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 (e.g., T4) has an icosahedral head (capsid) 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 surface receptors; some phages (e.g., filamentous M13) lack this complex tailed structure entirely.
Lytic cycle: attachment (tail fibres bind specific host receptors) → penetration (tail sheath contracts, injecting genome into the host) → eclipse phase/biosynthesis (viral genome directs synthesis of components, no complete virions exist yet) → maturation (assembly of new virions) → release (lysozyme-mediated lysis of the host, releasing progeny phage).
Lysogenic cycle: phage DNA integrates into the host chromosome as a prophage, replicating passively with the host without killing it; induction (UV light, stress) can trigger excision and entry into the lytic cycle.
Clinical/biological significance: bacteriophages mediate transduction (horizontal gene transfer, including toxin genes — e.g., diphtheria toxin, Shiga-like toxin — via lysogenic conversion, and antibiotic-resistance genes); increasingly explored as phage therapy for multidrug-resistant bacterial infections.