number of clearings, or plaques, visible against a lawn of bacterial growth on the surface of the agar.

In Living Animals

Animal inoculation may be used as a diagnostic procedure for identifying and isolating a virus from a clinical specimen.

After the animal is inoculated with the specimen, the animal is observed for signs of disease, or is killed so that infected tissues can be examined for the virus.

In Embryonated Eggs

Growing viruses in an embryonated egg can be a fairly convenient and inexpensive form of host for many animal viruses.

A hole is drilled in the shell of embryonated egg, and a viral suspension or suspected virus-containing tissue is injected into the fluid of the egg.

There are several membranes in an egg, and the virus is injected near the one most appropriate for its growth.

In Embryonated Eggs

Viral growth is signaled by the death of the embryo, by embryo cell damage, or by the formation of typical pocks or lesions on the egg membranes.

This method was once the most widely used method of viral isolation and growth, and it is still used to grow viruses for some vaccines.

FIGURE: Inoculation of an embryonated egg. The injection site determines the membrane on which the viruses will grow.

The Cell Cultures

Cell cultures have replaced embryonated eggs as the preferred type of growth medium for many viruses.

This cell deterioration is called cytopathic effect (CPE), can be detected and counted in much the same way as plaques caused by bacteriophages on a lawn of bacteria.

Primary cell lines, derived from tissue slices, tend to die out after only a few generations.

The Cell Cultures

Contain cell lines, called diploid cell lines, developed from human embryos can be maintained for about 100 generations and are widely used for culturing viruses that require a human host.

When viruses are routinely grown in the laboratory, continuous cell lines are used. These are transformed (cancerous) cells that can be maintained through an indefinite number of generations, and they are sometimes called mortal cell lines.

FIGURE: Cell cultures. Transformed cells can be grown indefinitely in laboratory culture.

Viral Identification

Viruses cannot be seen without the use of an electron microscope.

Serological methods, such as Western Blotting, are the most commonly used means of identification. In these tests, the virus is detected and identified by its reaction with antibodies.

Virologists can identify and characterize viruses by using such modern methods as restriction fragment length polymorphisms (RFLPs) and the polymerase chain reaction (PCR).

Viral Multiplication

The nucleic acid in a virion contains only a few of the genes needed for the synthesis of new viruses. These include genes for the virion’s structural components, such as the capsid proteins, and genes for a few of the enzymes used in the viral life cycle.

These enzymes are synthesized and functional only when the virus is within the host cell.

Thus, for a virus to multiply, it must invade a host cell and take over the host’s metabolic machinery.

Multiplication of Bacteriophages

Phages can multiply by two alternative mechanisms; the lytic cycle or the lysonegic cycle.

The lytic cycle ends with the lysis and death of the host cell, whereas the host cell remains alive in the lysogenic cycle.

T - Even Bacteriophages; The Lytic Cycle

The virions of T-even bacteriophages are large, complex, and nonenveloped, with a characteristic head-and-tail structure.

The multiplication cycle of these phages, like that of all viruses, occurs in five distinct stages: attachment, penetration, biosynthesis, maturation, and release.

FIGURE: The lytic cycle of a T-even bacteriophage

The Lytic Cycle

Attachment. The first step of the lytic process. During this process, an attachment site on the virus attaches to a complementary receptor site on the bacterial cell.

Penetration. After attachment, the T-even bacteriophage injects its DNA (nucleic acid) into the bacterium. To do this, the bacteriophage’s tail releases an enzyme, phage lysozyme, which breaks down a portion of the bacterial cell wall.

The Lytic Cycle

Biosynthesis. Once the bacteriophage DNA has reached the cytoplasm of the host cell, the biosynthesis of viral nucleic acid and protein occurs.

Maturation. In the next sequence of events, maturation occurs. Bacteriophage DNA and capsule are assembled into complete virions.

Release. The final stage of viral multiplication is the release of virions from the host cell.

Bacteriophage Lambda: The Lysogenic Cycle

Biosynthesis. Once the bacteriophage DNA has reached the cytoplasm of the host cell, the biosynthesis of viral nucleic acid and protein occurs.

Maturation. In the next sequence of events, maturation occurs. Bacteriophage DNA and capsule are assembled into complete virions.

Release. The final stage of viral multiplication is the release of virions from the host cell.

Bacteriophage Lambda: The Lysogenic Cycle

In lysogeny, the phage remains latent (inactive). The participating bacterial host cells are known as lysogenic cells.

Some viruses do not cause lysis and death of the host cell when they multiply.

FILE: The lysogenic cycle of bacteriophage in E. coli

Three results of Lysogeny:

Fist, Immunity to reinfection by the same phage.

The second result of lysogeny is phage conversion.

The third result of lysogeny is that it makes specialized transduction.

Multiplication of Animal Viruses

Adsorption. The virus becomes attached to the cells, and at this stage, it can be recovered in the infectious form without cell lysis by procedures that either destroy the receptors or weaken their bonds to the virions.

Penetration. Rapidly follows adsorption, and the virus can no longer be recovered from the intact cell.

