Lytic Cycle of Bacteriophages

Bacteriophages begin infection by attaching to a bacterium's surface. Proteins in the phage's tail fibers bind to specific receptor proteins on the bacterial cell. The tail then contracts, injecting the phage's DNA genome into the bacterial cytoplasm, leaving behind an empty capsid structure.

During the lytic cycle, the phage takes over the bacterium's cellular machinery to replicate its DNA and synthesize viral proteins. The phage protects its own DNA while degrading the host's DNA. Viral components self-assemble into new phage particles, and proteins degrade the cell wall, causing the cell to burst and release hundreds of new phages.

Bacteriophages produce proteins that degrade the host cell's wall, allowing water to enter the cell. This osmotic pressure causes the cell to expand and burst, or lyse. Cell lysis releases hundreds of newly assembled phage particles into the environment, enabling them to infect nearby bacterial populations.

Head-tail morphology is a distinctive structure found in many bacteriophages. The head contains the viral genome, typically double-stranded DNA, while the tail consists of fibers and a contractile shaft. This specialized structure allows the phage to attach to bacteria and inject its genetic material into the host cell.

Phage therapy uses bacteriophages that infect only specific bacterial strains, making it highly targeted compared to broad-spectrum antibiotics that kill both pathogenic and beneficial bacteria. This specificity limits bacterial resistance evolution to particular strains. However, different phages would be needed for each bacterial pathogen, and the specific bacterial strain must be identified before treatment.

Phage therapy faces several challenges. The high specificity of phages means different phages are needed for each bacterial species or strain, making large-scale production difficult. Additionally, clinicians must either identify the exact bacterial strain causing infection or use a cocktail of multiple phages. These limitations have slowed phage therapy's adoption despite ongoing research.

The lytic cycle is one mode of phage replication that destroys the host cell by taking over its machinery and releasing new phages through cell lysis. This contrasts with the lysogenic cycle of bacteriophages, where the phage DNA integrates into the host genome and replicates without immediately destroying the cell.