CLS (413)

LECTURE (2)

Objective

Comparison between viruses and bacteria

Atypical virus like agents.

Virus and cancer

Discuss antiviral therapy

Method of cultivation of viruses.

Comparison between viruses and bacteria

No Property Viruses Bacteria1 Size 20-300 nm 1000 nm

2 Genome(type of nucleic acid)

DNA or RNA butnot both

DNA and RNA

3 Cell wall Envelope presentin some viruses

Cell wall

4 Ribosomes No ribosomes Ribosomes

5 Sensitivity to antibiotics _ +

6 Growth in culture media Grow only inliving host cell

+

Atypical virus like agents1-DEFECTIVE VIRUSES:

Are composed of viral nucleic acid and proteins but

cannot replicate without a helper virus, which provides

the missing function.

These usually have a mutation or a deletion of part of

their genetic material.

Atypical virus like agents2- PSEUDOVIRIONS :

Contain host cell DNA instead of viral DNA within the

capsid. They are formed during infection with certain

viruses when the host cell DNA is fragmented and pieces

are incorporated within the capsid. Pseudovirions can

infect cell, but they don’t replicate.

Atypical virus like agents

3-PRIONS:

1) Are infectious particles that are composed solely of

protein.

2) They contain no detectable nucleic acid.

virus and cancer

Some DNA viruses that synthesize DNA during their

replication are capable to induce malignant transformation

of cells.CHARACTERS OF TRANSFORMED CELLS:

Increase rate of multiplication and metabolism.

Cells show morphological and chromosomal abnormalities

Virus TumorDNA Viruses: Human papiloma viruses(HPV)

Epstein Barr virus (EBV)

Hepatitis B virus (HBV)

Carcinoma of skin

Nasopharyngeal carcinoma

Hepatocellular carcinoma

RNA viruses:• Human T-cell Lymphotropic

Retrovirus (HTLV-1 )

• Hepatitis C virus (HCV)

Adult T-cell leukemia lymphoma

Liver cancer

Viruses implicated in human cancer

Antiviral therapy

Since viruses are obligate intracellular parasites, it is

difficult to find an antiviral drug that selectively inhibits the

virus without affecting the cell (selective toxicity).Few

antiviral drugs are in clinical use. They act by stopping virus

attachment, penetration, uncoating or intracellular

synthesis.

The potential points of inhibitory action of antiviral drugs

Interference with virus adsorption or attachment to the

receptor- binding site on the cell.

Inhibition of viral uncoating.

Inhibition of viral nucleic acid, transcription and

replication.

Prevention of virus budding or interference with virus

maturation

Interferon

Interferon :

are proteins made and released by host cell in response

to the presence of pathogen such as viruses, bacteria,

parasites and tumor cells.

Therapeutic uses of interferon:With recombinant DNA technology, interferon was now commercially produced. Interferon(INF-ὰ) treatment may be helpful in certain severe viral infections(herpes), in chronic virus infection (hepatitis Band C viruses).

Interferon

Method of cultivation of viruses:

Since viruses are obligate intracellular parasites, they have to be grown in living cells. There are three systems for their cultivation:

• Laboratory animals(1)•Embryonated eggs(2)•Cell culture(3)

Method of cultivation of viruses

Laboratory animals: Some animal viruses can be cultured only in living

animals, such as mice, rabbits, and pigs.

Most experiments to study the immune system's

response to viral infections must also be performed in

virally infected live animals.

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.

Method of cultivation of viruses Some human viruses cannot be grown in animals or can

be grown but do not cause disease.

Method of cultivation of viruses2.Embrionated eggs: If the virus will grow in an embryonated egg, this can be a

fairly convenient and inexpensive form of host for many

animal viruses.

A hole is drilled in the shell of the 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

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. For this reason, you may be asked if you are allergic to eggs before receiving a vaccination, because egg proteins may be present in the viral vaccine preparations.

Method of cultivation of viruses

3. In Cell Cultures: Cell cultures consist of cells grown in culture media in the

laboratory.

Cell culture lines are started by treating a slice of animal

tissue with enzymes that separate the individual cells.

These cells are suspended in a solution that provides the

osmotic pressure, nutrients, and growth factors needed

for the cells to grow.

Method of cultivation of viruses

3. In Cell Cultures: Normal cells tend to adhere to the glass or plastic

container and reproduce to form a monolayer.

Viruses infecting such a monolayer sometimes cause the

cells of the monolayer to deteriorates they multiply.

This cell deterioration is called cytopathic effect (CPE) ,

Diagnosis of virus infectionsThe laboratory procedures used in diagnosis of viral diseases include:

•Direct detection of viruses(A)

•Serologic detection of antiviral antibodies (B)

(A)Direct detection of viruses

Direct detection of viruses can be Achieved by different techniques:

1. Light microscopy:This can be used to visualize some large viruses e.g. poxviruses

2. Electron microscopy:Is used to demonstrate virus particles in vesicular fluid or tissue extracts treated with special stain. Its only successful if large numbers of particles are present.

3. Immunofluorescence microscopy:Detection of virus in smears from lesions using fluorescein labeled specific antisera

5. Polymerase chain reaction:This technique involves amplification of a short sequence of a target DNA or RNA (which may be in low concentration e.g. one copy) leading to accumulation of large amounts of that sequence, so it can be easily detected.PCR can be used to determine the quantity of viruses in patients blood i.e. virus load e.g. in HIV patients. This is used to monitor the course of the disease and to evaluate treatment and prognosis.

(B)Serologic detection of antiviral antibodies:

Serologic diagnosis used in virology:

1. Enzyme linked immunosorbent assay(ELISA):Enzyme linked immunosorbent assay (ELISA) can be used for detection of viral antigens in different clinical specimens e.g. detection of HBV ,HIV, HCV and Rota virus.

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