Microbiology Chapter 13: Viruses, Viroids, and Prions

Filterable agent

a virus; too small to be filtered out like bacteria

First human disease to be associated as a filterable agent and to be transmitted by a mosquito

yellow fever

Obligatory intracellular parasites

require living host cells to multiply

characteristics of viruses

contains either DNA or RNA, protein coat, multiply inside living cells, transfer viral nucleic acid to other cells

Bacteriophages (phages)

viruses that infect only bacteria

Viral attachment sites on host cell

part of cell wall and fimbriae of flagella, plasma membrane for animal viruses

Virion

complete, fully developed, infectious viral particle composed of nucleic acid and a protein coat

capsid

protein coat surrounding the nucleic acid core

capsomeres

protein subunits that compose the capsid

envelope

may be present around the outside of the capsid

envelope made up of

part of host cell membrane and proteins coded by virus

spikes

carbohydrate and protein complexes that project from the envelope surface

function of spikes

help attach the virus to host cells, identify virus, virulence factor

virulence factor of spikes

hemagglutination- clumps of RBCs

host reaction to viruses

production of antibodies to viral surface proteins and inactivation of virus

polyhedral virus

Icosahedron- 20 equilateral triangles

poliovirus (+ssRNA)

adenovirus (dsDNA, spikes)

Helical virus

Filovirus (Ebola) - ssRNA

Lyssavirus (Rabies) - ssRNA

Enveloped virus

Influenzavirus -ssRNA, multiple segments (8)

Influenza family, genus, capsid, envelope, genome

Family- Orthomyxoviridae

Genus- Influenzavirus

Capsid- protein coat

Envelope- lipid bilayer

Genome consists of 8 separate -RNA segments

Influenza surface spikes

neuraminidase and hemagglutinin spikes

Flu H spikes

allow virus to attach to specific host cell receptors (attachment)

Flu N spikes

help virus separate from infected cells after replication (release)

H 1-3 of flu strands

human infecting strans

H 4-5

infect swine and birds (5 avian not effective human to human)

significance of swine to influenza virus

mixing vessels, infected with both human and avian flu

antigenic shift

caused by major genetic recombination

pandemics occur when

virus acquires a new hemagglutinin and or neuraminidase spike and antibodies formed against one strain will not be protective due to antigenic shift

antigenic drift

reflects minor annual variations in the antigenic makeup of influenza virus. Often a missense mutations or response to selective pressure by antibodies

High mutation rates are characteristic of

RNA strands

flu categorized based on

the antigen of their protein coat

Three types of flue

Influenza A- regular outbreaks, domestic animals, and some wild birds

Influenza B- sporadic outbreaks of illness in limited areas

Influenza C- common, but seldom causes disease symptoms

main treatment for Influenza

neuraminidase inhibitors: relenza and tamiflu

secondary treatment for influenza

prevent uncoating reduce symptoms of type A

Enveloped icosahedral virus

Herpesviridae- dsDNA with envelope

Mono, shingles, cold sores, chicken pox, genital herpes

Complex Viruses

ds DNA, T-even bacteriophage, enveloped, large, brick shaped virus

Smallpox, cowpox

tests to identify viruses

Electron microscope, Western blot, (CPE), (PCR)

western blot test

virus is detected and identified by its reaction with antibodies

CPE

Observation of specific cytopathic effects

PCR

Polymerase chain reaction

two cycles for multiplication of bacterial viruses

the lytic and lysogenic cycle

The Lytic Cycle

T-even Phages, virulent phages, ends with death of cell

stages of lytic cycle

attachment, penetration, biosynthesis, maturation, release

Lysogenic cycle

Bacteriophage Lambda, temperate phages, host cell remains alive, phage remains latent, Phage DNA integrates with bacterial chromosome by recombination

stages of lysogenic cycle

attachment and injection, DNA enters lysogenic cycle and integrates with host DNA becoming prophage, cell divides or DNA excised and cycle starts again

results of lysogenic cycle

Cells immune to reinfection by same phage, phage conversion, makes specialized transduction possible

phage conversion

host cell exhibit new properties

specialized transduction

When prophage is excised from host chromosome, it can take with it a bit of the adjacent DNA and be inserted into another cell

