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Bacteriophage
Bacteriophage (Phage)
• Definition - Obligate intracellular parasites that
multiply inside bacteria by making use of some or
all of the host biosynthetic machinery
• Significance
– Models for animal cell viruses
– Gene transfer in bacteria
– Medical applications
• Identification of bacteria - phage typing
• Treatment and prophylaxsis???
Medical Applications of Phage
• “I strongly believe phage could become an
effective antibacterial tool” - Carl Merril, Chief of the
Laboratory of Biochemical Genetics, National Institute of Mental
Health, NIH.
• “It might be another string on the bow, such that
when (conventional antibiotics) fail, here’s
something that has a chance of working. But it’s
not going to be a panacea” - Joshua Lederberg, Sackler
Foundation Scholar at The Rockefeller University
Reassessment of Medicinal Phage Spurs Companies to Study Therapeutic Uses
American Society for Microbiology News 64:620-623, 1998
Medical Applications of Phage
• Exponential Biotherapies (Rockville, MD)
– Vancomycin resistant Enterococcus facium and Streptococcus
pneumoniae
• Phage Therapeutics (Bothell, WA)
– Staphylococcus aureus and Staphylococcus epidermidis
• Intralytix, Inc. (Baltimore, MD)
– Salmonella in meat and poultry
• Biopharm Ltd. (Tblisi, Georgia)
– Infections associated with burns
• University of Idaho
– Escherichia coli O157:H7 in cattle
Reassessment of Medicinal Phage Spurs Companies to Study Therapeutic Uses. American
Society for Microbiology News 64:620-623, 1998.
Phages eyed as agents to protect against harmful E. coli. American Society for
Microbiology News 65:666-667, 1999.
Bacteriophage
• T4
• Lambda ()
Composition and Structure
• Composition
– Nucleic acid
Head/Capsid
• Genome size
• Modified bases
– Protein
• Protection
• Infection
• Structure (T4)
– Size
– Head or capsid
– Tail
Contractile
Sheath
Tail
Tail Fibers
Base Plate
Infection of Host Cells
• Adsorption
– LPS for T4
• Irreversible attachment
• Sheath Contraction
• Nucleic acid injection
Types of Bacteriophage
• Lytic or virulent phage: Phage that can only
multiply within bacteria and kill the cell by
lysis. (e.g., T4)
• Eclipse
– Early genes
– Phage DNA
synthesis
– Late genes
• Intracellular
accumulation
• Lysis and Release
Number of Infectious Particles
Lytic Phage Multiplication Cycle
Total
Phage
Eclipse
Extracellular
Phage
Intracellular
accumulation
phase
Time after Infection
Lysis
Assay for Lytic Phage
Phage
• Plaque assay
– Method
– Plaque forming unit
(pfu)
– Measures infectious
particles
Bacteria
+
Phage
Types of Bacteriophage
• Lysogenic or temperate phage: Phage that
can either multiply via the lytic cycle or
enter a quiescent state in the bacterial cell.
(e.g., )
– Expression of most phage genes repressed
– Prophage
– Lysogen
Events Leading to Lysogeny
• Circularization of the phage chromosome
– Cohesive ends
Cohesive Ends
Lygase
Linear Double Stranded
Opened Circle
Closed Circle
Events Leading to Lysogeny
• Site-specific
recombination
– Phage coded
enzyme
gal
• Repression of the
phage genome
– Repressor protein
– Specific
– Immunity to
superinfection
bio
gal
bio
gal
bio
Termination of Lysogeny
• Induction
– Adverse conditions
• Role of proteases
bio
gal
– recA protein
– Destruction of
repressor
• Gene expression
• Excision
• Lytic growth
bio
gal
gal
bio
gal
bio
Lytic vs Lysogenic Cycle?
• Role of repressor
• Role of cro gene product
• Role of proteases
Significance of Lysogeny
• Model for animal virus transformation
• Lysogenic or phage conversion
– Definition: A change in the phenotype of a
bacterial cell as a consequence of lysogeny
• Modification of Salmonella O antigen
• Toxin production by Corynebacterium diphtheriae