Multiplication of Animal Viruses

Uncoating. The virus becomes attached to the cells, and at this stage, it can be recovered in the infectious form without cell lysis by procedures that either destroy the receptors or weaken their bonds to the virions.

Viral Nucleic Acid Replication. Virulent viruses, either DNA and RNA, shut off cellular protein synthesis and disaggregate cellular polyribosomes, favouring a shift to viral synthesis.

Comparison of Multiplication Cycles of Bacteriophage and Animal Viruses

Stage Bacteriophage Animal Viruses

Attachment Tail fibers attach to cell wall proteins

Attachment sites are plasma membrane proteins and glycoproteins

Penetration Viral DNA injected into host cell

Capsid enters by endocytosis or fusion

Uncoating Not required Enzymatic removal of capsid proteins

Biosynthesis (Eclipse)

In cytoplasm In nucleus (DNA viruses) or cytoplasm (R.NA viruses)

Chronic infection Lysogeny Latency; slow viral infections; cancer

Release Host cell lysed Enveloped viruses bud out; nonenveloped viruses rupture plasma membrane

FIGURE: The entry of herpes simplex virus (Simplexvirus) in an animal cell. (a) Attachment of the viral envelope to the plasma membrane. (b) The cell’s plasma membrane folds inward, forming a vesicle around the virus; this results in loss of the envelope. (c) The nonenveloped capsid penetrates the cytoplasm of the cell from the vesicle. (d), (e), (f) The nucleic acid core is uncoated by digestion of the capsid.

The Biosynthesis of DNA Viruses

Generally, DNA-containing viruses replicate their DNA in the nucleus of the host cell by using viral enzymes and they synthesize their capsid and other proteins in the cytoplasm by using host cell enzymes.

Then the proteins migrate into the nucleus and are joined with the newly synthesized DNA to form virions.

These virions are transported along the ER to the host cell’s membrane for release.

FIGURE: Multiplication of Papovavirus, a DNA – containing virus.

FIGURE: DNA-containing animal viruses. (a) Negatively stained adenoviruses that have been concentrated in a centrifuge gradient. The individual capsomers are clearly visible. (b) The envelope around this herpes simplex virus capsid has broken, giving a “fried egg” appearance.

Some DNA viruses

Adenoviridae. Named after adenoids, from which they were first isolated, adenovirus cause acute respiratory diseases – the common cold.

Poxviridae. All diseases caused by poxviruses, including small pox and cowpox, include skin lesions. Pox refers to pus-filled lesions.

Herpesviridae. Nearly 100 hepesviruses are known. They are named after the spreading appearance of cold sores. Species of human herpesviruses (HHV) include HHV-1 and HHV-2.

Some DNA viruses

Papovaviridae. Papovaviruses are named for papillomas (warts), polymoas (tumors), and vacuolation (cytoplasmic vacuoles produced by some of these viruses). Warts are caused by members of the genus Papillomavirus.

Hepadnaviridae. Hepadnaviridae are so named because they cause hepatitis and contain DNA. The only genus in this family causes hepatitis B.

The Biosynthesis of RNA Viruses

The multiplication of RNA viruses is essentially the same as that of DNA viruses, except the several different mechanisms of mRNA formation occur among different groups of RNA viruses.

RNA viruses multiply in the host cell’s cytoplasm. The major difference among the multiplication processes of these viruses lie in how mRNA and viral RNA are produced. Once viral RNA and viral proteins are synthesized, maturation occurs by similar means among all animal viruses.

Some RNA Viruses

Picornaviridae. Picornaviruses, such as poliovirus, are single-stranded RNA viruses. They are smallest viruses; and the prefix pico- (small) plus RNA gives these viruses their name.

The RNA within the virion is called a sense strand (or + stand), because it can act as mRNA.

After attachment, penetration, and uncoating are completed, the single-stranded viral RNA is translated into two principal proteins, which inhibit the host cell’s synthesis of RNA.

Some RNA Viruses

Picornaviridae. Picornaviruses, such as poliovirus, are single-stranded RNA viruses. They are smallest viruses; and the prefix pico- (small) plus RNA gives these viruses their name.

The RNA within the virion is called a sense strand (or + stand), because it can act as mRNA.

After attachment, penetration, and uncoating are completed, the single-stranded viral RNA is translated into two principal proteins, which inhibit the host cell’s synthesis of RNA.

Some RNA Viruses

Tagaviridae. Togaviruses, which include arthropod-borne arboviruses or alphaviruses, also contain a single + strand of RNA. Togaviruses are enveloped viruses; their name is from the Latin word for covering, toga.

Rhabdoviridae. Rhabdoviruses, such as rabiesvirus (genus Lyssavirus), are usually bullet-shaped. Rhabdo is from the Greek word for rod, is not really an accurate description of their morphology.

Some RNA Viruses

Reoviridae. Reoviruses were named for their habitats: the respiratory and enteric (digestive) – systems of humans. Their name comes from the first letters of respiratory, enteric, and orphan.

Retroviridae. Many retroviruses infect vertebrates. One genus of retrovirus, Lentivirus, includes the subspecies HIV-1 and HIV-2, which cause AIDS.

