1. Attachment: The first step in the infection process is the attachment of the bacteriophage to the surface of the bacterial cell. The bacteriophage has specific attachment proteins or structures that bind to receptors or ligands on the bacterial cell wall or membrane. This binding is highly specific, and each bacteriophage typically infects only particular bacterial strains or species.
2. Entry: Once the bacteriophage is attached to the bacterial cell, it must enter the cell to initiate infection. This can occur through various mechanisms:
- Injection: Some bacteriophages have a needle-like structure called the tail tube, which penetrates the bacterial cell wall or membrane. The phage's DNA or genetic material is then injected through the tail tube into the cytoplasm of the host cell.
- Enveloped phages: Certain bacteriophages, such as the lipid-containing enveloped phages, fuse their outer envelope with the bacterial cell membrane, releasing the phage's DNA into the host cell cytoplasm.
3. Replication: After entering the host cell, the bacteriophage's genetic material, usually double-stranded DNA or single-stranded RNA, is released from the phage protein coat. This genetic material serves as a template for replication using the host cell's machinery. The phage DNA or RNA is transcribed and translated, resulting in the production of new phage components and enzymes.
4. Assembly: New phage particles are assembled from the replicated DNA or RNA and protein coat components synthesized within the host cell. The structural components self-assemble, encasing the newly replicated genetic material to form new phage particles.
5. Lysis: The final stage of the infection cycle is cell lysis, which is the breaking open of the host cell to release the newly assembled phage particles. This can occur through two primary mechanisms:
- Lytic cycle: In the lytic cycle, the bacteriophage infection results in the host cell's destruction. The phage encodes enzymes called lysins, or endolysins, that degrade the bacterial cell wall or membrane. This leads to cell lysis, releasing the progeny phage particles into the environment to infect new host cells.
- Lysogenic cycle: In some cases, the bacteriophage does not immediately cause cell lysis. Instead, it integrates its genetic material into the host cell's genome, becoming a prophage. The prophage can remain dormant within the host cell, replicating and transmitting along with the host cell's DNA. Under specific conditions, the prophage can be induced to enter the lytic cycle and produce new phage particles, leading to cell lysis.
After the release of new phage particles from the lysed host cell, these progeny phages can go on to infect other bacterial cells, repeating the infection cycle and potentially causing widespread infection within a bacterial population.
