A bacteriophage is a virus that infects and replicates within bacteria. Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes. Phage replicate within bacteria following the injection of their genome into the cytoplasm. Bacteriophage are among the most common and diverse entities in the biosphere playing important roles in maintaining the natural abundance and distribution of microorganisms.

Phages are widely distributed in locations populated by bacterial hosts, such as soil or the intestines of animals. They have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Eastern Europe. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria.

Bacteriophages were discovered in 1915 by Frederick Twort. He described degenerative changes present in staphylococcal colonies isolated from calf lymph, which could be transmitted serially by application of culture filtrates from the original growth. Félix d'Hérelle in 1917 observed that filtrates of faeces culture from dysentery patients induced transmissible lysis of a broth culture of dysentery bacillus. He suggested that the lytic agent was a virus and gave it the name “Bactriophage”.

Intensive studies on the therapeutic use of phages for treating infectious diseases were taken up in 1920. Vibrio cholerae was the first bacteria against which phage therapy was tried but the activity of phage was found to be much higher in vitro than in vivo. These studies were later abandoned due to the introduction of broad spectrum antibiotics which were cheap and easy alternative.

Isolated from Western advances in antibiotic production in the 1940s, Russian scientists continued to develop already successful phage therapy to treat the wounds of soldiers in field hospitals. Russian researchers continued to develop and to refine their treatments and to publish their research and results. As a result of the development of antibiotic resistance since the 1950s and an advancement of scientific knowledge, there has been renewed interest worldwide in the ability of phage therapy to eradicate bacterial infections and chronic polymicrobial biofilm.

The importtance of phage therapy increased due to the occurance of multidrug resistant pathogens. Phage therapy has several advantages in comparison to antibiotics:

1. Bacteriophages are very specific, targeting only one or a few strains of bacteria.[26] Traditional antibiotics have more wide-ranging effect, killing both harmful bacteria and useful bacteria such as those facilitating food digestion. The specificity of bacteriophages might reduce the chance that useful bacteria are killed when fighting an infection.

2. Phages do not cause mutation of bacteria

3. Phages do not harm normal intestinal microflora. Antibiotics have side effects which can be serious but phages have been used in millions of patients without any reported side effects.

4. Phages can target against antibiotic-resistant bacteria

5. Phages can be used as disinfectant for food products without any influence on their characteristics

6. Phages can be used for surface processing in order to prevent of appearance and spread of bacterial infections in public places

In medicine today phages find many applications. They are used for typing of clinical bacterial strains for bacterial detection through labeled phages, (TB, Listeria), phage display system for vaccines, control of food pathogens and for drug and gene delivery using defective phage with targeted receptor. Phage therapy for eliminating multidrug resistant bacteria is gaining importance.