A lytic virus replicates within a host cell, causing it to burst. This process, known as lysis, destroys the infected cell. The release of new viral particles through lysis is fundamental for the virus to spread and continue its life cycle by infecting other cells.
Understanding the Lytic Cycle
The lytic cycle describes the sequence of events a lytic virus undergoes to reproduce within a host cell, culminating in the release of new viruses. The cycle begins with attachment, where the virus binds to specific receptors on the host cell surface. Following attachment, the viral genetic material enters the host cell through penetration.
Once inside, the virus takes control of the host cell’s machinery to replicate its components and synthesize new viral proteins, a phase known as replication. These newly synthesized viral components then assemble into mature viral particles in the maturation phase. The cycle concludes with the release of these new viruses.
Viral Release from Bacterial Cells
Bacteriophages, which are viruses that infect bacteria, employ a mechanism for lytic release from their host cells. This process relies on the coordinated action of two main types of viral proteins: holins and endolysins.
Holins are membrane proteins that accumulate in the bacterial inner membrane and form pores. These pores disrupt the membrane’s integrity, allowing endolysins, which are enzymes that degrade the bacterial cell wall, to access their target. Once endolysins reach the peptidoglycan layer of the cell wall, they break it down, leading to osmotic pressure imbalances that cause the bacterial cell to burst and release the viral particles. In some Gram-negative bacteria, additional proteins called spanins also play a role in disrupting the outer membrane.
Viral Release from Animal Cells
The mechanisms of lytic release from animal cells differ from those in bacteria due to the absence of a rigid cell wall. While many enveloped animal viruses release by budding, acquiring an outer membrane from the host without immediate cell lysis, some non-enveloped and even some enveloped viruses induce direct cell lysis. One way this occurs is through the accumulation of viral components, which can rupture the cell membrane. Another mechanism involves viral proteins called viroporins, which insert into host cell membranes. Viroporins form pores, disrupting the membrane’s permeability and ion balance, which can lead to osmotic lysis and the release of new virions.