Bacteriophages or phages, are bacterial viruses and therefore, they are their natural enemies.

Electron microscope images showing phage particles

The phage particle or virion has a fairly simple structure, made up of proteins and nucleic acid. Inside the icosahedral or elongated head (or capsid) is the genetic material (DNA or RNA). The head is joined by a neck to the tail of variable length; in the distal part of whihc is the basal plate with spicules and fibers.

Schematic representation of a phage particle


Phages are the most abundant biological entities on the planet and also the dominant predators in the biosphere. It is estimated that there are about 1031 viruses on Earth (10 – 100 viruses/cell). Between 106 and 109 phages/ml have been found in the sea. On the other hand, almost 30% of the proteins encoded by the human intestine metagenome are from phage origin. Consequently, it is estimated that there are around 1030 infections/day, which gives them a fundamental value in the evolution of natural ecosystems.


To carry out their life cycle, bacteriophages need a host bacterium within which they multiply. Bacteriophages can carry out two life cycles: lytic cycle and lysogenic cycle. Bacteriophages that are capable of performing both cycles are called temperate, while those that only follow a lytic cycle are called virulent.


Bacteriophages have an antimicrobial capacity that provides them great potential as alternative agents for the elimination of pathogenic bacteria, which is of special relevance in the current crisis of multi-resistance to antibiotics. However, this is only one of the many applications that phages are providing to biomedicine and biotechnology. The presence of bacteriophages in all known environments, including the human body, gives an idea of ​​the relevant role they play even in human health. The ability to modulate bacterial populations gives them a potential, still little exploited, in regulating the microbiota of living beings and industrial processes.

The study of phages, their genetic material and their ability to transfer it between their hosts is also of special interest because many of them encode virulence factors and other properties that can convert, for example, harmless bacteria into pathogens. Experimentally, phages can also be used as vectors to transfer specific properties between bacteria, using recombinant DNA methodologies. Thus, the gene pool of phages is considered a kind of common heritage that can pass from one to another, creating new combinations that will interact with the genome of their hosts and contribute to the generation of new microorganisms.

Finally, phages are excellent models for basic studies of macromolecular interactions (DNA, RNA, proteins), control of gene expression and DNA metabolism given their simplicity and ease propagation. Phages have been essential in the development of Molecular Biology and will be so in new biotechnological developments such as: (i) source of enzymes with antimicrobial activity; (ii) food preservatives, (iii) scaffolds to expose peptides; (iv) bacterial contamination detectors; (v) useful for re-evaluating lateral transfer and the Theory of Evolution; (vi) tools for Synthetic and Systems Biology; and (vii) donors of nanomolecules for biological detectors. The knowledge of its biology has made it possible to obtain tools, with a high degree of sophistication, for the genetic modification of higher organisms, or the creation of nanomaterials and nanomachines.


Frederick Twort in 1915 and Félix d’Herelle in 1917, independently discovered a new phenomenon that consisted of bacterial cultures that grew in liquid medium that disappeared if filtered residual water was added to them, which could only be interpreted as a consequence of a filterable virus, a parasite of bacteria. Félix d’Herelle called them bacteriophages, a name derived from the Greek phageton (φαγετον) or bacteria eaters. In the 1930s, phages were observed for the first time thanks to the development of the electron microscope.

Frederick Twort and Félix d´Herelle