Francisella tularensis is a pathogenic species of Gram-negative coccobacillus, an aerobic bacterium. It is nonspore-forming, nonmotile, and the causative agent of tularemia, the pneumonic form of which is often lethal without treatment. It is a fastidious, facultative intracellular bacterium, which requires cysteine for growth. Due to its low infectious dose, ease of spread by aerosol, and high virulence, F. tularensis is classified as a Tier 1 Select Agent by the U.S. government, along with other potential agents of bioterrorism such as Yersinia pestis, Bacillus anthracis, and Ebola virus. When found in nature, Francisella tularensis can survive for several weeks at low temperatures in animal carcasses, soil, and water. In the laboratory, F. tularensis appears as small rods (0.2 by 0.2 µm), and is grown best at 35–37 °C. History This species was discovered in ground squirrels in Tulare County, California in 1911. Bacterium tularense was soon isolated by George Walter McCoy (1876–1952) of the US Plague Lab in San Francisco and reported in 1912. In 1922, Edward Francis (1872–1957), a physician and medical researcher from Ohio, discovered that Bacterium tularense was the causative agent of tularemia, after studying several cases with symptoms of the disease. Later, it became known as Francisella tularensis, in honor of the discovery by Francis. The disease was also described in the Fukushima region of Japan by Hachiro Ohara in the 1920s, where it was associated with hunting rabbits. In 1938, Soviet bacteriologist Vladimir Dorofeev (1911–1988) and his team recreated the infectious cycle of the pathogen in humans, and his team was the first to create protection measures. In 1947, Dorofeev independently isolated the pathogen that Francis discovered in 1922. Hence it is commonly known as Francisella dorofeev in former Soviet countries. Classification Three subspecies (biovars) of F. tularensis are recognised (as of 2020): F. t. tularensis (or type A), found predominantly in North America, is the most virulent of the four known subspecies, and is associated with lethal pulmonary infections. This includes the primary type A laboratory strain, SCHUS4. F. t. holarctica (also known as biovar F. t. palearctica or type B) is found predominantly in Europe and Asia, but rarely leads to fatal disease. An attenuated live vaccine strain of subspecies F. t. holarctica has been described, though it is not yet fully licensed by the Food and Drug Administration as a vaccine. This subspecies lacks the citrulline ureidase activity and ability to produce