Bacteriophages, often referred to simply as phages, are viruses that specifically infect bacteria. They are among the most abundant and diverse biological entities on Earth, playing crucial roles in ecosystems and offering promising applications in medicine and biotechnology. Understanding the intricate structure of bacteriophages is essential for harnessing their potential in various fields.

Basic Structure of Bacteriophages

At the core of a bacteriophage is its genetic material, which can be either single-stranded or double-stranded DNA or RNA. This genetic material is enclosed within a protective protein shell known as the capsid. The capsid not only safeguards the genetic material from environmental hazards but also facilitates the attachment of the phage to bacterial hosts.

Figure 1. Structure of bacteriophage. (Mishra et al., 2024)

Capsid Structure

The capsid is the protein shell that encases the genetic material of a bacteriophage. It serves as a protective barrier, safeguarding the viral genome from environmental hazards until it reaches a suitable host. The architecture of the capsid is typically icosahedral, which provides strength and efficiency. This geometric configuration allows the capsid to withstand external pressures while maximizing internal volume for genetic material storage.

Capsids are constructed from protein subunits known as capsomers, which self-assemble into a precise geometric configuration. The arrangement of capsomers can vary among different phages, leading to diverse capsid sizes and shapes. This variation influences the phage's infectivity and host range. Some bacteriophages also have additional proteins on their capsids that play roles in host recognition and attachment, enhancing the phage's ability to infect specific bacterial cells.

Tail Fibers and Base Plate

Tail fibers and the base plate are essential components that facilitate the phage's interaction with its bacterial host. Tail fibers are elongated, flexible rods that are highly specialized to recognize unique molecular markers on target bacteria surfaces. This specificity ensures that the phage efficiently finds and attaches to its intended host, initiating the infection process.

Once the tail fibers bind to the bacterial surface, the base plate undergoes conformational changes that facilitate further steps in the infection cycle. This transformation often triggers the contraction of other components that drive subsequent steps of infection. The base plate's ability to undergo such changes underscores the complexity and precision of phage architecture.

Contractile Tails (Myoviridae)

Phages belonging to the Myoviridae family, such as the T4 phage, possess contractile tails that function like molecular syringes. The tail consists of a rigid sheath surrounding an inner tail tube. Upon attachment to a bacterial cell, the sheath contracts, driving the tail tube through the host's cell envelope and injecting the viral genome into the cell. This mechanism ensures efficient genome delivery, even in bacteria with thick peptidoglycan layers.

The contractile tail system of Myoviridae phages is highly dynamic. Structural studies using cryo-electron tomography have revealed the rearrangements that occur in the tail sheath during contraction. The robustness of this tail system allows Myoviridae phages to infect a broad range of bacterial species, making them valuable candidates for phage therapy applications

Non-Contractile Tails (Siphoviridae)

Siphoviridae phages, such as the lambda phage, have long, flexible, non-contractile tails that mediate host recognition and genome transfer through a more gradual process. The tail fibers of these phages establish initial contact with the bacterial surface, and the tail tip facilitates DNA entry. Unlike Myoviridae, which rely on mechanical force for genome injection, Siphoviridae phages utilize a diffusion-based process, where DNA transfers through the tail channel in response to osmotic gradients.

The structural flexibility of Siphoviridae tails enables adaptation to diverse receptor types, allowing for host specificity while maintaining infection efficiency. High-resolution imaging studies have shown that the length and flexibility of these tails influence their host range, with longer tails often conferring broader infectivity.

Short Tails (Podoviridae)

Podoviridae phages, such as T7, feature short, stubby tails that lack contractile elements. Their infection mechanism relies on enzymatic degradation of the bacterial cell surface to facilitate genome entry. The tail proteins of these phages often contain depolymerase or lysozyme-like activity, allowing them to breach bacterial defenses.

Podoviridae phages compensate for their short tails by employing specialized receptor-binding proteins that ensure precise attachment before enzymatic digestion begins. This streamlined infection process enables rapid DNA injection, making Podoviridae well-suited for infecting fast-growing bacterial populations.

Figure 2. The three-tailed phage families (Myoviridae, Siphoviridae and Podoviridae). (Elbreki et al., 2014)

Types of Bacteriophages Based on Structure

Bacteriophages can be classified into several types based on their structural characteristics. The most common classification divides them into three general structural groups: filamentous, icosahedral with tail, and icosahedral without tail.

Filamentous Bacteriophages

Filamentous bacteriophages, including species like Pf1, fd, and M13, possess a rod-like structure with a length of 800–2000 nm. These phages have several thousand copies of an α-helical coat protein arranged in a helical array around the single-stranded DNA. Filamentous phages are unique in that they do not lyse their host cells upon infection. Instead, they release their progeny through a continuous process, allowing the host cell to survive.

