What do Bacteriophage Diagrams Look Like? (Morphological classification of bacteriophages)

What do bacteriophages look like?

In both academic and non-academic contexts around the world, a well-shaped particle with a clearly separated head, tail (neck, sheath, base plate, and pins), and tail fibers (very perfect body) has been used to depict bacteriophages. The shape that comes to mind when someone mentions bacteriophages is not the only shape that phages can be. T-4 bacteriophages are the source of the most well-known illustration morphology. Regardless of the fact that other bacteriophages have similar or nearly identical shapes (having slightly deviations like lacking some parts or having different body parts proportions), there are some with quite a peculiar shape. Other morphologies include "pipe/thread" like structures (e.g., Inoviridae), round (e.g., Cytoviridae), and icosahedral (e.g., Leviviridae); morphologies can be used to classify bacteriophages in the absence of other parts.

T4 bacteriophage. Photo by Omar CotsFernandez for The phage

There are various theories as to why the T-phage shape is the most commonly used to graphically represent bacteriophages. Many scientists have different theories about how it came to be that way, with some claiming it was the first bacteriophage to be discovered and visualized, others claiming it's easier to study because of its distinct body parts, and still others claiming that a significant percentage of phages look like that, while others claim its shape resembles "predator (killing machine)" compared to other morphologies. The majority of the recommendations are based on their structure, which makes studying their mechanisms and availability easier (which comprises a large percentage of the phage population).

Scientists are revealing a lot of important information that we would have missed in previous decades as sophisticated improvements in science and technology positively impact research. Although the age of Antimicrobial Resistance (AMR) has reignited interest in bacteriophage research, many scientists around the world are working together to find antibiotic alternatives. Much information has been released/revealed during this time, and humanity is now becoming more acquainted with their close "ally" in the microscopic universe (bacteriophages). The less prioritized phage morphologies are now in the spotlight as well.

Bacteriophage classification by morphology (shapes)

The International Committee on Taxonomy of Viruses is in charge of virus (including phage) classification (ICTV). This body has proposed a morphology-based classification system. This version of classification is simpler than Bradley's (which will be discussed later) and contains only four groups: polyhedral or cubic, filamentous or pleomorphic phages.

Classification of bacteriophages based on morphologies as per The International Committee on Taxonomy of Viruses (ICTV)

Tailed phages (equivalents to 96% of the phages discovered)

Classified as order Caudovirales, divided into three families: Myoviridae, Siphoviridae, and Podoviridae.

P1 bacteriophage. Photo by Omar CotsFernandez for The phage

Bacteriophage family	Example
Myoviridae phages- with icosahedral heads, contractile tails, double-stranded DNA (dsDNA)	Phage T4 Phage P1
Siphoviridae phages -with icosahedral heads, long and non-contractile tails, double-stranded DNA (dsDNA)	Phage λ Lactococcus phage C2
Podoviridae phages- have an icosahedral head, short tails, double-stranded DNA (dsDNA)	Phage T7 Phage P22

Polyhedral or cubic phages

-classified into Microviridae, Corticoviridae, Tectiviridae, Leviviridae, and Cystoviridae.

Bacteriophages family	Example
Microviridae phages- icosahedral head, virion size 27 nm, with 12 capsomers, single-stranded DNA (ssDNA)	Phage φX174
Corticoviridae phages- no envelope, 63 nm in size, complex capsid, lipids, dsDNA	Phage PM2
Tectiviridae phages- no envelope, 60 nm, flexible lipid vesicle, pseudo-tail, dsDNA	Phage PRD1
Leviviridae phages- no envelope, 23 nm, poliovirus-like, ssRNA	Phage MS2
Cystoviridae phages-with enveloped, icosahedral head, 70-80 nm, lipids, dsRNA	Pseudomonas ɸ6

Filamentous phages

It-Made up of three families known as Inoviridae, Lipothrixviridae, and Rudiviridae.

Bacteriophage family	Example
Inoviridae phages- no envelope, long flexible filament or short straight rods, ssDNA	Phage M13
Lipothrixviridae phages- enveloped, rod-shaped capsid, lipids, dsDNA	Phage TTV1
Rudiviridae phages- Straight uncoated rods, TMV-like, dsDNA	Phage SIRV-1

Pleomorphic phages

Phages containing dsDNA are classified into several families: Plasmaviridae, Fuselloviridae, Guttaviridae, Bicaudaviridae, Ampullaviridae, and Globuloviridae.

Bacteriophages family	Example
Plasmaviridae phages- enveloped, 80nm, with no capsid, lipids	Phage MVL2
Fuselloviridae phages- enveloped, tapered capsid with short spikes end, lipids.	Phage SSV1
Ampullaviridae phages- enveloped, bottle-shaped virion, 230 nm in length	Phage ABV
Guttaviridae phages- droplet-shaped	Phage SNDV
Bicaudaviridae phages- Lemon-shaped virions, 120X 80 nm, long tails	Phage ATV

Many scientists came up with different ways of classifying phages. However, the most popular style was the one published by Bradley in 1967which resulted in six bacteriophage-based on six morphological groups, with the seventh group added later by other scientists.

Classification of bacteriophages based on morphologies as described by Bradley (1967)

Type A: Bacteriophage with hexagonal head and tail with contractile sheath

These viruses have a "tadpole shape," meaning they have a hexagonal head, a rigid tail with contractile sheath and tail fibers dsRNA, and T-even (T2, T4, T6) phages. Most of the phages are T-shaped.

Siphoviridae phage with a long 
non-contractile tail. Drawn by
Omar Cots Fernandez for
The Phage

Type B: Bacteriophage with a hexagonal head and long, flexible tail

Unlike Type-A, these phages contain a hexagonal head, but they lack a contractile sheath. Its tail is flexible and may or may not have tail fiber, such as dsDNA phages, e.g., T1 and T5 phages.

Type C: Bacteriophage with a hexagonal head and short, non-contractile tail

Type C is characterized by a hexagonal head and a tail shorter than the head. The tail lacks contractile sheath and may or may not have tail fiber, such as dsDNA phages, e.g., T3 and T7.

Type D: Bacteriophage with only hexagonal head in symmetry with large capsomere on it

Type D contains a head made up of capsomers but lacks a tail, for example, ssDNA phages (e.g., φX174). The capsomeres are subunits of the capsid, an outer covering of protein that protects the genetic material.

Type E: Bacteriophage with a simple regular hexagonal head

This type consists of a head made up of small capsomers but contains no tail, for example, ssRNA phages (e.g., F2, MS2).

Type F: Bacteriophage with no head but with long flexible filament virion

These phages are named for their filamentous shape, a worm-like chain, about 6 nm in diameter and about 1000-2000 nm long.

Type G: No detectable capsid (This group was added later after Bradley's original study publication)

This group has a lipid-containing envelope and has no detectable capsid, for example, a dsRNA phage, MV-L2.

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