Microbiology Chapter 13

Viroids

They consist solely of a single-stranded RNA molecule that varies in size from 246 to 375 nucleotides and forms a closed ring. It is about one-tenth the size of the smallest infectious viral RNA genome known. A great deal of hydrogen bonding exists between complementary bases in the viroid RNA so that the single-stranded molecule appears to be a double-stranded structure. Have been identified to infect only plants. Enter through wound sites.

Provirus

The silent viral genome. A provirus cannot be eliminated from the body, and means that the disease can recur even after an extended period without symptoms

Persistent infections

These infections can continue for years, or even the life of the host with or without symptoms. Viruses have mechanisms that evade some of the defenses against viruses

How can you use cell and tissue culture to study animal viruses?

They are commonly used to cultivate most animal viruses. Animal cells - grown in a liquid medium contained in special screw-capped flasks - are used as host cells for virus culture

Prions

They are composed solely of protein, which is reflected in the name (derived from proteinaceous infections agent). These agents have been linked to a number of slow, always fatal, human diseases. They accumulate in neural tissue. Neurons die and brain function deteriorates as the tissues develop characteristic holes

Chronic infections

Persistent infections characterized by the continuous production of low levels of viral particles. In some cases, it is similar to what occurs in filamentous bacteriophage infections with each infected cell surviving and slowly releasing viral particles. In other cases, the infected cell lyses, but only a small proportion of cells is infected at any given time, resulting in a low number of viral particles being continuously released.

How do prions cause diseases?

Prions violate the central dogma of replication that requires nucleic acid act as a template for replication of macromolecules. It is hypothesized that PrPsc interacts directly with PrPc and converts its folding properties from PrPc to PrPsc. In most cases, the disease is transmitted only to members of the same species, however, it depends on the strain.

Transmissible mink encephalopathy

Ranched mink

Scrapie

Sheep and goats

Transmissible spongiform encephalopathies

The characteristic spongelike appearance of the brain tissues gave rise to this general term, which refers to all prion diseases

PrPc

The normal cellular form (prion protein, cellular) and is readily destroyed by host cell proteases, as a normal turnover process, with older molecules being destroyed as new ones are synthesized

What was the only way to study animal viruses back in the day?

The only way was to inoculate live animals with a suspension of virus particles. Later, embryonated chicken eggs were used to grow virus, which made research much easier. Today, influenza viruses are grown in embryonated chicken eggs to manufacture vaccines against influenza

Primary culture

1) cut tissue into small pieces and incubate with a protease (trypsin) to separate cells. 2) place cells into flask with growth medium. 3) allow cells to settle to bottom of flask and grow into a single layer (a monolayer). It is one way to obtain animal cells for culture. one problem is that normal cells can divide only a limited number of times, so new primary cultures must be made regularly

Cytopathic effect

A virus propogated in tissue culture often causes distinct morphological alterations in infected cells, called a cytopathic effect. The host cells may change shape, detach from the surface, or lyse. Infected cells may fuse into a giant multinuclear cell, a mechanism of viral spread. Several viruses, such as HIV and measles, cause this effect

Tumor

An abnormal growth of tissue resulting from a malfunction in the normally highly regulated process of cell growth. Some are benign, others are cancerous or malignant

Latent infection

Analogous to lysogeny by bacteriophages. The viral genome remains silent within a host cell, yet can reactivate to direct a productive infection. Some viruses do not integrate into the host cell chromosome; rather, they replicate independently of the host genome, much like a plasmid

Exotic ungulate encephalopathy

Antelope in South Africa

What are some examples of prion diseases?

CWD: deer at a research facility in Colorado were first animals to show signs and symptoms, within 40 years the disease spread from Colorado to wild deer populations through their natural migration and transport of captive farmed animals located in more than a dozen states and parts of Canada

How can you recognize a viral infection in a plant?

Can be recognized through various outward signs, including yellowing of foliage with irregular lines appearing on the leaves and fruits. Individual cells or specialized organs of the plant may die, and tumors may appear. Usually these plants become stunted in growth

Feline spongiform encephalopathy

Cats

Bovine spongiform encephalopathy

Cattle

Inclusion body

Certain viruses cause an infected cell to form this distinct region which is the site of viral replication. The position in a cell depends on the type of virus.

Acute infections

Characterized by the sudden onset of symptoms of a relatively short duration. Can be compared to productive lytic infection by bacteriophages, resulting in a burst of virions being released from infected host cells. Difference is that the host cell may survive

Chronic wasting disease

Deer and elk

Oncogene

Gene whose activity is involved in turning a normal cell into a cancer cell. their entry into cells can interfere with the cell's own control mechanisms, leading to tumor formation

Proto-oncogenes

Genes that stimulate cell growth. Genes that inhibit cell growth are called tumor suppressor genes. They work together to regulate growth and cell division

Examples of latent infections

HSV-1 causes cold sores. It causes an acute infection in mucosal epithelial cells, leading to the typical symptoms of cold sores. From there, the virus can spread to sensory nerve cells where it remains latent. Later, latent virus can reactivate to cause another episode of cold sores. Not clear what reactivates the cold sores

Examples of chronic infections

Hepatitis B virus infection. A person can transmit the virus to others even in the absence of symptoms. Some people infected with hepatitis B virus develop a chronic infection, they become carriers of the virus, able to pass it to other people through blood and body fluids

Kuru

Humans (caused by cannibalism)

Variant Creutzfeldt-Jakob disease

Humans (caused by consumption of prion-contaminated beef)

Creutzfeldt-Jakob disease

Humans (inherited)

Fatal familial insomnia

Humans (inherited)

Gerstmann-Straussler Scheinker syndrom

Humans (inherited)

How can you use an electron microscope to count animal viruses?

