Bacteriophage

2008-09-03T17:39:00.000-07:00

BACTERIOPHAGE

Definition:

These are viruses that infect bacteria. They are highly specific.

Structure:

Head contains ~ Nucleic acid

~ Protein coat called capsid

Tail ~ Contractile, has fibers at the base, used to attach to host and transfer nucleic acids.

~ Considerable variability in the number and length of fibers.

Icosahedral symmetry.

Stable structure ~ 20 triangular shaped sides.

T – even phages: T2, T4 and T6 infect E. Coli.

Usually contains double stranded DNA.

Application of Phages:

Identification and typing of bacteria for epidemiological purposes, such as that for mRNA.

Studying gene regulation and expression when used as models.

Lysogenic conversion. Acquisition of new phenotypic characteristics following entry of temperate phage.

Cloning vector in recombinant DNA technology.

Modes of Propagation or Multiplication:

Lytic Phage: A Bacteriophage produces many copies of itself as it kills its host cell.

Temperate Phage: A Bacteriophage is able to enter a non-lytic prophage state, in which replication of its nucleic acid is linked to replication of its host cell DNA.

1. Lytic Cycle: (Vegetative Cycle)

Known as virulent phages, because they lead to lysis of host cell.

There are several stages in this cycle:

1. Adsorption: Phage must recognize a host cell and infect it. It binds to specific receptor sites on the bacterial cell surface.

2. Penetration: It injects nucleic acid into host cell and leaves the protein coat outside the cell.

3. Replication: It can be plotted as a one step growth curve.

4. Eclipse Period: This is when the infectivity temporarily disappears.

5. Transcription: There’s variation between the different types of phages. Several distinct stages: Immediate early, delayed early and late gene products.

6. Assembly & Release: Phage’s structural proteins and nucleic acids are assembled in definite pathways to form mature progeny. These accumulate inside the cell up to the time of cell lysis.

7. Lysis by Enzymatic Action: Mature phages are released from the cell following its lysis.

8. Latent period: Time from infection until lysis. It usually takes 20 ~ 40 min.

9. Burst Size: The yield of phage per infected cell. Usually about 100 ~ 300 phages per cell.

Temperate Phages: (Lysogenic cycle)

Doesn’t cause lysis of cell.

Temperate phages have the ability to enter lytic or lysogenic cycles.

A Prophage integrates with the host cell’s genome, and replicates using the bacterial chromosome.

Bacteria carrying prophages are termed lysogenic because prophages retain the ability to lyse its host.

Lysogenic strains are immune to super infections.

Temperate phages are commonly found in clinical isolates of Gm +ve & Gm –ve bacteria. In some cases, they contribute to the pathogenicity of the organism.

The lysogenic state can be maintained indefinitely.

Spontaneous induction of a prophage is rare, but methods which interfere with DNA replication such as UV light or antibiotics can trigger the conversion from lysogenic to lytic cycle.

During induction, the phage may carry with it a part of the bacterial chromosome, and transmit it to a new cell adding a new character. This is called Specialized Transduction.

Microbiology Practical Guide

2008-09-03T17:19:00.000-07:00

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Bacteriophage

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