FIGURE: Pathways of multiplication used by various RNA-containing viruses. a) After uncoating, ssRNA viruses with a + strand genome are able to sythesize proteins directly from their + strand. Using the + strand as a template, they transcribe – strands to produce additional + strands to serve as mRNA and be incorporated into copied proteins as the viral genome. B) The ssRNA viruses with a – strand genome must transcrible a + strand to serve as mRNA before they begin synthesizing proteins. The mRNA transcribes additional strands for incorporation into capsid protein. Both ssRNA and (c) dsRNA must use mRNA (+strand) to code for proteins, including capsid proteins.

Mutation and Cancer

The first step in viral mutation is the assembly of the protein capsid.

The envelope protein is encoded by the viral genes and is incorporated into plasma membrane of the host cell.

The envelope develops around the capsid by a process called budding.

After the sequence of attachment, penetration, uncoating, and biosynthesis or viral nucleic acid and protein, the assembled capsid containing nucleic acid pushes through the plasma membrane.

Mutation and Cancer

As a result, a portion of the plasma membrane, now the envelope, adheres to the virus.

This extrusion of a virus from a host cell is one method of release. Budding does not immediately kill the host cell, and in some cases the host cell survives.

Nonenveloped viruses are released through ruptures in the host cell plasma membrane. In contrast to budding, this type of release usually results in the death of the host cell.

Viruses and Cancer

Several types of cancer are now known to be caused by viruses. Molecular biological research shows that the mechanisms of the diseases are similar, even when a virus does not cause cancer.

The viral cause of cancer can often go unrecognized because the particles of some viruses infect cells but do not induce cancer. Also, cancer might not develop until long after viral infection. And cancers do not seem to be contagious, as viral disease usually are.

The transformation of Normal Cells into Tumor Cells

Almost anything that can alter the genetic material of a eukaryotic cell has the potential to make a normal cell cancerous.

These cancer-causing alterations to cellular DNA affect parts of the genome called oncogenes.

Viruses capable of inducing tumors in animals are called oncogenic viruses, or oncoviruses.

Tumor cells undergo transformation; they acquire properties that are distinct from the properties of unaffected cells.

After transformation, tumor cells contain virus-specific antigens called T-antigen.

DNA Oncogenic Viruses Oncogenic viruses are found within

several families of DNA-containing viruses. These groups Include the Adenoviridae, Herpesviridae, Poxviridae, Papovaviridae, and Hepadnaviridae.

RNA Oncogenic Viruses Among RNA viruses, only the

oncoviruses in the family Retroviridae cause cancer. The human T-cell leukemia viruses (HTLV-1 and HTLV-2) are retroviruses that cause adult T-cell leukemia and lymphoma in humans.

Latent Viral Infections

A virus can remain in equilibrium with the host and not actually produce disease for a long period, often many years.

All of human herpesviruses can remain in host cells throughout the life of an individual. When the viruses are reactivated by immunosuppression like AIDS, the resulting infection may be fatal.

An example of latent infection in viruses is the infection of the skin by herpes semplex virus, which produces cold sores.

Persistent Viral Infections

A persistent viral infection is a disease process that occurs gradually over a long period caused by a virus.

A persistent viral infection is different from a latent viral infection in that detectable infectious virus gradually builds over a long period, rather than appearing suddenly.

The measles virus is responsible for persistent viral infections like sclerosing panencephalitis (SSPE).

PRIONS

Prion Disease

Other infectious diseases that have not been found to have a viral cause might be caused by prions. Such prion diseases include:

Mad Cow Disease Scrapie Kuru Creutzfeldt–Jakob Disease (CJD)

All prion disease include:

Involve neurodegeneration

Are caused by proteins that misfold in the brain

Are also called Transmissible Spongiform Encephalopathis (TSE)

- Mad cow disease is also known as Bovine Spongiform Encephalopaty (BSE)

These diseases are caused by the conversion of a nomal host halogen into an infectious form.

The specific protein that misfolds during prion disease is called PrP

Normally found in membrane of cells Thought to play a role in normal

transduction Normal form of PrP is Called PrPC (prion

protein, cellular form) Misfolded form of PrP is called PrPRes

(prion protein, resistant to enzyme degradation)

A.k.a PrPSe (prion protein, scrapie form) Prion disease occurs when PrPC is

converted to PrPRes

FIGURE: How a protein can be infectious. If an abnormal prion protein (PrPSc) enters a call, it changes a normal prion protein to PrPSc which can now change another normal PrP, resulting in an accumulation of the abnormal PrPSc.

Resources:

Torota, Gerard J.; Funke, Berdell R.; Case Christine .L; 2004. Introduction to Microbiology; Viruses, Viroids, and Prions. Pearson Edcuation Inc.

Cortan, Ramzi S.; Kumar, Vinay; Robbins, Stanley L.; 1994. Pathologic Basis of Disease 5th Edition, Infectious Diseases. W.B Saunders Company.

Reporter:

Louis Carlo V. Lim

BS. Biology IV, Liceo de Cagayan University

Virtual Assistant, Digital Marketing Archive, San Antonio TX

Instructor: Joy B. Pabillaran, Ph.D Biology (on-going)