Microbio Lecture 3: Survey of Microorganisms (virus and other infectious agents)

prokaryotic and eukaryotic cells have

DNA and RNA

DNA

primary genetic material

RNA

plays a secondary role

viruses contain either

DNA or RNA but not both

viruses are referred to by

the type of nucleic acid they contain

ex DNA virus or RNA virus

DNA can be

linear or circular, double- or single-stranded

RNA is usually

single stranded but can be double stranded

some viruses like influenza can have

nucleic acid in several segments

viral replication

Multiply only in living, actively metabolizing cells

viruses have no mitochondria or ribosomes meaning they

Can not harvest energy or synthesize proteins

Must take over host cell structures in order to replicate

viruses also have little (5)

nucleic acid; only have genes required to:

Make the viral coat

Replicate the viral nucleic acid

Move the virus in and out of the host cell

Make viral enzymes needed for replication that are not present in the host cell

types of viral replication by dsDNA Bacterial viruses (2)

dsDNA; bacteriophages

1. Lytic infection

2. Lysogenic infection

(latent infection- in animal cells)

lytic infection and example

Virus completely "takes over" host cell metabolism

• Virus replicates

• Host cell is lysed (killed) as new virions are released •

e.g. bacteriophage T4, infects E. coli

lytic infection steps (5)

1. Attachment/ Absorption

2. Penetration/ Injection

3. Biosynthesis

4.Maturation

5. Release

attachment

phage attaches by tail fibers to host cell

penetration

phage penetrates host cell and injects its DNA, leaving the page coat outside

biosynthesis

phage DNA directs synthesis of viral components by the host cell

maturation

viral components are assembled into virions

release

host cell lyses and new virions are released

In a one-step growth experiment

-Mix bacterial host and phage

-Brief incubation (attachment occurs)

-Dilute greatly (released viruses can't infect new cells)

-Over time, collect sample and enumerate viruses

one step growth curve

-following adsorption, infectivity is abolished "eclipsed", this stage is set for viral replication

-the appearance of the first mature progeny viral particle marks the beginning of "rise" period or "maturation"

-according to nucleic acid, there are early period and late period

-the time between the late period and rise period is the time for viral encapsidation

one step growth curve

Eclipse period: attachment

latent period: genome in just not replicated yet

lysogenic infection

virus integrates its DNA into the genome of the bacteria they infect or the DNA replicates as a plasmid

Phage replicates as the bacterial DNA replicates.

A "latent" infection - may be no sign that the cells are infected.

Phage is called "temperate" (controlled)

Infection can modify

properties of the host cell

Induction to a lytic cycle occurs when

host cell is stressed

e.g. of temperate phage:

lambda (λ), infects E. col

dynamic nature of viruses

once in cells they use enzymes to integrate their genome with hosts genome: so everytime ecoli replicates so does the lysogenic genome

temperate bacteriophage infect a host cell

lambda cycle

take a detour after step two and if stress like ethanol, lysol or temperature affect them then they return to step 3

integrase

lambda genome is integrated into the host genome in a reaction catalyzed by integrase

excisionase

-binds integrase

-enables integrase to reverse integration process

Transduction in bacteria

...type of bacterial recombination (create genetic diversity) in which there is an exchange of bacterial DNA through bacteriophages -

gal gene

gene for galactose metabolize

gal gene transduction

temperate phages have

two reproductive options

1. reproduce lytically as virulent phages do

2.Remain within host cell without destroying it

- Remain within host cell without destroying it, is done by

many temperate phages by integration of their genome with the host genome in a relationship called lysogeny

lysogeny: prophage

w/o capsid

integrated phage genome

lysogeny: lysogens

- infected bacterial host

-temperate phages: phages are able to establish lysogeny

distinctive characteristics of Lysogenic bacteria

• They are immune to superinfection

• Under appropriate conditions they will lyse and release phage particles

(- This occurs when conditions in the cell cause the prophage to initiate synthesis of new phage particles, a process called induction - Environmental factors such as UV induce lysogenic infection to lytic_

possible inducers

chemical, physical, biological

chemical inducers

heavy metals, chemical carcinogens

physical inducers

UV radiation

biological inducers

other viruses

lysogenic conversion

change in host phenotype induced by lysogeny

lysogenic conversion ex

e.g., modification of Salmonella lipopolysaccharide structure

• e.g., production of diphtheria toxin by Corynebacterium diphtheriae

epsilon phage

A bacteriophage that integrates into the genome of Salmonella and modifies the LPS

b phage

The β-phage integrates into the bacterial genome and carries the tox gene, which codes for the diphtheria toxin in corynebacterium diphtheriae.

bacterial defense against phage infection

1. restriction- modification system in bacteria

2. cells also have suicide defense by premature cell death abort the infection

3. secret weapon

1. restriction- modification system in bacteria

Bacterium chemically modified its own DNA at particular restriction sites. Newly injected virus will be stopped.

• But, virus can become modified or

• Use two-step injection process, first a small portion of phage DNA enters, encode proteins that antagonize restriction enzymes, and the rest of DNA can escape the host defense

secret weapon

crispr:

CRISPR

Clustered Regularly Interspaced Short Palindromic Repeats

acquired immunity

a mechanism that generates small RNA in bacteria provides protection against bacteriophage

Anti-CRISPR proteins

-encoded by bacteriophage genomes

-typically inhibit function of one of the Cas proteins

Plaque isolation and assay of bacteriophages

viruses grown with host cells as food

bacteria in culture

animal cells in tissue culture

host cells form confluent lawn

viruses form plaques where host cells killed

plaque is

a colony of viruses

tissue culture of animal viruses

animal viruses can be cultured within whole animals by serial inoculation

Ensures that the virus strain maintains its original virulence, but process is expensive and laborious.

They can also be grown in human cell tissue culture

Plaque Isolation and Assay of Animal Viruses

1. infect monolayer with virus

2. remove liquid medium

3. add gelatin medium

4. virus reproduces. host cells lyse forming plaques