Bacteriophage: Lytic Phage Infections: Videos & Practice Problems

The lytic cycle of bacteriophages involves five key steps: attachment, genome entry, synthesis, assembly, and release. In the attachment step, the phage attaches to specific receptors on the bacterial cell. During genome entry, the phage injects its DNA into the host cell, leaving the protein coat outside. In the synthesis step, phage enzymes degrade the host's DNA, allowing for the production of new phage components. These components are then assembled inside the host cell during the assembly step. Finally, in the release step, the host cell lyses, releasing new phages to infect neighboring cells. This cycle is crucial for understanding viral replication and host interactions in microbiology.

During the lytic cycle, a bacteriophage attaches to a bacterial cell by binding to specific receptors on the bacterial cell envelope. This attachment is highly specific, as the phage recognizes and binds to particular molecules on the surface of the host bacterium. This specificity ensures that the phage infects only certain types of bacteria. The attachment step is the first crucial step in the lytic cycle, setting the stage for the subsequent injection of the phage genome into the host cell.

During the genome entry step of the lytic cycle, the bacteriophage injects its genetic material, typically DNA, into the host bacterial cell. The protein coat of the phage remains outside the host cell. This injection process is facilitated by the phage's tail structure, which punctures the bacterial cell envelope, allowing the phage DNA to enter the host cell. Once inside, the phage DNA can take over the host's cellular machinery to begin the synthesis of new phage components.

During the synthesis step of the lytic cycle, phage enzymes play a critical role in degrading the host bacterial DNA. This degradation inactivates the host's genetic material, preventing it from interfering with the production of new phage components. Additionally, the phage enzymes facilitate the synthesis of viral genomes and proteins, which are necessary for assembling new phages. By hijacking the host's cellular machinery, the phage ensures the efficient production of its own components, leading to the eventual assembly of new infectious phages.

During the assembly step of the lytic cycle, newly synthesized phage components, including viral genomes and proteins, are put together inside the host bacterial cell. This process involves the precise assembly of various phage parts, such as the head, tail, and tail fibers, to form complete, functional phages. The assembly occurs in a highly organized manner, ensuring that each new phage is correctly constructed and capable of infecting other bacterial cells. Once assembled, these new phages are ready for release during the final step of the lytic cycle.

During the release step of the lytic cycle, the host bacterial cell undergoes lysis, or rupture, which releases the newly assembled phages into the surrounding environment. This lysis is typically caused by phage-encoded enzymes that break down the bacterial cell wall. As the cell lyses, it releases numerous new phages, which can then go on to infect neighboring bacterial cells, initiating new lytic cycles. This release step is crucial for the propagation of the phage population and the continuation of the infection process.