Viral Replication: Lytic Cycle

Bacteriophages, or phages, are viruses that specifically infect bacteria. Among them, T-even bacteriophages, such as T4, exhibit a well-characterized lytic replication cycle in Escherichia coli (E. coli). This process ensures the rapid proliferation of the virus while ultimately leading to the destruction of the bacterial host.

Attachment and DNA Injection

The infection process begins with the recognition and binding of the T4 phage to the E. coli cell surface. Tail fibers of the phage specifically interact with receptor molecules on the bacterial outer membrane, ensuring host specificity. Upon attachment, the baseplate of the phage releases gp5, an enzyme that locally degrades the peptidoglycan layer of the bacterial cell wall, weakening its structure and facilitating DNA entry. As the tail sheath contracts like a spring, the tail tube punctures the weakened cell wall, creating a direct pathway for viral DNA entry into the bacterial cytoplasm. The empty protein capsid remains outside the cell.

Hijacking Host Machinery

Once inside the host, the phage DNA rapidly takes control of cellular functions. Early viral genes are transcribed first, producing enzymes that degrade bacterial DNA and modify transcription factors to favor viral replication. This prevents interference with viral gene expression and allows for the efficient synthesis of new viral proteins and genomic DNA using the host’s ribosomes and nucleotides. The phage redirects the host's transcription and translation machinery entirely toward viral replication.

Assembly of New Virions

After the production of viral components, a highly coordinated process organizes structural proteins into new phage particles. Self-assembly of viral components follows a precise order, assisted by scaffolding proteins and chaperones, ensuring the correct attachment of capsids, tails, and fibers. Fully assembled, infectious virions form within the bacterial cytoplasm, ready to infect new hosts.

Cell Lysis and Virion Release

To release progeny virions, the phage produces holin proteins that create membrane pores, facilitating the action of lysozyme in degrading the bacterial cell wall. As the enzymatic degradation progresses, the bacterial cell swells before bursting due to osmotic lysis. Within approximately 25–30 minutes of initial infection, the bacterial cell ruptures, releasing dozens to hundreds of new phage particles. These virions then disperse to infect other susceptible E. coli cells, perpetuating the lytic cycle.