Classification and morphological types
Depending on the type of nucleic acid, bacteriophages are subdivided into DNA-containing (which includes the majority of known species) and RNA-containing.
Morphologically, six main types of virion structure are distinguished:
- Filamentous.
- Small without a tail.
- Cubic with a tail analogue.
- With a short tail.
- With a long non-contractile tail.
- With a long contractile tail.
The main phage families differ in genome type and shape:
| Family | NA type | Morphology |
|---|---|---|
| Myoviridae | dsDNA | Head and contractile tail |
| Siphoviridae | dsDNA | Head and long non-contractile tail |
| Podoviridae | dsDNA | Head and short tail |
| Microviridae | ssDNA | Rounded shape without tail |
| Leviviridae | plus-RNA | Spherical shape |
Spectrum of activity and types of interaction
According to the breadth of their activity spectrum, bacteriophages are divided into three groups:
- Type-specific (T-phages) — capable of interacting only with specific types of bacteria within a single species.
- Monovalent — lyse strictly a single species of bacteria.
- Polyvalent — can infect bacteria of several related species.
According to the mechanism of interaction with the host cell, phages are virulent and temperate. Virulent phages trigger the lytic cycle, which inevitably ends in the destruction (lysis) of the bacterium. Temperate phages implement the lysogenic cycle: they integrate their DNA into the bacterial genome without causing immediate lysis and cell death.
Life cycle of virulent bacteriophages
The life cycle of a virulent phage is characterized by a productive type of infection and proceeds in three main stages.
Stage I: Adsorption The phage attaches to specific receptors on the bacterial cell wall. Lipopolysaccharides (LPS), lipoproteins, teichoic acids, surface proteins, or sex pili can serve as targets for reception.
Stage II: Penetration (injection) The mechanism of nucleic acid delivery depends on virus morphology:
- Phages with a contractile sheath (the most studied type) attach to the cell by the free end of the process — the basal plate or fibrils. The sheath contracts using ATP energy. The phage releases the enzyme lysozyme, which locally dissolves the cell wall. Then the tail core mechanically "drills" through the envelope, and the DNA from the capsid is injected through the core channel into the cytoplasm. The protein capsid remains outside.
- Small cubic phages adsorb onto the bacterial sex pili and introduce their nucleic acid directly through the inner channel of the pilus.
- Filamentous phages penetrate the cell interior differently: their DNA enters the cytoplasm in a complex with one of their own capsid proteins.
Stage III: Onset of reproduction Once inside the cell, the phage nucleic acid completely suppresses the biosynthesis of the host bacterium. All cellular resources are switched to the synthesis of viral nucleic acids and proteins. The outcome of this cycle is the assembly of 200–300 new virions and complete lysis of the bacterial cell.
Ecological role: microbial antagonism
The relationship between a bacteriophage and a bacterium is a form of microbial antagonism realized as parasitism. The phage uses the bacterial cell as a temporary habitat and the sole source of resources for replication.
As with any true parasitism, two opposing processes are observed here: the pathogenic effect of the virus (suppression of host biosynthesis and lysis) and defense attempts on the part of the bacterium. At the same time, the parasite's reproductive potential is colossal: the size of the phage population significantly exceeds the host bacterial population, which is the main ecological difference between parasitism and predation.
To memorize the morphology of tailed DNA phages by family, use associations with the first letters: Myoviridae — Mighty (contractile) tail, Siphoviridae — Siphon (long non-contractile), Podoviridae — Stump (short).
Class and exam questions
How do virulent bacteriophages differ from temperate ones?
Virulent phages cause a productive infection (lytic cycle) that always ends in the death and destruction of the bacterium. Temperate phages are capable of lysogeny — they integrate their genome into the bacterial chromosome and can coexist with it for a long time without lysis.
Does the phage virion itself penetrate inside the bacterium?
In most phages (e.g., those with a tail appendage), the protein capsid remains outside on the cell wall. Only the nucleic acid is injected into the cytoplasm. An exception is filamentous phages, whose DNA penetrates in a complex with the capsid protein.
What role does lysozyme play during infection?
Lysozyme is released by phages with a contractile sheath at the penetration stage. This enzyme locally dissolves a section of the bacterial cell wall, allowing the tail core to mechanically pierce the envelope for DNA injection.
What else to study on this topic
- History of bacteriophage discovery
- Structure and chemical composition of virions
- Mechanisms of bacterial resistance to phages
- Integrative (lysogenic) type of infection
- Molecular mechanisms of microbial antagonism
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