1.1 What is a Bacteriophage?
Bacteriophages, also named phages, are viruses that infect and replicate within bacteria. These viruses are characterized by their structural simplicity as they can consist of only a DNA or RNA genome encapsulated in a protein shell, known as a capsid. Some may contain major appendixes, called tails. No external lipidic membrane is observed, unlike in many animal viruses. These features imply that in terms of composition, structure and morphology, phages share many fundamental properties with human viruses.
The whole viral particle is called a virion
Note that the mentioned groups (families) are not the only ones, but they are the more commonly found in samples of any kind. The rest of the bacteriophage groups are rather rare to find. Electron microscope images of natural samples show that Siphoviridae and Myoviridae are the predominant ones.
1.2 Bacteriophages in the biosphere
Bacteriophages are the most abundant living organisms on the planet, with an estimated number of 1031 total viral particles (virions). They outnumber bacteria in most ecological niches where they have been studied (Weinbauer, 2004).
Bacteriophages have different roles in the biosphere:
Regulating bacterial populations
First, bacteriophages help to maintain the equilibrium of bacterial populations in the environment as they infect more individuals of a given bacterium when its population starts to grow and establishes itself as one of the dominant ones in the environment. This is known as the kill the winner hypothesis, which says that phages tend to infect and lyse the most abundant bacteria in the media so they can reproduce more.
A second implication is that when a bacterial cell is lysed, all of the material that it contained is released into the media, thus making it possible for other bacteria to re-use these components. This role is very important in the oceans where nutrients are usually scarce.
Providing variability to bacterial populations
Bacteriophages play a major role in gene transfer between bacteria. This is achieved through a process known as transduction. Genetic material transfer can happen due to the fact that cell DNA can be incorporated into phage particles and introduced into new bacteria by phage infection (Bott, 2014).
1.3 Uses for bacteriophages
Discovered over 100 years ago, bacteriophages have played a crucial role in the progress of biotechnology (Marks and Sharp, 2000).
Their early isolation appeared to offer the first therapy for controlling bacterial infections. The discovery of antibiotics in the 1940s eclipsed bacteriophage‐based therapies. However, with the rise of drug‐resistant pathogens, phages are being re‐assessed as the basis of new therapeutic strategies.
Their host specificity facilitated their application in the typing and identification of bacteria. Bacteriophage typing schemes, known as phage-typing, were developed for many groups of pathogenic bacteria and more recently their host specificity has been applied to the development of bacterial detection and diagnostic strategies.
The advance in molecular biology over the second half of the last century was founded on the study of phage structure and genetics carried out in the 1950s and 1960s. Restriction endonucleases, DNA polymerases and ligases which form the basis of molecular cloning were developed following studies of phage infection and at present many phage enzymes provide tools for molecular biology.
Phage display has more recently provided a powerful technique for the identification and optimization of ligands for antibodies and other biomolecules.
Bacteriophages are being evaluated as delivery vehicles for protein and nucleic acids.
In the environment they have been widely applied as tracers, as indicators of pollution and in the monitoring and validation of micro and ultrafilters.
Studies worldwide support the value of phages as practical and economic tools for monitoring the safety of water supplies along with the efficiency of water treatment and disinfection processes regarding fecal pollution as well as human and animal viruses (IAWPRC, 1991; Grabow, 2001; Jofre, 2007; Jebri et al., 2017).
1.4 Bacteriophage diversity
Bacteriophages are classified according to their morphology and type on nucleic acid. There is a great variety of combinations and consequently a wide range of bacteriophage types. However, the following six families are by far the most common among bacteriophages (Krupovic et al., 2016).
This kind of phage is characterized by having a cubic capsid (which can be elongated or icosahedral) and a long contractile tail. Their capsid diameter can vary from 65 nm up to 100 nm. As for the genetic material, the members of this group present linear double stranded (ds) DNA. A typical member would be the bacteriophage T4.
They present an icosahedral capsid and a long non-contractile tail. They usually have a capsid with a diameter between 50 nm and 70 nm. Like the Myoviridae, they have linear dsDNA. Examples of this group are phage T5 and phage lambda.phage.
In this case, the characteristic morphology of the group consists of an icosahedral capsid and a short non-contractile tail. Podoviridae usually have a width of 60 nm up to 65 nm. Just like the previous groups, they present linear dsDNA. A representative member of this group is the bacteriophage T7.
They are bacteriophages that only have an icosahedral capsid and present no tail. The diameter of each virion is around 25-30 nm. In this case, their nucleic acid is circular single stranded (ss) DNA. Within the Microviridae we can find the bacteriophage ᶲX174.
This group presents a filamentous or rod-shaped morphology which is very different from the rest of groups, as the diameter is around 7 nm but they are up to 2000 nm long. They also present circular ssDNA. A typical member of this group is the phage M13.
The Leviviridae are a phage group characterized by having an icosahedral capsid and linear ssRNA as nucleic acid. Their diameter is around 25 nm. The phage MS2 would be a representative member of this group.
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