Download Lecture 3

Branches of Microbiology •
Bacteriology •
Virology •
Mycology •
Parasitology •
Immunology •
Recombinant DNA technology •
viruses
Are the smallest infectious agents •
Intracellular parasite•
The distinguishing properties are •
***Size •
****Nucleic acid content & structure
****Metabolic capabilities & reproduction •
Size –electron microscope •
20-300 nm •
Nucleic acid content – contain only a •
single type of nucleic acid ether DNA or
RNA (never both)
They are either single or double •
stranded
They are + or - sense •
structure
A complete virion is composed of nucleic •
acid core surrounded by protein coat ==
capsid
Some viruses are surrounded by an •
envelope consisting of lipid, protein,
carbohydrate
structure
viruses
DNA
enveloped
nonenveloped
RNA
enveloped
nonenveloped
Metabolic capabilities & reproduction •
Viruses have no metabolic machinery of •
their own
They are obligatory intracellular parasites •
Only growing within living cells •
replication
The DNA or RNA genome may be : •
ss – single stranded –
or
ds – double stranded –
DNA & RNA
Genomes may be either: •
(+) sense: Positive-sense viral RNA is identical to viral –
mRNA and thus can be immediately translated into protein
by the host cell.
OR
(-) sense: Negative-sense viral RNA is complementary to –
mRNA and thus must be converted to positive-sense RNA
by an RNA polymerase before translation.
11
Steps of viral replication
Adsorption ( Attachment) – binding of .1
virus to specific molecule on host cell
Penetration–genome enters host cell .2
uncoating- getting rid of protein coat
Biosynthesis replication – viral .3
components produced
assembly - viral components assembled .4
maturation – completion of viral formation .5
release – viruses leave cell to infect other .6
cells
Attachment •
specific binding of a virion protein (the anti-receptor) –
to a constituent of the cell surface (the receptor)
e.g. hemagglutinin of influenza virus •
Penetration •
energy-dependent step –
occurs almost instantaneously after attachment –
13
Host range
Spectrum of cells a virus can infect •
cell has to have a specific structure (receptor) on –
its surface for viral attachment
cell has to contain all of the enzymes and –
materials needed to produce new virions
May be one species or many •
HIV (only humans) vs rabies (many animals) –
May be one tissue or many within a host •
Hepatitis (liver) vs polio (intestinal & nerve cells) –
14
Entry by:
Pinocytosis
Enveloped viruses by fusion
15
Uncoating •
at same time as penetration or shortly after –
separation of viral nucleic acid (NA) from outer structural –
components ( by cellular enzymes)
Released as: •
Free nucleic acid inside the cytoplasm of the infected –
cell
Expression of viral genome and •
synthesis of viral components
16
Viral replication
After the viral nucleic acid is released inside the host cell: •
The transcription and translation processes of the host cell are 
redirected for the production of viral proteins and nucleic acids
The different types of nucleic acid genomes are expressed and 
replicated in several ways:
DNA genomes undergo replication-using processes similar to •
cellular replication
RNA genomes may be +ssRNA; Can be read directly as an •
mRNA or reverse transcribed by reverse transcriptase into DNA
RNA genomes may also be -ssRNA; The RNA must first be used as •
a template to form +mRNAs
18
Assembly and Release
Components of capsid synthesis directed by late genes •
•
Assembly of enveloped viruses needs interaction with plasma
membrane which has been modified
Final stage of infection •
Enveloped viruses released gradually by budding or
•
exocytosis
Naked viruses accumulate in cytoplasm and released during
•
lysis
19
Viral Replication
When a virus infects a cell, nucleic acid must be uncoated and gain
•
access to metabolic machinery of cell.
Virus life cycle is characterized by:
•
attachment –
penetration, with entry of nucleic acid into cell –
early expression of virus genes (either directly by translation, if –
virus contains "+" RNA, or indirectly after transcription and then
translation)
replication of virus nucleic acid –
synthesis of new virion components –
packaging and assembly of new virions
exit from cell –
–
20
Effect of virus on cells
The production of new viral particles •
generally results in
***death of the host cell (cytopathic effect) •
***transformation •
Cytopathic effects- virusinduced damage to cells
changes in size & shape .1
cytoplasmic inclusion bodies .2
nuclear inclusion bodies .3
cells fuse to form multinucleated cells .4
cell lysis .5
alter DNA .6
transform cells into cancerous cells .7
22
How do we grow viruses?
Obligate intracellular parasites
require appropriate
cells to replicate.
Growing animal viruses
Living animals .1
Bird embryonated eggs – chicken, duck; .2
intact, self-supporting unit, sterile, selfnourished
Cell culture .3
25
26
Cell culture
27
Diagnosis of viral diseases
More difficult than other agents •
Consider overall clinical picture •
Take appropriate sample •
Infect cell culture- look for characteristic –
cytopathic effects
Screen for parts of the virus –
Screen for immune response to virus –
(antibodies)
28
Direct detection of viruses •
E.M. •
Agglutination == influenza •
Fluorescent antibody test •
ELISA=enzyme linked immunosorbent •
assay
RIA=radio immuno assay •
Hybridization --- in Situ hybridization •
===detection of DNA sequence in •
cancer cells