Question
Life cycle of bacteriophage.
Answer
Bacteriophages (viruses that infect bacteria) replicate via two alternative life cycle strategies — the lytic (virulent) cycle and the lysogenic (temperate) cycle.
Lytic cycle (used by virulent phages, and available to temperate phages after induction):
- Attachment (adsorption) — the phage’s tail fibres bind specifically to receptors on the bacterial cell surface, determining host specificity.
- Penetration — the phage’s contractile tail sheath contracts, driving the tail core through the bacterial cell wall, and viral DNA (or RNA) is injected into the host cytoplasm; the empty protein capsid remains outside.
- Eclipse phase (biosynthesis) — the phage genome directs synthesis of early proteins (which shut down host metabolism/degrade host DNA) followed by late proteins (structural components) and replication of the phage genome, using host cell machinery; no complete infectious virions exist during this phase.
- Maturation (assembly) — newly synthesized phage genomes and structural proteins are assembled into complete, mature virions.
- Release (lysis) — phage-encoded lysozyme degrades the bacterial cell wall, causing the cell to lyse and release numerous new progeny phage particles, which can then infect fresh host cells, repeating the cycle.
Lysogenic cycle (used by temperate phages):
- After penetration, instead of immediately entering the lytic pathway, the phage DNA integrates into the host bacterial chromosome, becoming a prophage.
- The prophage is replicated passively along with the host chromosome at each bacterial cell division, being passed to daughter cells without producing new phage particles or killing the host — the bacterium is said to be lysogenized.
- Induction — certain stimuli (UV light, chemical mutagens, stress) can trigger the prophage to excise from the chromosome and enter the lytic cycle, producing and releasing new phage particles.
Clinical/biological significance: the lysogenic cycle underlies lysogenic conversion — a prophage can carry genes (e.g., toxin genes) that alter the phenotype of the lysogenized bacterium, such as diphtheria toxin in toxigenic Corynebacterium diphtheriae, erythrogenic toxin in Streptococcus pyogenes, and Shiga-like toxin in certain E. coli strains — all examples where the toxin gene is carried and expressed only because the bacterium harbours a specific lysogenic prophage. Specialized transduction (transfer of specific bacterial genes near the prophage integration site) also occurs upon imprecise excision during induction, contributing to horizontal gene transfer, including of virulence and antibiotic-resistance genes.