Lytic vs. Lysogenic: Understanding Bacteriophage Life Cycles (2023)

Bacteriophages (phages) are obligate intracellular viruses that specifically infect bacteria. They were discovered independently by two researchers, Frederick William Twort¹at the University of London in 1915 and Félix d'Herelle²who confirmed the discovery and coined the term bacteriophage in 1917 and have been widely studied since then.

Lytic vs Lysogenic: What's the Difference?

The lytic cycle, or virulent infection, involves a virus taking over a host cell and using it to produce its viral progeny, killing the host in the process. The lysogenic cycle, or non-virulent infection, involves the virus assimilating its genome with the host cell's genome to achieve replication without killing the host.

What are the stages of the lytic cycle?

The lysis cycle consists of four phases:

1. Attachment of fago
2. invasion of bacterial cells
3. Phagenreplikation
4. The birth of a new phage

For more information on these steps, see our article:

Understanding the Lytic Cycle - what are the steps?

Figure 2.Diagram of the stages of the lytic cycle of bacteriophage.

See the lytic cycle in actionhere.

lysogenic cycle

Figure 3.Diagram of the stages of the lysogenic cycle of bacteriophage.

Transition from lysogenic to lytic

When a prophage-containing bacterium is exposed to stressors such as ultraviolet light, nutrient-poor conditions, or chemicals such as mitomycin C, the prophage can spontaneously extract itself from the host genome and enter the lytic cycle in a process called induction.

However, this process is not perfect and the prophages can sometimes leave behind parts of their DNA or take parts of the host's DNA with them when they recirculate. When they infect a new host cell, they can transfer bacterial genes from one strain to another in a process called transduction. This is a method by which antibiotic resistance genes, genes encoding toxins and superantigens, and other virulence traits can spread in a bacterial population.

Bacterial immunity to phage infection

Not all bacteria are defenseless against phage attack, as they have an "immune system" that allows them to fight back.CRISPR-Cas, which is now synonymous with genetic modification, was first proposed by Francisco Mojica as a bacterial "adaptive immune system".⁵and independent of a group from the Université Paris-Sud⁶in 2005. The CRISPR locus is an array of short repeated sequences separated by spacers with unique sequences. These spacer sequences have been found to share homology with viral and plasmid DNA, including phage. When attacked by a phage not previously encountered, new spacers are added to one side of the CRISPR, making the CRISPR a chronological record of the phage encountered by the cell and its ancestors. In response to phage invasion, CRISPR sequences are transcribed and, in cooperation with Cas proteins, target and destroy phage sequences homologous to the spacer sequences.

Phages as tools in molecular biology and genetics

The lambda phage originally isolated fromEscherichia coli, is one of the best-studied phages and has formed the basis of many genetic tools. It has been said that using phage as a tool eventually led to the development of molecular biology as a discipline.⁷🇧🇷 In the 1950s, the ability of phages to recombine with host DNA was first used to manipulate the genomes ofsalmonellaSpecies and therefore the process oftransmissionwas born⁸🇧🇷 Since then it has been used as a vehicle to move genetic material between many organisms, including genetic manipulation by fungi.⁹and even human genes. That was thanks to the humble phagehuman insulinwas produced safely and inexpensively for the first time. It has also opened applications in high-throughput clone screening and nanomaterial development¹⁰,antibacterial food treatment, As adiagnostic tooland drug discovery and delivery systems¹¹.

The ϕX174 phage became an unknowing pioneer in 1977 when, thanks to Fred Sanger and colleagues, it became the first organism to have its entire nucleotide sequence determined¹².

Phagentherapie

Before Alexander Fleming discovered antibiotics in 1928, the phage was explored as a method of treating bacterial infections. In the post-antibiotics era, the practical broad spectrum activity of antibiotic treatment has led to the abandonment of phage therapy research in most organizations. However, in many countries of the former Soviet Union lacking Western antibiotics, research into phage therapies necessarily continued. With the growing global problems ofantibiotic resistancethe field of phage therapy has experienced an upswing in recent years. Although phages are capable of infecting and destroying bacteria, they have been used successfully to treat life-threatening infections¹³, their species and even strain specificity, and the potential for pre-existing immunity of some bacteria mean that phage treatment is currently not a trivial process and must be tailored to the individual infection. This makes it expensive and time consuming. Consequently, this is currently a last resort and much work is still needed in this area.

The phage family tree

With the increasing availability and accessibility of nucleotide sequencing, the number of phage genomes submitted to databases has exploded over the past two decades.¹⁴.

Phages are classified byInternational Committee on the Taxonomy of Viruses(ICTV), as of its 2017 update, there are 19 phage families infecting bacteria and archaea (Table 1), but as more samples from more remote areas are sequenced, this is likely to increase in the future.

For mobile users, scroll left and right to view the data in the table below.

Table 1.ICTV taxonomic classification of bacteriophages infecting bacteria and archaea.

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