Bacteriophages are ubiquitous in ecosystems microbial, of the largestthe oceans – at the smallest – the digestive tract of a microscopic insect, for example. We even host impressive quantities, both at the surface of our skin only in our digestive system. Bacteriophages are thus the entities the most abundant and the most diverse on Earth.
The estimated amount of 1030 bacteriophages is a dizzying number when compared to that of all the cells making up some 7 billion humans, or “only” 1022 cells! In terms of diversity, more difficult to grasp, it is estimated that it there would be at least 100 million bacteriophage genomes in nature.
This diversity is put in parallel with the very great specificity of bacteriophages, which make them viruses capable of infecting only a limited number of bacteria within the same species, at some exceptions. As host spectrum of a bacteriophage is a lot narrower than that of a narrow spectrum antibiotic like penicillin. This narrowness of bacterial spectrum can, in a therapeutic approach, be compensated by the association
of several bacteriophages in cocktail, each with complementary spectra and not
Bacteriophages to counter bacteria resistant to antibiotics
Antibiotics are less and less effective because bacteria are more and more resistant. The Institut Pasteur therefore works daily on solutions, one of which uses bacteriophages, viruses that only infect bacteria.
A bacteriophage, or phage, is a virus that infects bacteria. The phages were brought to light by the British Frederick Twort in 1915 and the French-Canadian Félix d´Hérelle in 1917.
At the time of this discovery, it was not possible to see the bacteriophage under the microscope. The researchers highlighted it by the fact that it could destroy bacteria and that it crossed the Chamberland filter, which proved that it was smaller than a bacterium. It was not until the early 1940s that this virus was seen under the electron microscope. The term bacteriophage, coined by Félix d’Hérelle, comes from the Greek and means “eater of bacteria”.
There are two kinds of bacteriophages:
- lytic phages, like phage T4, which infect the bacteria, hijack the cellular machinery to reproduce and destroy the cell by releasing dozens of new phages;
- lysogenic or temperate phages, such as phage Lambda, which insert their DNA into that of the bacterium in the form of a prophage, and can confer new properties on it (manufacture of toxins, etc.).
Use of bacteriophages in medicine and molecular biology
The phages are used for molecular biology experiments, for example as cloning vectors. The Phage group, researchers gathered around Max Delbrück, is at the origin of important discoveries which served as a basis for molecular biology. For this work, Max Delbrück, Alfred Hershey and Salvador Luria obtained the Nobel Prize in physiology or medicine in 1969.
As lytic phages destroy bacteria, they can be used to fight against bacterial infections: this is the principle of phagotherapy.
Invented by Félix d’Hérelle in Paris, phage therapy is still practiced in certain countries of the former Soviet bloc (Georgia, Russia …).
Because of the problems of resistance to antibiotics, more and more researchers are interested in phage therapy because it appears as an alternative to antibiotics. The phages have the advantage of being specific to a bacterium: unlike antibiotics which destabilize the intestinal flora, they only kill their target bacteria.