Stages for multiplication of animal viruses

attachment, entry, uncoating

attachment of animal viruses to host cells

to plasma membrane proteins and glycoproteins

2 ways of entry for animal viruses

By pinocytosis (Receptor-mediated endocytosis) or fusion (enveloped)

Receptor-mediated endocytosis

cell's plasma membrane folds inward to form vesicles

fusion

enveloped viruses, Viral envelope fuses with the plasma membrane and releases the capsid into the cell's cytoplasm

uncoating

separation of the viral nucleic acid from its protein coat once the virion is enclosed within the vesicle and digestion of protein coat

Biosynthesis of DNA viruses

replication of DNA viruses occurs

in nucleus of host cell by using viral enzymes

during replication of DNA viruses, the capsid and proteins are synthesized ____ and then transported to the _____

in the cytoplasm using host cell enzymes and then transported to nucleus

assembly of newly synthesized DNA and proteins takes place in the ______ and forms ________

nucleus and forms virions

virions travel through the ____________ to the _____________

endoplasmic reticulum to the cell membrane for release

POX viruses are synthesized completely in _______ because _______

cytoplasm because they are too large

biosynthesis of RNA viruses

replication of RNA viruses occurs in

cell's cytoplasm

sense strand

+ strand

antisense strand

- strand

in RNA viruses, + strands can

serve as mRNA

in RNA viruses, - strands are

complementary and can transcribe + strands

Retroviridae

HIV-1 and -2

Retroviridae carry

reverse transcriptase which can be inserted into host cell chromosome as provirus

reverse transcriptase

which uses viral RNA as a template to produce complementary double-stranded DNA

provirus

provides protection from host immune system and antiviral drugs, can remain latent and replicate with host cell or produce new viruses to infect adjacent cells

first step of viral maturation is

assembly of protein capsids

For enveloped viruses during release, the envelope protein is incorporated into the

host cell membrane through budding

budding

this may not cause death of the host cell

nonenveloped viruses are released through

ruptures in the host cell membrane; always kills cell

oncogenic viruses

Viruses capable of inducing tumors in animals

tumors can be activated by

Mutagens, viruses, high energy radiation

tumors activated to produce

cancer

what percent of cancers are known to be caused by viruses

10%

transformation of tumor cells

They acquire properties that are distinct from the properties of uninfected cells or from infected cells that do not form tumors

after being transformed by viruses, many tumor cells contain a virus-specific antigen on their cell surface called

Tumor-specific transplantation antigen (TSTA) or T antigens

DNA oncogenic viruses

Herpesviridae, Burkitt's lymphoma, Hepatitis B, liver disease

RNA oncogenic viruses

T-cell leukemia, sarcoma, feline leukemia, retroviridae

Latent Viral Infections

virus remains in host cell for long periods without producing and infection

example of Latent Viral Infections

human herpes viruses

persistent viral infections

disease processes that occur over a long period of time and are generally fatal

persistent viral infections caused by

conventional viruses that accumulate over long period

prions

infectious proteins involving degeneration of brain tissue

prions cause

scrapie in sheep, mad cow disease, in humans kuru C-L disease

prions all produce

spongiform encephalopathy

prion infectivity is reduced by treatment with

proteases, but not with radiation

how do proteins become infectious

if an abnormal prion protein enters cell, it changes normal prion proteins

place the following in the most likely order for biosynthesis of a bacteriophage

1. phage lysozyme

2. mRNA

3. DNA

4. viral proteins

5. DNA polymerase

2. mRNA

5. DNA polymerase

3. DNA

4. viral proteins

1. phage lysozyme

a virus with the RNA-dependent RNA polymerase

synthesizes double-stranded RNA from an RNA template

the ability of a virus to infect an organism is regulated by

the host species, the type of cells, the availability of an attachment site, cell factors necessary for viral replication