Bacteriophages
1.1 What is a bacteriophage?
1.2 Bacteriophages in the biosphere
1.3 Uses for bacteriophages
1.4 Bacteriophage diversity
1.1 What is a Bacteriophage?
Bacteriophages, also named phages, are virus that infect and replicate within bacteria. This virus are characterized by their structural simplicity, as they can consist, only, of a DNA or RNA genome encapsulated in a protein shell, known as capsid. Some may contain major appendixes, named tails. No external lipidic membrane is observed, unlike it occurs in many animal virus. These features imply that in terms of composition, structure and morphology, phages share many fundamental properties with human viruses.
The whole viral particle is named virion
There are six types of bacteriophages which are the most common. According to their morphology and nucleic acid they are classified in:
• Myoviridae.
• Siphoviridae.
• Podoviridae.
• Microviridae.
• Inoviridae.
• Leviviridae.
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.
According to their way of infecting the cell, bacteriophages, can be classified in:
- Somatic bacteriophages
- F-specific bacteriophages
Video content: Bacteriophages as indicators
1.2 Bacteriophages in the biosphere
Bacteriophages are the most abundant living organisms over 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 serve 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 stablishes itself as one of the dominants in the environment. This is known as the kill the winner hypothesis, which says that the 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 to the media making it possible for other bacteria to re-use these components. This role is very important in the oceans where nutrients are usually scarce.
Endowing variability to bacterial population
Bacteriophages play a major role in gene transfer between bacteria. This is achieved through a process known as transduction. The genetic material transfer can happen due to the fact that cell DNA can be incorporated in 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 of 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 past century was founded on the study of phage structure and genetics carried out through 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 the molecular biology.
The technique of 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.
Bacteriophages as indicators
Studies worldwide support the value of phages as practical and economic tools for monitoring the safety of water supplies and 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).
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