Nature and Significance of Bacteriophages
Bacteriophages are a unique category of viruses whose sole target is prokaryotic cells. Unlike viruses that cause diseases in higher organisms, phages are unable to enter plant, fungal, or animal cells. Their natural habitat is closely associated with sites where bacteria accumulate. In particular, a huge number of bacteriophages permanently inhabit the intestines of humans and various animals.
From a biological and medical standpoint, these viruses are considered extremely beneficial. Their main function is the destruction of pathogenic bacteria. This very feature formed the basis of phage therapy—a medical approach that uses bacteriophages for the targeted treatment of severe bacterial infections. Today, phage-based drugs are actively used to combat the causative agents of dysentery, typhoid fever, and cholera.
Complex Structure of the Virion
The architecture of a bacteriophage is highly complex and exhibits what is known as a mixed type of symmetry. The viral particle visually resembles a microscopic syringe and consists of two main parts: the head and the tail apparatus.
- The head has icosahedral symmetry (polyhedral shape). It is a unique safe box inside of which the virus's genetic material (nucleic acid) is tightly packed.
- The tail has helical symmetry and is a complex apparatus for attachment and injection of the genome. It includes several structural elements:
- Collar, connecting the head to the tail section.
- Hollow cylindrical process, serving as a channel for the passage of genetic material.
- Sheath, consisting of contractile proteins. It closely surrounds the cylindrical process and is capable of contracting.
- Base plate, located at the very end of the tail.
- Tail fibers, extending from the base plate and necessary for recognizing receptors on the bacterial surface.
Stages of Replication in the Host Cell
The reproduction (replication) process of a bacteriophage is strictly ordered and leads to the inevitable death of the infected bacterium. The life cycle includes the following sequential stages:
- I. Adsorption. The viral particle uses its tail fibers to recognize specific receptors and firmly attaches to the surface of the target cell.
- II. Injection. The contractile protein sheath contracts, and the hollow cylindrical process punctures the cell envelope. The viral nucleic acid is injected directly into the cytoplasm of the bacterium (bypassing the cell wall and periplasmic space).
- III. Replication. Once inside, the phage genetic material hijacks the cell's systems, triggering multiple rounds of viral nucleic acid molecule production.
- IV. Synthesis. Active production of specific viral proteins and enzymes required to build new particles begins on the ribosomes of the host cell.
- V–VI. Assembly and Lysis. New, fully formed bacteriophages are assembled from the synthesized proteins and nucleic acids. The cycle concludes with the release of numerous new viral particles, which is accompanied by the destruction (lysis) and ultimate death of the bacterial cell.
Briefly: answers to topical questions
What types of nucleic acids (DNA, RNA) can make up the genome of a bacteriophage?
The genome of bacteriophages can be represented by either DNA or RNA. Nucleic acids vary in structure and form:
- DNA can be single-stranded (e.g., in families Microviridae, Inoviridae) or double-stranded (most phages, e.g., families Myoviridae, Siphoviridae, Podoviridae). Double-stranded DNA is most commonly circular.
- RNA can be segmented double-stranded (Cystoviridae) or plus-strand RNA (Leviviridae).
How does the life cycle of virulent bacteriophages differ from that of temperate ones?
The life cycle of virulent and temperate bacteriophages differs in the type of interaction with the host cell.
| Type of bacteriophage | Possible cycle/pathway | Features |
|---|---|---|
| Virulent | Lytic (productive) | Cause a productive infection: a new generation of phages is produced, followed by lysis (death) of the bacterium. |
| Temperate | Lysogenic (integrative) or productive (lytic) | During lysogeny, the phage DNA integrates into the bacterial chromosome as a prophage, replicating synchronously with the bacterial genome; phage progeny are not produced, and lysis does not occur. Temperate phages can also follow the productive pathway, which concludes with lysis. |
What enzyme does a bacteriophage use to destroy the bacterial cell wall?
A bacteriophage destroys the bacterial cell wall using the enzyme lysozyme. This enzyme is contained in the distal part of the tail process and enzymatically dissolves a section of the cell wall to allow the nucleic acid to penetrate inside the cell.
What are the specific receptors on the surface of bacteria to which the phage tail fibers bind called?
Specific receptors (reception targets) on the bacterial cell wall can include lipopolysaccharides (LPS), lipoproteins, teichoic acids, surface proteins, and pili. Some phages (particularly small cubic ones) adsorb to sex pili (F-pili).
To remember the structure of the bacteriophage tail, use the mnemonic: "Cat Sat On Big Fence" (Collar, Sheath, Hollow cylinder, Base plate, Fibers).
Class and exam questions
Are bacteriophages capable of infecting human or animal cells?
No, bacteriophages are strictly specific. They infect exclusively bacteria and pose no threat to animal, plant, or fungal cells.
Into which part of the bacterial cell is the phage genetic material injected?
During the injection stage, the viral nucleic acid enters directly into the cytoplasm of the target bacterial cell.
Where does the synthesis of viral proteins take place?
The synthesis of viral proteins and enzymes is carried out on the ribosomes of the infected host cell, whose operation is completely hijacked by the virus.
What type of symmetry does the bacteriophage head possess?
The head, which contains the genetic material, possesses icosahedral symmetry, whereas the tail section has helical symmetry.
What else to study on this topic
- Mechanisms of receptor recognition by tail fibers
- Biochemical process of protein sheath contraction
- Characteristics of enzymes synthesized on host cell ribosomes
- Differences between lytic and lysogenic life cycles
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