Notes

Bacteriophage, also called phage or bacterial virus , any of a group of viruses that infect bacteria and replicates within it. They are among the most common and diverse entities in the biosphere. Bacteriophages were discovered independently by Frederick W. Twort in Great Britain (1915) and Félix d’Hérelle in France (1917). D’Hérelle coined the term bacteriophage, meaning “bacteria eater,” to describe the agent’s bacteriocidal ability. Bacteriophages also infect the single-celled prokaryotic organisms known as archaea. The two scientists observed that bacterial colonies sometimes dissolved and disappeared because their component cells Lysed, or burst. More interesting had an observation that this lytic effect could be transmitted from colony to colony. Even highly diluted material from a lysed colony that had been passed through a bacterial filter could lyse other bacteria. However, heating this filtrate destroyed its lytic property. From this observation, Twort cautiously suggested that the lytic agent might be a filterable infectious agent.


When d’Herelle discovered his phenomenon (hence the term Twort-d’herelle phenomenon), he coined the word Bacteriophage, which means “bacteria eater”. He concluded that the filterable agent was an invisible entity- a virus- that was parasitic toward 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. It’s estimated there are more than 1031 bacteriophages on the planet, more than every other organism on Earth, including bacteria.

Phages are widely distributed in locations populated by bacterial hosts, such as soil or the intestines of animals. One of the densest natural sources for phages and other viruses is sea water in where they have been found in microbial mats at the surface, and up to 70% of marine bacteria may be infected by them. They have been also used for over 90 years as an alternative to antibiotics in the former Soviet Union and Central Europe, and in France. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria. Nevertheless, phages of Inoviridae have been shown to complicate biofilms involved in pneumonia and cystic fibrosis, and shelter the bacteria from drugs meant to eradicate disease and promote persistent infection. In 1896, Ernest Hanbury Hankin reported that something in the waters of the Ganges and Yamuna rivers in India had marked antibacterial action against cholera and could pass through a very fine porcelain filter. In 1915, British bacteriologist Frederick Twort, superintendent of the Brown Institution of London, discovered a small agent that infected and killed bacteria. He believed the agent must be one of the following:

1) a stage in the life cycle of the bacteria 2) an enzyme produced by the bacteria themselves 3) a virus that grew on and destroyed the bacteria. Twort's work was interrupted by the onset of World War I and shortage of funding. Independently, French-Canadian microbiologist Félix d'Hérelle, announced on 3 September 1917, that he had discovered "an invisible, antagonistic microbe of the dysentery bacillus". For d’Hérelle, there was no question as to the nature of his discovery: "In a flash I had understood: what caused my clear spots was in fact an invisible microbe … a virus parasitic on bacteria." D'Hérelle called the virus a bacteriophage or bacteria-eater (from the Greek phagein meaning to eat). He also recorded a dramatic account of a man suffering from dysentery who was restored to good health by the bacteriophages. It was D'Herelle who conducted much research into bacteriophages and introduced the concept of phage therapy.

In 1969, Max Delbrück, Alfred Hershey and Salvador Luria were awarded the Nobel Prize in Physiology or Medicine for their discoveries of the replication of viruses and their genetic structure.