Figure 3. The Ff bacteriophage structure and virion proteins used most commonly in phage display. (A) Ff virion visualized by atomic force microscope. (B) Schematic diagram of Ff bacteriophage. (C) Ribbon representations (top and side view) of the pVIII coat protein (RCSB PDB database accession number 2cOw; arranged around bacteriophage single stranded DNA (not shown). (D) Ribbon representation of N1 and N2 domains of pIII (RCSB PDB database accession number1g3p. (Gagic et al., 2016)

Icosahedral Bacteriophages with Tail

Icosahedral bacteriophages with tails, such as T4 and T7, are characterized by their DNA stored inside an icosahedral capsid, connected to a tail structure. The tail is equipped with fibers or spikes that recognize and bind to specific receptors on the bacterial surface. This specificity allows phages to target particular bacterial strains, making them highly selective agents.

The T4 phage is a well-studied example of this group. It has a complex tadpole-shaped structure, with an elongated icosahedral head and a contractile tail. The head contains the phage's double-stranded DNA, protected by a two-layered protein wall. The tail consists of a hollow core surrounded by a contractile sheath, which drives the hollow core into the bacterial cell during infection.

Figure 4. Cryo-electron micrographic tomogram of the WX174-like phage ST-1 infecting E. coli mini cells. a–c: Slices of tomograms showing three states of the infection process. d–h, Enlarged images taken from a–c. d: The virus has attached to the outer membrane (OM). One of the pentameric spikes of an icosahedral particle has recognized a lipopolysaccharide (LPS) molecule in the outer membrane of the E. coli cell wall. e, f: After attachment, the virus extrudes a tube for DNA penetration. A tube can be seen (white arrow) crossing the periplasmic space, lodged in the outer and inner membrane (IM). g, h: After DNA has been injected into the cell, the extended tail starts to disassemble. I: Schematic model of WX174 infection. (Sun et al., 2014)

Icosahedral Bacteriophages without Tail

Icosahedral bacteriophages without tails are less common and typically infect specific bacterial hosts. These phages rely on alternative mechanisms for attaching to and entering bacterial cells. Their structure and infection process can vary significantly, reflecting adaptations to specific ecological niches and host environments.

Figure 5. (Left) X-ray crystallographic structure of a virion of Pseudoalteromonas phage PM2 at 7 Å resolution, viewed along two-fold axis of symmetry (Middle) A schematic presentation and (right) negative stain electron micrograph of Pseudoalteromonas phage PM2 particles. The bar represents 50 nm. (Corticoviridae, 2012)

Structural Variations and Evolutionary Adaptations

Bacteriophages exhibit remarkable structural diversity, reflecting their evolutionary adaptations to different bacterial hosts and environments. This diversity is evident in the size, shape, and composition of their capsids, tails, and other structural components.

Host-Phage Coevolution

The ongoing arms race between bacteria and phages drives the evolution of both parties. Bacteria develop resistance mechanisms, such as altering surface receptors or producing enzymes that degrade phage components. In response, phages evolve structural adaptations that enhance their ability to recognize and infect resistant hosts. For example, some phages acquire new receptor-binding proteins or modify their tail fibers to interact with alternative bacterial receptors.

Structural Plasticity

Some phages exhibit structural plasticity, allowing them to switch between different forms in response to environmental cues. This plasticity enables phages to adapt to changing conditions and optimize their infectivity. For example, certain phages can alter the length or composition of their tail fibers to target different bacterial hosts or evade host defenses.

Environmental Influences

Environmental factors, such as temperature, pH, and salinity, also shape phage structure and function. Phages that thrive in extreme environments, such as hot springs or deep-sea vents, often possess unique structural features that confer stability and functionality under harsh conditions. These adaptations include specialized proteins that resist denaturation and mechanisms for efficient genome delivery in low-water environments.

Figure 6. Factors influencing the coevolution of phages and bacteria. The factors can be categorized into three main groups: factors related to the presence of specific phages and bacteria, factors concerning the conditions under which coevolution occurs, and genomic factors, such as mutational load. ARD: arms race dynamics, FSD: fluctuating selection dynamics, LPS: lipopolysaccharides. (Jdeed et al., 2025)

Bacteriophages are intricate viral parasites with diverse structural features that enable them to infect and manipulate bacterial hosts. Their capsids, tails, and other components have evolved to facilitate specific interactions with bacterial surfaces, ensuring efficient genome delivery and replication. Understanding the structure of bacteriophages not only enhances our knowledge of viral biology but also unlocks their potential for various applications, from combating antibiotic-resistant infections to advancing nanotechnology.

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