Bacteriophage Types: A Guide to the Diversity of These Viral Entities

Bacteriophages, or phages, are viruses that specifically infect bacteria. They are incredibly diverse and abundant, playing crucial roles in shaping bacterial populations, driving microbial evolution, and influencing various ecosystems. Understanding the diversity of bacteriophages is essential for grasping their ecological impact, developing phage-based therapies, and managing microbial processes in diverse environments. This comprehensive guide explores the different types of bacteriophages, highlighting their similarities and differences in structure, life cycles, and ecological roles.

3D illustration of bacteriophage.

Classification of Bacteriophages

Morphological Classification

Bacteriophages can be classified based on their morphological characteristics, which include the shape of their capsid (head) and the presence or absence of a tail. The International Committee on Taxonomy of Viruses (ICTV) recognizes several families of bacteriophages based on these features:

Caudovirales (Tailed Phages)	Myoviridae: These phages have contractile tails that function like a syringe, injecting their genetic material into bacterial cells. Examples include the T4 phage, which infects Escherichia coli.
	Siphoviridae: These phages have long, non-contractile tails that facilitate a more gradual injection of genetic material. Examples include the lambda phage, which also infects E. coli.
	Podoviridae: These phages have short, non-contractile tails and rely on other mechanisms for DNA injection. Examples include the T7 phage, which infects E. coli.
Filamentous Phages	These phages have a filamentous or rod-like structure and are typically non-lytic, meaning they do not cause the bacterial cell to burst. Examples include the M13 phage, which infects E. coli.
Icosahedral Phages	These phages have an icosahedral capsid and may or may not have a tail. They are often found in environments with high bacterial density. Examples include the PRD1 phage, which infects Enterobacteriaceae.

Genetic Classification

Bacteriophages can also be classified based on their genetic material:

DNA Phages	Double-Stranded DNA (dsDNA)	These phages have a double-stranded DNA genome. Examples include the T4 phage (Myoviridae) and lambda phage (Siphoviridae).
DNA Phages	Single-Stranded DNA (ssDNA)	These phages have a single-stranded DNA genome. Examples include the phiX174 phage, which infects E. coli.
RNA Phages	Double-Stranded RNA (dsRNA)	These phages have a double-stranded RNA genome. Examples include the phi6 phage, which infects Pseudomonas.
RNA Phages	Single-Stranded RNA (ssRNA)	These phages have a single-stranded RNA genome. Examples include the MS2 phage, which infects E. coli.

Classification of bacteriophages: Microviridae (phiX174), Inoviridae, Myoviridae (T4), Podoviridae (T7), Ackermannviridae (AG3), Siphoviridae (lambda), Corticoviridae (PM2), Tectiviridae (PRD1), Plasmaviridae (MVL2), Cystoviridae (phi6), and Leviviridae (MS2).

Life Cycle Differences

Lytic Cycle

In the lytic cycle, the phage infects the bacterial cell, hijacks its machinery to replicate its genetic material and produce new phage particles, and eventually causes the bacterial cell to lyse. This process is rapid and results in the death of the bacterial host. Lytic phages are often used in phage therapy due to their ability to kill bacterial cells.

Lysogenic Cycle

In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome and is replicated along with the bacterial DNA during cell division. The phage remains dormant within the bacterial cell until certain conditions trigger the switch to the lytic cycle. Lysogenic phages can carry genes that confer adaptive advantages to the bacterial host, such as antibiotic resistance or virulence factors.

Life cycle of bacteriophages: lytic cycle and lysogenic cycle. Figure 2. Lytic cycle, compared to lysogenic cycle.

Ecological Roles

Environmental Phages: Phages found in environmental niches such as soil, water, and air play crucial roles in regulating bacterial populations and influencing nutrient cycling. They can help maintain ecological balance by preventing any single bacterial species from dominating an ecosystem. For example, phages in marine environments contribute to the "viral shunt," where they lyse bacterial cells and release organic matter back into the water column.
Host-Associated Phages: Phages found in host-associated environments such as the human gut, skin, and respiratory tract can influence the composition and function of microbial communities. They can help maintain a healthy microbiome by controlling the abundance of specific bacterial species. For example, phages in the gut can prevent the overgrowth of pathogenic bacteria and promote the growth of beneficial microbes.

Applications of Bacteriophage Diversity

Phage Therapy: The diversity of bacteriophages allows for the development of targeted therapies against specific bacterial pathogens. Lytic phages are particularly useful for treating infections caused by antibiotic-resistant bacteria. By selecting phages that specifically infect the pathogenic bacteria, phage therapy can minimize the impact on beneficial microbiota.
Bioremediation: Phages can be harnessed for bioremediation purposes, where they are used to degrade pollutants and contaminants in the environment. By targeting specific bacterial species involved in the degradation of pollutants, phages can enhance the efficiency of bioremediation processes.
Agricultural Applications: In agriculture, phages can be used to control bacterial pathogens that infect crops, reducing the need for chemical pesticides. Phage-based biocontrol agents can target specific plant pathogens without harming beneficial microbes or the environment.
Environmental Management: Phages can play a role in environmental management and conservation by influencing bacterial populations and ecosystem processes. Understanding phage diversity and transmission dynamics can inform strategies for managing microbial processes in natural and engineered environments.

Bacteriophages exhibit remarkable diversity in their morphology, genetics, and ecological roles. Understanding the differences and similarities among different types of bacteriophages is essential for harnessing their potential in various applications, from phage therapy to environmental management. The ongoing research and technological advancements in phage biology offer exciting opportunities to explore and utilize the vast diversity of bacteriophages.

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Reference:

Dion MB, Oechslin F, Moineau S. Phage diversity, genomics and phylogeny. Nat Rev Microbiol. 2020;18(3):125-138. doi:10.1038/s41579-019-0311-5