Bacteriophages ( phages ) are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery . They are viruses that infect bacteria.
Composition of Bacteriophage • Nucleic acid: either DNA or RNA but not both • ds DNA,ss RNA,ss DNA • unusual or modified bases • encode 3-5 gene products to over 100 gene products • Protein: function in infection and protect the nucleic acid
Structure of Bacteriophage • different sizes and shapes icosahedral 二十面体 filamentous
Capsid DNA Contractile Sheath Tail Fibers Head Tail Base Plate
Types of Bacteriophage • Lytic or Virulent Phages 毒性噬菌体 • Lysogenic or Temperate Phage 溶原性噬菌体或温和噬菌体
Lytic or Virulent Phages • Definition: Lytic or virulent phages are phages which can only multiply in bacteria and kill the cell by lysis at the end of the life cycle.
Life Cycle • Adsorption吸附 • Penetration穿入 • Biological Synthesis生物合成 • Maturation and Release成熟和释放
Bacteriophage binding to and injecting their DNA into a bacterial cell
bacteriophage after contraction of sheath normal bacteriophage
Assay for Lytic Phage • Plaque assay噬斑测定
Lytic phage are enumerated by a plaque assay. A plaque is a clear area which results from the lysis of bacteria. Each plaque arises from a single infectious phage. The infectious particle that gives rise to a plaque is called a pfu (plaque forming unit).
Lysogenic or Temperate Phage溶原性噬菌体或温和噬菌体 • Lysogenic or temperate phages are those that can either multiply via the lytic cycle or enter a quiescent state in the cell. In this quiescent state most of the phage genes are not transcribed; the phage genome exists in a repressed state. The phage DNA in this repressed state is called a prophagebecause it is not a phage but it has the potential to produce phage. In most cases the phage DNA actually integrates into the host chromosome and is replicated along with the host chromosome and passed on to the daughter cells. The cell harboring a prophage is termed a lysogenic bacterium.
Animation summarizing the lysogenic life cycle of a temperate bacteriophage.
lysogenic conversion溶原性转换 • When a cell becomes lysogenized, occasionally extra genes carried by the phage get expressed in the cell. These genes can change the properties of the bacterial cell. This process is called lysogenic conversion.
Application of Bacteriophage • Used in treatment of bacterial infections • Used for the identification of pathogenic bacteria (phage typing 噬菌体型分型) • Used in molecular biology