BACTERIOPHAGE Bacteriophage Phage Definition Viruses that infect and

BACTERIOPHAGE

Bacteriophage (Phage) Definition - Viruses that infect and multiply inside bacteria by making use of some or all of the host biosynthetic machinery

Figure Bacteriophage

Composition and Structure Composition ◦ Nucleic acid Head/Capsi d Genome size Modified bases ◦ Protein Protection Infection • Structure (T 4) – Size – Head or capsid – Tail Contractil e Sheath Tail Fibers Tail Base Plate

Each phage attaches to a specific cell surface molecule (usually a protein) and so only cells bearing that “receptor” can be infected by a given phage.

Two Basic Types 1. Lytic phage: eg. T phage infect a bacterial cell DNA replication coat proteins expression host cell lysed to release the new phage

The lytic growth cycle Figure 21 -1

2. Temperate phage: eg. Phage λ Lysogeny —the phage genome integrated into the bacterial genome and replicated passively as part of the host chromosome, coat protein genes not expressed. • The phage is called a prophage. • Daughter cells are lysogens.

Figure 21 -2 The lysogenic cycle of a bacteriophage

The lysogenic state can switch to lytic growth, called induction. Excision of the prophage DNA replication Coat proteins expression Lytic growth

Figure 16 -24 Growth and induction of λ lysogen

Progagate phage ØFind a suitable host cell that supports the growth of the virus. ØThe mixture of viruses and bacteria are filtered through a bacterial-proof filter.

Assay for Lytic Phage • Plaque assay – Method – Plaque forming unit (pfu) – Measures infectious particles Bacteria + Phage

As the viruses replicate and are released, they spread and infect the nearby cells. This circle-of-death produces a hole or PLAQUE in a lawn of living cells. These plaques can be easily seen and counted so that the numbers of virus can be quantitated.

The Single-Step Growth Curve Bacteriophage Latent periodthe time lapse between infection and release of progeny. Burst size-the number of phage released Figure 21 -4

Therapy and prevention Phage therapy is the application of bacteriophages to bacterial infections of humans (or other animals) reducing bacterial load Phages can be delivered topically, orally, directly into body tissues, or systemically To treat bacterial infections that do not respond to conventional antibiotics

Therapeutic use of lytic bacteriophages Phage therapy of staphylococcal infections (including MRSA) may be less expensive than antibiotic treatment To control biofilms, form on the surfaces of embedded medical devices such as catheters.