A bacteriophage (or simply 'phage') is a virus that infects and replicates inside bacteria. The name comes from Greek: 'bacterio' (bacteria) + 'phagein' (to eat). The most studied bacteriophage is the T4 phage, which infects E. coli (Escherichia coli). The T4 phage has a complex, tadpole-like structure with distinct parts: head, collar, tail sheath, tail tube, base plate, tail fibers, and spikes.
Bacteriophage = virus that infects bacteria ('bacteria eater').
T4 phage (infects E. coli) is the most studied bacteriophage.
Parts: head (dsDNA), collar, tail sheath (contractile), tail tube, base plate, tail fibers, spikes.
Head contains dsDNA — the genetic material of the phage.
Tail fibers attach to specific receptors on the bacterial cell.
Lytic cycle: Attachment → Injection → Replication → Assembly → Lysis.
Hershey-Chase experiment (1952) used T2 phage to prove DNA is the genetic material.
Phage therapy: using bacteriophages to kill antibiotic-resistant bacteria.
Parts of a T4 Bacteriophage (top to bottom):
HEAD (Capsid) • Icosahedral (20-faced polyhedral) protein coat • Contains the viral genetic material: double-stranded DNA (dsDNA) • Typically ~100 nm in diameter • Made of protein subunits called capsomeres
COLLAR (Neck) • Short cylindrical structure connecting the head to the tail • Helps transfer DNA from head to tail
TAIL SHEATH • Contractile protein tube surrounding the tail tube • During infection, the tail sheath contracts to inject DNA into the host cell • Made of contractile proteins (similar in mechanism to a syringe)
TAIL TUBE (Tail Core) • Inner hollow tube through which DNA is injected into the bacterial cell • Runs through the tail sheath
BASE PLATE • Hexagonal protein structure at the bottom of the tail • Attachment point for tail fibers and spikes • Triggers sheath contraction when properly bound to host
TAIL FIBERS (Long tail fibers) • 6 long, thin protein fibers extending from the base plate • Primary site of recognition and attachment to the host bacterial cell surface • Each fiber ends in a 'foot' region that binds to specific receptors on E. coli
SPIKES (Short tail fibers / pins) • Short spikes at the base plate • Help in irreversible attachment to the host
Types of bacteriophages:
By DNA type: • dsDNA phages: T4, lambda (λ), T7 (most complex) • ssDNA phages: φX174, M13 • ssRNA phages: MS2, Qβ • dsRNA phages: phi6
By replication cycle: • Lytic phages (virulent): directly replicate and lyse (burst) the host cell — T4 phage is lytic — 5 stages: Attachment → Injection → Replication → Assembly → Lysis • Lysogenic phages (temperate): integrate into host chromosome as prophage, replicate with host — Lambda (λ) phage is lysogenic — Can switch to lytic cycle under stress
Lytic Cycle of T4 Phage (5 stages):
Scientific importance: • Bacteriophages were used to prove that DNA (not protein) is the genetic material — Hershey-Chase experiment (1952) • Alfred Hershey and Martha Chase used T2 phage labelled with radioactive ³²P (DNA) and ³⁵S (protein) • Only ³²P entered bacteria → DNA is the genetic material ✓
Applications of Bacteriophages:
Phage therapy: • Bacteriophages can be used to kill antibiotic-resistant bacteria (drug-resistant infections) • Used as an alternative or supplement to antibiotics
Genetic research: • Tools in molecular biology • Used as vectors to introduce foreign genes into bacteria (gene cloning)
Phage display: • Used to study protein-protein interactions and develop new drugs and vaccines
Biocontrol: • Used in agriculture to protect plants from bacterial diseases • Added to food packaging to prevent bacterial contamination (e.g., Listeria)
Size and structure comparison: • Bacteriophage: ~150-200 nm tall • Average bacterium (E. coli): ~2 µm long — phage is much smaller • For comparison: 1 µm = 1000 nm
A bacteriophage (phage) is a virus that specifically infects bacteria. It uses the bacterial cell's machinery to replicate. The most studied phage is T4, which infects E. coli. Phages have a complex structure: head (with DNA), collar, contractile tail sheath, tail tube, base plate, tail fibers, and spikes.
T4 bacteriophage parts (top to bottom): (1) Head/Capsid — icosahedral, contains dsDNA; (2) Collar — connects head to tail; (3) Tail sheath — contractile protein tube; (4) Tail tube — hollow inner tube for DNA injection; (5) Base plate — hexagonal, connects tail to fibers; (6) Tail fibers (6 long fibers) — attach to host cell receptors; (7) Spikes — short pins at the base for firm attachment.
The lytic cycle of bacteriophage (T4) has 5 stages: (1) Attachment — tail fibers bind to specific receptor on E. coli surface; (2) Injection — tail sheath contracts and injects DNA into host; (3) Replication — host cell makes new viral DNA and proteins; (4) Assembly — new phage particles assembled; (5) Lysis — host cell bursts, releasing 100–200 new phages.
Hershey and Chase (1952) used T2 bacteriophage labelled with radioactive ³²P (in DNA) and ³⁵S (in protein). They infected E. coli bacteria and found that only ³²P (DNA) entered the host, while ³⁵S (protein) stayed outside. This proved that DNA — not protein — is the genetic material of bacteriophage and by implication, of all organisms.
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