Infectious agents
Eukaryotic- parasites, protists, fungi
Prokaryotes
Nonliving- viruses, viroids, prions
Viruses apparently infect every living thing
(bacterio)phages- bacteria
plant and animal viruses
All viruses have a protein capsid
Capsid shape is characteristic of the virus
Viruses are naked
Or enveloped
Much smaller
than cells
General features of viruses, continued
All have nucleic acid (DNA or RNA, but not
both)
genome is very small
gene sequences sometimes overlap
Must have genes to:
make the capsid
replicate itself (special polymerases)
get into and out of the cell
Viruses use the host replication processes
What do viruses do to cells?
Some viruses kill the host cell (lytic; virulent)
Some viruses are released from living cells
(not lytic, but still productive)
Some viruses become integrated into the DNA
of the host
(temperate: latent infection. Infection
is lysogenic)
Viral “life cycles” especially well-studied
in bacteriophages, esp. T4
Bacteriophage are classified according to
host range
attachment site
genome (DNA or RNA)
shape
effect on host cell
Temperate phages can either lyse their host
or replicate with it
Bacteriophage lambda () is especially well
analyzed
Why lytic? Why lysogenic?
Lytic if bacteria are actively dividing; otherwise
lysogenic
How are viruses maintained this way?
Virus becomes prophage (integrated into
host genome)
has gene sequences homologous to sequences
in E. coli, and integrates at that site
Is excised in a reverse process
In lysogen, some viral genes are repressed
(prevented from being expressed)
How does phage become lytic again?
loss or inactivation of repressor
damage to bacterium
Bacteria that already are lysogenic are
immune to infection by the same type of
phage
Lysogenic conversion
Transduction- phage-mediated genetic
transfer
Generalized- any bacterial gene can be
transferred
Specialized- virus inserted in a specific site
so only genes adjacent to it are
transferred (i.e., temperate phages)
Why can phages infect some bacterial strains
but not others?
Phage must attach to specific receptors on host
cell
Restriction-modification- prevents the “right”
phage DNA from being degraded
Infectious agents of animals and plants
Analogous biologically to bacteriophages
Taxonomic grouping according to:
mode of replication (genome structure)
shape of capsid
presence of envelope
Both RNA and DNA genomes; wide variety
of arrangement of nucleic acid
RNA viruses even more diverse (p. 343)
Some other grouping may be more useful,
e.g., mode of transmission
Growing and studying viruses
Tissue culture
propagation
cytopathic effect
Quantitation
plaque assay
titration
hemagglutination
antibody titer
adenovirus
Herpes simplex
Viruses may be pathogenic or have no apparent
effect
May cause acute or persistent infections
Entry of enveloped viruses into animal cells
Many viruses have genomes that cannot
be transcribed by host enzymes
They must encode their own
How are viruses released?
from dead cells (not actually lysed by virus)
budding
shed from host
In persistent infections, viruses are continuously
produced by budding
Infected person is a carrier
Retroviruses are analogous to bacteriophage
such as
Can be integrated (randomly) into host genome
Reverse transcriptase
Retroviruses (and other, DNA viruses) can cause
tumors in susceptible animals
Viruses and tumors
When viral genome gets inserted into a host
genome, it can cause changes
Activation of host genes
Mutation of host genes
If these genes regulate the cell cycle, this
can lead to tumors (e.g., proto-oncogenes)
Papilloma, herpes, Epstein-Barr
Retroviruses and cell transformation
Viruses are integrated into host DNA and later
excised
May incorporate regulatory gene
Virus polymerase is error-prone; may mutate
gene
This gene may then be transferred to a new
host
Host range: the type of organism (or tissue)
that a virus can infect
Some can infect many species (zoonoses)
Some can acquire that wide range through
modifications
Phenotypic mixing
Mutation
Probably how
influenza virus
changes so
rapidly
Viruses infect plants, too
Wounds
Contaminated soil
Vectors
Spread through plasmodesmata in cell walls
Viruses can cause a variety of pathologies
in plants
Other, novel infectious agents
Prions- infectious proteins
Viroids- naked, small RNA molecule
replicate autonomously
single viroid can infect cell
resistant to proteases and nucleases
only infect plants (as fas as we know)
Replication of prions?
Prions are species-specific
Some apparently can cross species, e.g.,
mad cow disease