Medical Microbiology Chapter 12 Viruses



Medical Microbiology Chapter 12 Viruses

I. History of Viruses

A. Louis Pasteur

1. 1884 - rabies vaccine

2. proposed term virus (Latin: poison)

B. Filtrate infectious

C. Billions of years old

D. Most abundant microbes in world

E. Interact with host cell

F. Infectious particles

G. Obligate intracellular parasite

II. General Structure of virus

A. Size

1. > 0.2 µm

2. 50 million polioviruses in 1 human cell

3. must use electron microscope

B. Viral Parts

1. capsid (shell)

2. nucleic core - either DNA or RNA

a. double stranded (ds) DNA or ds RNA

b. single stranded (ss) DNA or ss RNA

c. surrounded by capsid

3. nucleocapsid = nucleic acid + capsid

4. naked viruses - only have nucleocapsid

5. enveloped viruses - nucleocapsid plus another

outer covering - comes from host cell

6. functions of capsid/envelope

a. protection

b. helps virus bind to/enter host cell

7. enzymes for specific functions within host cell

C. Helical or polyhedral symmetry

D. Phage or Bacteriophage

1. infects bacteria

2. head - capsid and nucleic acid

3. tail - collar, sheath, base plate, tail fibers

4. example: T4 bacteriophage

5. often make infected bacteria more pathogenic for humans

III. Multiplication Cycles in Animal Viruses

A. Adsorption (attachment) and host range

1. virus attaches to cell membrane of host cell

2. attaches at specific receptor sites

3. host range

a. restricted - specific site, specific cell, specific species (example: Hepatitis B

human liver cell only)

b. intermediate - specific site, multiple cells, several related species (example:

poliovirus - primate nerve and intestinal cells)

c. broad - specific site, specific cell but in large population (example: rabies - nerve cells of most mammals)

B. Penetration

1. endocytosis

a. virus engulfed by cell

b. enters host cell as vesicle/vacuole

2. direct fusion

a. envelope merges with host cell membrane

b. nucleocapsid injected into cell's interior

C. Uncoating

1. enzyme (lysosyme) secreted

2. vacuole/vesicle/capsid dissolved releasing nucleic acid core

D. Replication and protein production - lytic cycle

1. nucleic acid takes over machinery of cell

a. DNA virus replicates in nucleus

b. RNA virus replicates in cytoplasm

c. viral mRNA made

-redirects ribosomes to make only viral proteins

d. capsid made from viral proteins

2. eclipse period - 1st 10-15 minutes after penetration of host cell no viruses

detected.

E. Maturation - Assembly of Viruses

F. Release

1. lysis (rupture)

a. enzyme released - weakens host cell membrane

b. host cell ruptures/dies

c. viral particles released

d. viral particles now attack other cells

e. example: cold

2. budding or exocytosis

a. virus particles shed gradually

b. host cell does not immediately die

c. example: HIV virus

3. virion - fully formed extracellular virus particle that is virulent

4. can cause acute infections or persistent infections

G. Damage to Host Cell and Persistent Infections

1. Cytopathic effects (CPE's)

-virus-induced damage to microscopic appearance of host cell

2. persistent infections - few weeks to lifetime - lysogeny - lysogenic cycle

-latent state - virus can remain inactive for long periods of time then reappear

only to "disappear" again

-herpes simplex virus

cold sores, genital herpes

-herpes zoster virus, chicken pox, shingles

-allows virus to spread without killing host

3. loss of cell function

-poliovirus - nerve cells - causes polio

4. damage to immune system

-HIV virus causes AIDS

5. begin process of tumor formation

-warts (benign tumors)

-carcinomas (malignant tumors)

6. oncogenic/oncoviruses

a. viruses that lead to cancer

b. transformation

-virus permanently alters genetic material of host cell

-viral nucleic acid becomes part of host cell DNA

-papillomavirus - HPV - genitals warts and cervical cancer

-herpesviruses - Epstein-Barr virus causes Burkitt's Lymphoma

-hepatitis B virus - liver cancer

IV. Cell (Tissue) Culture Techniques

A. Bird embryo cultures

B. Cell culture

C. Live Animal inoculation - rats, mice, guinea pigs, hamsters, rabbits

V. Viral Diseases

Mumps, measles, shingles, Epstein-Barr, smallpox, chickenpox, fever blisters, genital warts, genital herpes, rabies, flu, cold, Equine Encephalitis, AIDS, strep throat, Hepatitis, polio, Ebola, warts, SARS, West Nile Virus

VI. Treatment

A. Antibiotics do not work

B. Antiviral drugs - block virus replication, but can cause severe side effects

C. Interferon

D. Vaccines

VII. Other noncellular agents of disease

A. Prions

1. protein fibers

2. cause spongiform encephalopathies

3. spread by direct contact, contaminated food

4. mammalian examples: scrapie in sheep, mad cow disease (bovine spongiform encephalopathy), wasting disease in elk, deer, and mink

5. human example: Creutzfeldt-Jakob syndrome (CJS), kuru, fatal familial insomnia

-can be transmitted by consuming infected meat

6. diseases fatal

B. Viroids

1. small pieces of the protein-coated nucleic acid RNA

2. in plants

C. Satellite viruses

1. depend on other viruses for replication

2. can cause host virus to be more pathogenic

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