A bacteriophage or phage is a virus which infects a bacterium.
Bacteriophages belong to the simplest and most primitive form of life – viruses. They are the most abundant organisms in the biosphere and are a ubiquitous feature of prokaryotic existence.
They have been of interest to scientists as tools to understand fundamental molecular biology, as vectors of horizontal gene transfer and drivers of bacterial evolution, as sources of diagnostic and genetic tools and as novel therapeutic agents.
Unravelling the biology of phages and the relationship with their hosts (bacteria) is key to understanding these complex microbial systems and how we can exploit them.
How do bacteriophages work?
The bacteriophage lifecycle has two components- the lytic cycle and the lysogenic cycle. Here we briefly describe the process at a very basic level:
1) Once the phage comes into contact with the specific bacterium, the phage attaches to the surface of the cell.
2) The phage injects its genetic information into the bacterial cell when the core of the phage is driven through the bacterial wall into the membrane.
3) The virus’ genetic information is integrated into the bacterial chromosome.
4) Every time the infected cell replicates, the viral genes are replicated along with it. At this stage, the phage genes are repressed. If at any point these genes stop being repressed, the lytic cycle takes over.
3) DNA replication and protein synthesis occurs to produce early proteins which assist in replicating phage genes.
4) More proteins are produced, amongst them those which make up the virus’ structure.
5) The new viruses are assembled.
6) The cell is lysed by enzymes produced during the later stages of protein production, releasing these new phages into the surroundings.
Advantages of bacteriophages in fighting bacterial infection:
- the specificity of the phage to the exact strain of bacteria causing the infection means they don’t damage other cells in the human body or other useful bacteria which are useful to the body.
- they can kill not only one bacterium, but a whole colony quickly and effectively.
- they have been found to be able to make genetic changes to the bacteria, which makes the harmful bacteria more vulnerable to current treatments.
- the killing capacity of phages is not affected by antibiotic resistance.
- they are so small that they can penetrate biofilms to kill harmful bacteria in areas which are not easily accessible for antibiotic treatments. Such biofilms are a major defence mechanism used by bacteria against antibiotics and hence an important contributor to antibiotic resistance. Phages’ ability penetrate biofilms allows them to reproduce in the localized centres of bacterial infection and to destroy the bacteria within them, producing a strong, but highly localized therapeutic effect.