Question
Morphology of bacteriophage
Answer
Bacteriophages are viruses that specifically infect bacteria, and their morphology (most extensively characterized in the well-studied T-even coliphages, e.g., T4) is among the most structurally complex of all viruses.
Structural components:
- Head (capsid) — an icosahedral (or, in some phages, elongated/prolate icosahedral) protein shell enclosing the viral genome (double-stranded DNA in most well-characterized phages, though ssDNA and RNA phages also exist).
- Collar — a narrow neck-like structure connecting the head to the tail.
- Tail sheath — a contractile protein sheath surrounding the tail core, capable of contracting during the infection process to drive the tail core through the bacterial cell wall.
- Tail core (tube) — a hollow inner tube through which the viral genome is injected into the host bacterium.
- Base plate — a structure at the distal end of the tail, to which tail fibres are attached, involved in initial attachment to the host cell surface.
- Tail fibres — thin, filamentous appendages extending from the base plate, mediating specific recognition and attachment to receptors on the bacterial cell surface — the basis of a phage’s characteristic host specificity (a given phage typically infects only a narrow range of bacterial strains/species).
Morphological variation: not all phages possess this full complex structure — some (e.g., filamentous phages such as M13) are simple helical filaments without a tail apparatus at all, and some have short, non-contractile tails.
Functional significance of morphology: the tail fibres and base plate mediate specific host recognition and attachment; the contractile tail sheath mechanically drives the tail core through the bacterial cell envelope, after which viral genetic material is injected into the host cytoplasm through the hollow tail core, initiating infection — while the empty protein capsid (the “ghost”) remains outside the cell.
Biological/clinical significance: bacteriophages mediate transduction (a mechanism of horizontal gene transfer, including transfer of toxin/virulence and antibiotic-resistance genes), and are of growing therapeutic interest as phage therapy for treating multidrug-resistant bacterial infections, given their high specificity for particular bacterial targets.