Phage therapy is the therapeutic use of bacteriophages, or short phages, to treat bacterial infections. Phages are viruses that specifically infect and kill bacteria without infecting, or further impacting animal cells. Each phage-type is able to infect and kill a narrow range of bacterial strains, which makes them suitable for development into highly precise antimicrobials. Phages have been successfully used to remove undesirable bacteria both in medicine, agriculture and other industries

Phages have been used to remove pathogenic bacteria from fish and seafood including shrimp , oysters , lobsters, sea cucumbers, salmon and cod. Data consistently demonstrate that phages cause a significant reduction in pathogen load whilst having minimal impact on the environment. Recently, ACD Pharma, a Norwegian aquaculture pharmaceutical company, launched its first phage-based product CUSTUS®yrs, which effectively uses Yersinia ruckeri-specific phages to control the contagious disease yersiniosis in Atlantic salmon. In aquaculture, phage therapy is a particularly attractive option as applying in liquid conditions suits the natural biology of phages, increasing their ability to find and kill their target bacterial hosts.

Bacteriophages are much more specific than antibiotics. They are typically harmless not only to the host organism but also to other beneficial bacteria, such as the gut microbiota, reducing the chances of opportunistic infections. Antibiotics usually kill many dfferent or all bacteria, phages are usually targeting only one specific strain of bacteria. This makes them highly specific and precise weapons against pathogenic bacteria.

Phages are particularly good at clearing biofilms, which are where bacteria grow as a static film within a matrix, rendering them impenetrable to standard cleaning agents and antibiotics, and causing a particular problem in the aquaculture industry. In many biofilm studies phages have been demonstrated to penetrate established biofilms and eradicate bacteria. Phages encoding enzymes (such as depolymerase) which can chemically degrade the extracellular polymeric substances found in biofilms, have been shown to be particularly useful for these purposes. Importantly, phages are highly effective in pre-treatment studies at preventing biofilm formation of bacteria.

Bacteriophages are only dangerous for bacteria because they need bacteria to reproduce. They specifically infect and kill bacteria without infecting, or further impacting animal cells.

As phages are highly specific, it can be difficult to find the right type of phages targeting specifically the bacteria of interest.

Bacteriophages are the most abundant organisms in the biosphere with an estimated 10,000,000,000,000,000,000,000,000,000,000 (10e31) phage particles on the planet, more than every other organism on Earth, including bacteria, combined. If we were to lay all phage particles in the world side by side, the length of that line would be about 200 million light years.

Phages have been isolated from every environment in which bacteria exist, and it is believed that at least one type of phage – and most likely many more than one – infects every strain of bacteria.