If a sample contains a high enough concentration of viruses to e seen with an electron microscope, direct counts can be used to determine the number of viral particles in suspension

Quantal assay

In this method, several dilutions of the virus preparation are administered to a number of animals, cells, or chick embryos, depending on the host specificity of the virus. The titer of the virus, or endpoint, is the dilution at which 50% of the inoculated hosts are infected

PrPsc

Infectious prion proteins (prion protein, scrapie) are less susceptible to degradation by proteases and become insoluble, leading to aggregation. They are unusually resistant to heat and chemical treatments that are commonly used to inactivate infectious agents

ID50

Infective dose for quantal assay

How can a plaque assay be used to find animal virus concentration?

It is one of the most precise methods for determining the concentration of animal viruses. It is similar in principle to the method described for quantitating bacteriophages, but in the case of animal viruses, a monolayer of tissue culture cells is the host. Again, clear zones surrounded by uninfected cells are counted to determine the viral titer.

LD50

Lethal dose for quantal assay

How do viruses infect plants?

They do not attach to specific receptors. Instead, they enter through wound sites in the cell wall, which is otherwise very tough and rigid. Infection in the plant can then be spread from cell to cell through openings (the plasmodesmata) that interconnects cells. Plant viruses can be transmitted through soil contaminated by prior growth of infected plants, and by growers themselves. A small percentage is transmitted through contaminated seeds, tubers, or pollen. Can also spread through grafting of healthy plant tissue onto diseased plants. The most important transmitters of plant viruses are probably insects.

What is an example of a plant that has been maintained in a virus-infected state?

Tulips are kept in a virus-infected state; a virus transmitted through the bulbs can cause a desirable color variegation of the flowers. The infecting virus was transmitted through bulbs or a long time before the cause of the variegation was suspected

Epstein-Barr

Type of Nucleic Acid: DNA; Kind of Tumor: Burkitt's lymphoma; nasopharyngeal carcinoma; B-cell lymphoma

Human papillomaviruses (HPVs)

Type of Nucleic Acid: DNA; Kind of Tumor: Different kinds of tumors, caused by different HPV types

Hepatitis B

Type of Nucleic Acid: DNA; Kind of Tumor: Hepatocellular carcinoma

Human herpesvirus type B

Type of Nucleic Acid: DNA; Kind of Tumor: Kaposi's sarcoma

HTLV-1

Type of Nucleic Acid: RNA (retrovirus); Kind of Tumor: Adult T-cell leukemia (rare)

Hepatitis C

Type of Nucleic Acid: RNA; Kind of Tumor: Hepatocellular carcinoma

Human immunodeficiency virus (HIV)

Type of infection: Chronic; Cells involved: Activated helper T cells, macrophages; Disease: AIDS/Type of infection: Latent; Cells involved: Memory helper T cells

Hepatitis C virus

Type of infection: Chronic; Cells involved: Hepatocytes; Disease: Hepatitis, cirrhosis, hepatocellular carcinoma

Hepatitis B virus

Type of infection: Chronic; Cells involved: Hepatocytes; Disease: Hepatits, cirrhosis, hepatocellular carcinoma

Epstein-Barr virus

Type of infection: Latent; Cells involved: B cells, which are involved in antibody production; Disease: Burkitt's lymphoma

Herpes simplex virus type 2

Type of infection: Latent; Cells involved: Neurons of sensory ganglia; Disease: Genital herpes and recurrent genital herpes

Herpes simplex virus type 1

Type of infection: Latent; Cells involved: Neurons of sensory ganglia; Disease: Primary oral herpes and recurrent herpes simplex (cold sores)

Cytomegalovirus (CMV; Herpesviridae family)

Type of infection: Latent; Cells involved: Salivary glands, kidney epithelium, leukocytes; Disease: CMV pneumonia, eye infection, mononucleosis, congenital CMV infection

Varicella zoster (Herpesviridae family)

Type of infection: Latent; Cells involved: Satellite cells of sensory ganglia; Disease: Chickenpox and herpes zoster (shingles)

Hemagglutination

When certain viruses cause red blood cells to agglutinate (clump). It occurs when individual viral particles attach to surface molecules of multiple red blood cells simultaneously, connecting the cells to form an aggregate. It is visible only at high concentration of viruses, and can be used to determine only the relative concentration of viral particles. It is measured by mixing serial dilutions of the viral suspension with a standard amount of red blood cells. The highest dilution showing maximum agglutination is the titer of the virus. Orthomyxoviruses (influenza is a member) can agglutinate red blood cells