5.1 Structure and classification of bacteriophages
3 min read•august 1, 2024
Bacteriophages, viruses that infect bacteria, come in various shapes and sizes. Their structure, from protein capsids to , plays a crucial role in how they attack and infect their bacterial hosts. Understanding these structures is key to grasping phage behavior.
Classifying bacteriophages helps scientists organize and study them better. Based on their shape, genetic material, and strategies, phages are grouped into families. This classification system aids in predicting phage behavior and potential applications in research and medicine.
Bacteriophage Structure
Capsid and Genetic Material
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- Bacteriophages consist of a protein enclosing genetic material
- Capsid (head) typically in shape
- Composed of protein subunits arranged symmetrically
- Phage genetic material varies
- DNA or RNA
- Single-stranded or double-stranded
- Depends on specific phage type
- Capsid size and shape determine amount of genetic material packaged
- Affects potential for carrying additional genes
- Influences host range or virulence
Tail Structure and Components
- Most bacteriophages possess a tail structure
- Facilitates host attachment and DNA injection
- Tail includes hollow tube
- Allows genetic material injection into host cell
- Tail fibers or spikes located at tail base
- Responsible for host recognition
- Attach to specific receptors on bacterial cell surface
- Some phages have collar or neck region
- Connects head to tail
- Provides structural support
- Facilitates DNA passage during infection
- Contractile tail sheaths present in some phage types
- Assist in penetrating host cell wall during infection
- Tail structure length and flexibility influence infection efficiency
- Affects ability to penetrate host cell wall
- Impacts delivery of genetic material
Bacteriophage Classification
Morphology-based Classification
- International Committee on Taxonomy of Viruses (ICTV) classifies bacteriophages into 14 families
- Based on morphology and nucleic acid type
- Tailed phages (order Caudovirales) comprise majority of known bacteriophages
- Further divided into three families (, , )
- Morphological classifications include
- Tailed phages (most common)
- Polyhedral phages
- Filamentous phages
- Pleomorphic phages
- Tailless phages less common
- Include icosahedral and filamentous morphologies
Nucleic Acid and Genomic Classification
- Phages categorized as DNA or RNA viruses
- DNA phages more common
- Further classified as single-stranded or double-stranded
- system groups phages based on mRNA production method
- Ranges from Class I (dsDNA) to Class VII (reverse-transcribing dsDNA)
- Genomic organization and replication strategies contribute to classification
- Distinguishes between temperate and virulent phages
- DNA phage examples (T4, lambda)
- RNA phage examples (MS2, Φ6)
Bacteriophage Families
Tailed Phage Families
- Myoviridae phages
- Have contractile tails
- Typically larger in size
- Well-studied example (T4 phage)
- Siphoviridae phages
- Possess long, non-contractile tails
- Model organism (lambda phage)
- Podoviridae phages
- Have short, non-contractile tails
- Exemplified by T7 phage
Tailless and Unique Phage Families
-
- Small, tailless phages
- Single-stranded DNA genomes
- Example (ΦX174 phage)
-
- Filamentous phages
- Single-stranded DNA genomes
- Used in molecular biology applications (M13 phage)
-
- Small, spherical RNA phages
- Prominent example (MS2 phage)
-
- Unique segmented double-stranded RNA genome
- Type species (Φ6 phage)
Structure vs Host Specificity
Structural Elements Influencing Host Range
- Tail fibers and baseplate proteins crucial for host recognition and attachment
- Determine host range specificity
- Composition and arrangement of tail fiber proteins allow binding to specific receptors
- Influences host tropism
- Capsid structure affects phage particle stability
- Impacts survival in various environmental conditions
- Influences potential for infection
Infectivity and Host Interactions
- Contractile tail sheaths in Myoviridae provide additional force for DNA injection
- Potentially increases infectivity in certain host species
- Structural proteins on phage surface interact with host immune systems
- Affects phage's ability to persist and infect in complex environments
- Tail structure's length and flexibility influence penetration of host cell wall
- Impacts efficiency of genetic material delivery
- Size and shape of phage head determine amount of packaged genetic material
- Affects potential for carrying genes influencing host range or virulence
Key Terms to Review (24)
Adsorption: Adsorption is the process by which viruses, specifically bacteriophages, attach themselves to the surface of host cells prior to infection. This step is crucial as it determines the efficiency of viral entry and subsequent replication within the host. During adsorption, specific interactions between viral surface proteins and host cell receptors play a key role, setting the stage for the next phases of viral life cycles and their applications in research and biotechnology.
