Frederick William Twort FRS[1] (22 October 1877 – 20 March 1950) was an English bacteriologist and was the original discoverer in 1915 of bacteriophages (viruses that infect bacteria).

He researched into Johne's disease, a chronic intestinal infection of cattle, and also discovered that vitamin K is needed by growing leprosy bacteria.

In 1909, Twort became the superintendent of the Brown Animal Sanatory Institution, a pathology research centre, and remained there for the duration of his career.

If this be so, one might expect them to be constantly varying, losing old characters and gaining new ones according to the conditions under which they are grown, and it was with the object of testing this hypothesis that further series of experiments were undertaken."

Twort wrote, "It appeared highly probable that these two organisms would require the same chemical substances for building up their protoplasm, which could be elaborated from the ordinary media only by the tubercle bacillus.

"[9] Twort therefore incorporated dead tubercle bacilli in the growth medium and succeeded in culturing leprosy.

In 1914, Twort set out to identify the elusive (now known to be nonexistent) "essential substance" that would allow vaccinia virus to grow in vitro.

At the time, smallpox vaccines had to be made in the skin of calves and was almost always contaminated with the bacterial genus Staphylococcus.

He plated some of the smallpox vaccines on nutrient agar slants and obtained large bacterial colonies of several colours.

Upon closer examination of the colonies with a magnifying glass, he found minute glassy areas that would not grow when subcultured.

Further, he became interested in the Royal Army Medical Corps and actually left for Salonika, where he was in charge of the base laboratory, a few weeks after his phage paper was published.

Following the war, the recently formed Medical Research Committee (Council) supplemented Twort's salary as a university professor by an annual grant but he never was given an assistant to help with the great number of experiments he had in mind.

His prime idea was to devise conditions for the cultivation of viruses from abiotic precursors or hypothetical pre-virus forms which might exist in nature.