Multiplication of Bacteriophages - The Lysogenic Cycle

Multiplication of Bacteriophages - The Lysogenic Cycle

Bacteriophages multiply inside a host bacterium by one of two alternative routes, the lytic cycle and the lysogenic cycle, both of which begin the same way: the phage attaches to the surface of a susceptible bacterium using its tail fibres, and then injects its nucleic acid into the host cell, leaving its empty protein capsid outside.

In the lytic cycle, the injected phage DNA immediately takes control of the host cell's machinery, directing it to replicate the phage DNA and to manufacture the protein components of new phage particles (new heads, tails and tail fibres). These components then self-assemble into complete new phage particles inside the host cell. Once a large number of new phages have been assembled, the phage-encoded enzyme lysozyme breaks down the host bacterium's cell wall from within, causing the cell to burst open - a process called lysis - and releasing all the newly made phage particles into the surroundings, ready to infect fresh host cells. The number of new phage particles released from a single infected cell in this way is called the burst size. Phages that always follow this destructive lytic pathway, killing their host cell at the end of every infection cycle, are called virulent phages; the T-even phages (such as T4) are typical virulent phages.

The lysogenic cycle offers an alternative, non-destructive route. Instead of immediately taking over the host cell to make new phage particles, the injected phage DNA integrates itself into the host bacterium's own chromosome, becoming what is called a prophage. While it remains a prophage, the phage DNA causes no harm to its host and produces no new phage particles; instead, it is simply replicated passively, along with the rest of the host chromosome, every time the host bacterium divides, so that every descendant of the original infected cell also carries the same prophage. Under certain conditions - such as exposure to stress or damage - a prophage can be induced to excise itself from the host chromosome and switch over into the lytic cycle, at which point it directs the production of new phage particles and lyses its host exactly as a virulent phage would. Phages capable of this dual lifestyle, alternating between a harmless, integrated prophage state and an active lytic state, are called temperate phages, of which the bacteriophage lambda is a well-known example. …