Baltimore Classification: The Baltimore Classification is a system that categorizes viruses based on their type of genetic material and their method of replication. This classification divides viruses into seven groups, helping scientists understand their relationships and mechanisms of infection, including how they interact with host cells and the host's immune response. By organizing viruses in this way, researchers can predict virus behavior and identify potential treatment strategies more effectively.
Biocontrol: Biocontrol, or biological control, refers to the use of living organisms, such as predators, parasites, or pathogens, to manage pest populations and diseases in agricultural systems. This approach aims to reduce the reliance on chemical pesticides, promoting a more sustainable method of crop protection that can enhance biodiversity and ecosystem health.
Capsid: A capsid is the protein shell of a virus that encases and protects its genetic material. This structure is crucial for the stability of the virus outside a host cell and plays an essential role in the viral life cycle, including attachment to host cells and delivery of the viral genome. Capsids can vary in shape and size, influencing how viruses interact with their environments and how they are classified.
Cystoviridae: Cystoviridae is a family of bacteriophages known for their unique structure and the ability to infect bacteria, particularly those of the genus Pseudomonas. These viruses are characterized by their double-stranded RNA genomes and a complex capsid, which provides protection to their genetic material. The study of Cystoviridae contributes to our understanding of viral diversity, mechanisms of infection, and the potential for biotechnological applications.
Felix d'Hérelle: Felix d'Hérelle was a French-Canadian microbiologist known for his discovery of bacteriophages, viruses that infect bacteria. His groundbreaking work in the early 20th century laid the foundation for understanding the structure and classification of these viral entities, showcasing their potential as therapeutic agents and valuable tools in research and biotechnology.
Frederick Twort: Frederick Twort was a British bacteriologist known for his discovery of bacteriophages, viruses that infect bacteria, in the early 20th century. His work laid the foundation for understanding the role of these viral entities in bacterial infections and opened new avenues for research on viral genomes and phage therapy, particularly related to DNA and RNA viral structures.
Host Specificity: Host specificity refers to the ability of a virus to infect and replicate within certain host organisms while having little or no capacity to infect others. This characteristic is crucial in understanding viral behavior, adaptation, and interactions with various biological systems, which can influence everything from viral evolution to the potential use of viruses in medical applications.
Icosahedral: Icosahedral refers to a specific geometric shape that has 20 equilateral triangular faces, 12 vertices, and 30 edges. This structure is significant in virology because many viruses adopt the icosahedral shape to maximize stability and efficiency in packaging their genetic material. The symmetry and uniformity of the icosahedral form allow viruses to effectively assemble and protect their nucleic acids, making it a common characteristic among many viral families.
ICTV Classification: The ICTV (International Committee on Taxonomy of Viruses) Classification is a comprehensive system that categorizes viruses based on their characteristics, including their morphology, genetic material, and replication strategies. This classification plays a crucial role in understanding the diversity and relationships among various viruses, aiding in the study of virology and the development of antiviral therapies.
Infection: Infection is the invasion and multiplication of pathogenic microorganisms, such as bacteria, viruses, or parasites, within a host organism. This process often leads to damage or disruption of normal cellular functions, resulting in disease. Understanding how infection occurs is crucial in exploring the structure and classification of bacteriophages, as these viruses specifically target bacterial cells to initiate the infection process.
Inoviridae: Inoviridae is a family of bacteriophages characterized by their unique filamentous structure, which consists of a single-stranded circular DNA genome encased in a long, flexible protein coat. These viruses are important in the study of microbial genetics and can be used in various biotechnological applications due to their ability to transfer genetic material between bacteria.
Leviviridae: Leviviridae is a family of bacteriophages that specifically infects bacteria, particularly those belonging to the genus Escherichia. These viruses have a simple structure, consisting of a single-stranded RNA genome and a protein coat, or capsid, which protects the genetic material. Leviviridae plays an important role in the classification of bacteriophages, showcasing the diversity of viral life and their interactions with bacterial hosts.
