Bacteriophage – Lytic vs Lysogenic Cycle

Even bacteria can get a virus! To be more specific a bacteriophage,the viruses that infect bacteria are called bacteriophages,In this article, we'll take a look at two different cycles that bacteriophages may use to infect their bacterial hosts.The lytic cycle and The lysogenic cycle.

to summarize, The lytic cycle; is where the phage hijacks the bacterium to make lots of copies of itself, and then kills it by making it explode (lyse).

The lysogenic cycle; is where The phage infects a bacterium and inserts its DNA into the bacterial chromosome, allowing the phage DNA (now called a prophage) to be copied and passed on along with the cell's own DNA.

before we go in depth into each cycle lets first take a closer look at the bacteriophage. 

So bacteriophage, or phage for short, is a virus that infects bacteria. And just like other types of viruses, bacteriophages vary a lot in their shape and genetic material.

The phage genomes can consist of either DNA or RNA, and can contain as few as four genes or as many as several hundred. The capsid of a bacteriophage can be icosahedral, filamentous, or head-tail in shape. The head-tail structure seems to be unique to phages and their close relatives (and is not found in eukaryotic viruses).

like any other viruses bacteriophages must infect a host cell in order to reproduce. The steps that make up the infection process are collectively called the lifecycle of the phage.

Some phages can only reproduce via a lytic lifecycle, in which they burst and kill their host cells. Other phages can alternate between a lytic lifecycle and a lysogenic lifecycle, in which they don't kill the host cell (and are instead copied along with the host DNA each time the cell divides).

So lets get into the bacteriophages life cycles in more details:

The Lytic cycle:

In the lytic cycle, a phage acts like a typical virus: it hijacks its host cell and uses the cell's resources to make lots of new phages, causing the cell to lyse (burst) and die in the process.

By, using Proteins in the "tail" the phage bind to a specific receptor on the surface of the bacterial cell. It then injects its double-stranded DNA genome into the cytoplasm of the bacterium. And using the Bacterium machinery the phage's DNA is copied and its genes are expressed to make proteins, such as capsid proteins. After exhausting the host cell's resources the capsids assemble from the capsid proteins and are stuffed with DNA to make lots of new phage particles. Late in the lytic cycle, the phage expresses genes for proteins that poke holes in the plasma membrane and cell wall. The holes let water flow in, making the cell expand and burst.

Cell bursting, or lysis, releases hundreds of new phages, which can find and infect other host cells nearby. In this way, a few cycles of lytic infection can let the phage spread like wildfire through a bacterial population.

Lysogenic cycle:

The lysogenic cycle allows a phage to reproduce without killing its host. In the lysogenic cycle, the first two steps (attachment and DNA injection) occur just as they do for the lytic cycle. However, once the phage DNA is inside the cell, it is not immediately copied or expressed to make proteins. Instead, it recombines with a particular region of the bacterial chromosome. This causes the phage DNA to be integrated into the chromosome. 

The integrated phage DNA, called a prophage, is not active: its genes aren't expressed, and it doesn't drive production of new phages. However, each time a host cell divides, the prophage is copied along with the host DNA, getting a free ride. The lysogenic cycle is less flashy (and less gory) than the lytic cycle, but at the end of the day, it's just another way for the phage to reproduce.

Under the right conditions, the prophage can become active and come back out of the bacterial chromosome, triggering the remaining steps of the lytic cycle (DNA copying and protein synthesis, phage assembly, and lysis).

So what triggers a prophage to pop back out of the chromosome and enter the lytic cycle? At least in the laboratory, DNA-damaging agents (like UV radiation and chemicals) will trigger most prophages in a population to re-activate. However, a small fraction of the prophages in a population spontaneously "go lytic" even without these external cues.