GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

10. VIRUSES: DISTRIBUTION AND STRUCTURE

10.3. BACTERIAL VIRUSES (BACTERIOPHAGES)

10.3.2. Development of Temperate Phages: Lysogeny

The Bacteriophages described above typically lyse their host Bacteria, which is why they are referred to as virulent. Some phages infect bacteria but do not replicate autonomously within them and do not cause lysis. Such phages are called temperate, and the bacteria they infect are termed lysogenic. The multiplication of temperate phages occurs synchronously with bacterial Replication. Only very rarely—in about one out of every 10²-10³ such lysogenic bacteria—does the phage spontaneously begin to replicate, leading to Cell lysis. In this case, detecting the release of infectious phage requires an indicator bacterial strain to which the phage is virulent. If lysogenic and excess indicator bacteria are mixed and plated onto an Agar medium, both lysogenic and indicator bacteria will grow. However, from time to time, certain lysogenic Cells will undergo lysis, and the released phage particles will infect susceptible indicator cells. This results in The formation of plaques within the confluent bacterial lawn. Nevertheless, a colony of the lysogenic bacterium survives in the center of each such plaque.

Lysogenic bacteria possess the intrinsic potential to produce phages, yet this capability cannot be detected either morphologically or serologically. A phage in this infectious state, which is transmitted to daughter bacterial cells during division, is known as a prophage. Similar to other characteristics of a bacterial cell, the presence of a prophage is heritable. Because all progeny of a lysogenic bacterium are also lysogenic, the prophage must obviously replicate synchronously and regularly along with the bacterial chromosome (see Fig. 10.5).

Lysogenic bacteria are immune (resistant) to infection by phages that are already present within them as prophages. This phage-mediated Immunity is not due to an inability to adsorb phage particles (unlike resistance to virulent phages), but rather to the synthesis of a specific cytoplasmic repressor protein that prevents vegetative phages from multiplying. This same repressor prevents the prophage from reverting to a vegetative state and inhibits the synthesis of phage Proteins. Thus, the ESTABLISHMENT OF THE lysogenic state is directly associated with The production of this repressor protein.

Lysogenic bacteria rarely undergo spontaneous lysis in the absence of external factors. However, a variety of agents—such as UV irradiation, mitomycin C, and alkylating agents—can induce prophage development in individual cells, leading to the production and release of infectious phage. This induction is evidently triggered by the inactivation or destruction of repressor molecules.

How is the prophage associated with the bacterial chromosome? This question was answered by studies conducted with phage λ (lambda), which is lysogenic for Escherichia coli K-12. The chromosome length of phage λ is only 2% of the bacterial chromosome. Research has shown that in lysogenic cells, the prophage is tightly bound to the host chromosome at a specific attachment site. Genetic studies demonstrated that during lysogenization, the phage does not merely attach to the bacterial DNA, but actually incorporates into it; in other words, the prophage DNA becomes integrated into the bacterial DNA.

In the integrated state, the phage DNA replicates alongside the bacterial DNA and undergoes the same regulatory controls as bacterial Chromosome replication (see Fig. 10.5). The Genetic information encoded in the phage DNA remains unexpressed during this time. Only when the prophage transitions into the vegetative state is the autonomy of the phage DNA restored (transitioning to an autonomous state), allowing phage replication to commence. The excision of the phage DNA from the bacterial chromosome is extremely precise, occurring at the exact same site where integration took place. Only on rare occasions (about one in 100,000 cases) does aberrant excision occur.

As soon as the prophage transitions to the vegetative state via excision, it regains its autonomy and can replicate within the bacterial cell like a virulent phage. Thus, excision ultimately leads to bacterial lysis and the release of progeny phage.