Lysogenic cycle: The lysogenic cycle is a method of viral reproduction in which the viral genome integrates into the host cell's DNA, allowing the virus to replicate along with the host cell without immediately causing cell death. This cycle enables the virus to persist in a dormant state, becoming a part of the host's genetic material and can later switch to the lytic cycle, where it actively produces new viruses and destroys the host cell.
Lysogenic phages: Lysogenic phages are a type of bacteriophage that can integrate their viral genome into the host bacterium's DNA, forming a stable relationship with the host. Unlike lytic phages that immediately hijack the host's cellular machinery to produce new viruses and cause cell lysis, lysogenic phages enter a dormant state, allowing the viral DNA, or prophage, to be replicated along with the bacterial DNA during cell division. This process can lead to a stable coexistence, but under certain conditions, the prophage can reactivate and enter the lytic cycle.
Lytic Cycle: The lytic cycle is a viral replication process in which a virus infects a host cell, hijacks the cell's machinery to produce new viral particles, and ultimately leads to the destruction of the host cell. This cycle results in the release of newly formed virions, which can go on to infect additional cells, making it a crucial aspect of viral propagation.
Lytic phages: Lytic phages are a type of bacteriophage that infects bacterial cells, leading to the destruction of the host cell through a process known as lysis. These phages follow a specific life cycle that results in the rapid replication of their viral components, culminating in the release of new phage particles and the death of the bacterial cell. This lytic cycle distinguishes them from lysogenic phages and underlines their significance in various fields including medicine and biotechnology.
Microviridae: Microviridae is a family of small, single-stranded DNA viruses that primarily infect bacteria, especially those in the genus Escherichia. These viruses are characterized by their icosahedral shape and relatively simple structure, making them a classic example of bacteriophages, which are crucial for understanding viral interactions with bacterial hosts and the role of viruses in ecosystems.
Myoviridae: Myoviridae is a family of bacteriophages characterized by their contractile tails and icosahedral heads. This family of viruses is known for its unique structure, which enables them to effectively infect bacterial cells by injecting their genetic material through the tail into the host. The distinct morphology and infection mechanism of Myoviridae plays a critical role in their classification among bacteriophages.
Phage Therapy: Phage therapy is the therapeutic use of bacteriophages to treat bacterial infections, utilizing these viruses that specifically infect bacteria to target and kill pathogenic strains. This approach leverages the unique structure and classification of bacteriophages, alongside their specific life cycles and replication strategies, to offer a potential alternative to traditional antibiotics, especially in an era of rising antibiotic resistance. Phage therapy has also opened up numerous applications in research and biotechnology, as well as contributing to advances in synthetic virology and genome engineering.
Podoviridae: Podoviridae is a family of bacteriophages characterized by their short, stubby tails and icosahedral capsids. This family is significant in the classification of bacteriophages, as it includes viruses that infect bacteria and play a vital role in microbial ecology and gene transfer. The unique structure and infection mechanisms of Podoviridae highlight their diversity within the larger world of bacteriophages.
Siphoviridae: Siphoviridae is a family of bacteriophages characterized by their long, non-contractile tails and an icosahedral head. These viruses primarily infect bacteria, playing an essential role in controlling bacterial populations and contributing to gene transfer through transduction.
Tail Fibers: Tail fibers are elongated protein structures found on the tail of certain bacteriophages that play a crucial role in the virus's ability to attach to and infect bacterial host cells. These fibers extend from the tail and facilitate specific interactions with receptors on the bacterial surface, ensuring successful penetration and subsequent injection of the viral genome into the host cell. The structure and composition of tail fibers can vary among different bacteriophage families, impacting their host specificity and infection mechanisms.
Transduction: Transduction is the process by which DNA is transferred from one bacterium to another via a bacteriophage, a type of virus that infects bacteria. This mechanism allows for genetic exchange and diversity among bacterial populations, playing a crucial role in horizontal gene transfer. It can result in the incorporation of new traits into the bacterial genome, impacting functions such as antibiotic resistance or metabolic capabilities.