T4 BACTERIOPHAGE LIFE CYCLE
Introduction
Imagine a tiny predator: a virus that
specifically targets a bacterium and uses it like a factory to make hundreds of
new copies of itself, then bursts the host open. That’s the story of the T4
bacteriophage. The T4 bacteriophage, formally known as Escherichia virus T4, is
one of the most studied viruses in microbiology and virology. Its life cycle
and structure make it a model for understanding virus-host interactions.
In this post, we’ll explore:
• Classification of T4 phage
• Structure overview
• Step-by-step life cycle and
reproduction
• Why it matters (applications)
• Colorful diagrams you can use for your
blog or lecture
Let’s dive in!
Classification & Structure
Classification
Order: Caudovirales
Family: Myoviridae
Genus: Escherichia coli (E. coli)
T4 is considered a virulent (strictly
lytic) phage. It does not integrate into the host genome, meaning there is no
lysogenic cycle under normal conditions.
Structure
• The head (capsid) is prolate (elongated)
and contains about 168,903 base pairs of double-stranded DNA (about 169 kbp)
and roughly 289 protein-encoding genes.
• Connected to the head is a 925 Å-long
contractile tail sheath, a baseplate, and six long tail fibers that recognize
bacterial surface receptors.
• The tail fibers initiate attachment to the host cell. The tail sheath contracts and injects the viral DNA into the host.
Life Cycle & Reproduction of T4 Bacteriophage
Since T4 follows the lytic life cycle
exclusively, we’ll go through that in detail. This will also benefit your
SEO-rich blog section on “T4 bacteriophage life cycle.”
Step 1: Adsorption (Attachment)
• The T4 phage's long tail fibers recognize
and bind to specific receptors on the surface of E. coli cells, typically
lipopolysaccharides (LPS) and outer-membrane proteins like OmpC.
• First, the attachment is reversible,
followed by irreversible binding of the short tail fibers.
Step 2: Penetration (Injection)
• Once bound, changes in the baseplate
trigger contraction of the tail sheath. The tail tube punctures the bacterial
outer membrane and peptidoglycan layer.
• A lysozyme domain within a phage tail protein helps break down the bacterial cell wall, allowing the DNA to be injected into the cytoplasm of the host.
Step 3: Hijack & Early Gene Expression
• Once inside the host cytoplasm, the phage
stops host gene expression and redirects the host’s machinery to produce phage
proteins.
• Early enzymes degrade host DNA and modify
host RNA polymerase so that phage genes are transcribed preferentially.
Step 4: DNA Replication & Late Gene Expression
• The phage makes multiple copies of its
DNA using mechanisms that are very efficient with a low error rate.
• Structural proteins (capsid, tail,
baseplate, fibers) are then made.
• Assembly of phage particles begins: heads form, tails form separately, then they are joined.
Step 5: Maturation & Assembly
• DNA is packaged into capsids, and tails,
baseplates, and tail fibers attach.
• Fully mature virions accumulate within the host.
Step 6: Release (Lysis)
• Phage-encoded lysis proteins (holins,
endolysins) cause the bacterial cell wall to break. This leads to host cell
lysis and the release of newly formed phage particles.
• For T4, the entire cycle, from adsorption to release, takes about 30 minutes at 37 °C and typically produces around 100–150 phage particles per host.
• Model system: T4 has been used for
decades in molecular biology to study gene regulation, DNA replication,
recombination, and virus-host interactions.
• Virulence & specificity: As a
virulent phage, it’s ideal for studying lytic phages.
• Applications: With the rise of antibiotic
resistance, phages like T4 and related phages are being explored for phage
therapy.
• Energy usage: Interestingly, despite its
size, the energy cost of building a T4 phage inside a host is about a third of
the host’s energy budget based on some estimates.
• Burst size & timing: Knowing the ~30-minute life cycle and burst size of ~100-150 helps in lab quantifications and modeling phage dynamics in bacterial populations.
- T4 bacteriophage life cycle
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- Lytic cycle of T4 phage
- Stages of T4 bacteriophage
- T4 bacteriophage diagram
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- Bacteriophage T4 steps
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- Structure and function of T4 bacteriophage
- What are the steps of T4 bacteriophage life cycle?
- How does T4 bacteriophage reproduce?
- Diagram of T4 bacteriophage with labeled parts
- Lytic cycle of T4 bacteriophage explained with diagram
- T4 phage infection mechanism in E. coli
- Complete life cycle of T4 virus PDF
- T4 bacteriophage classification and structure notes for B.Sc.