Question
Bacteriophage
Answer
Bacteriophages are viruses that specifically infect bacteria, replicating via either a lytic (virulent) or lysogenic (temperate) life cycle.
Morphology: a typical phage has an icosahedral head enclosing the nucleic acid genome, a collar, a contractile tail sheath, a hollow tail core, a base plate, and tail fibres mediating specific attachment to bacterial receptors.
Lytic cycle: attachment → penetration (genome injection) → eclipse phase/biosynthesis → maturation (assembly) → release (lysozyme-mediated lysis).
Lysogenic cycle: phage DNA integrates into the host chromosome as a prophage, replicating passively with the host; induction (UV light, stress) can trigger excision and entry into the lytic cycle.
Clinical significance: mediates transduction (horizontal gene transfer, including toxin genes via lysogenic conversion — e.g., diphtheria toxin, Shiga-like toxin — and antibiotic-resistance genes); explored as phage therapy for multidrug-resistant infections.