Francisella tularensis Francisella tularensis is a pathogen ic species of gram - negative bacteria and the causative agent of tularemia or rabbit fever . It is a facultative intracellular bacterium . Subspecies . This species was discovered in ground squirrels in Tulare County , California in 1911 ; " Bacterium tularense " was soon isolated by George Walter McCoy of the US Plague Lab in San Francisco and reported in 1912 . Four subspecies ( biovar s ) of " F . tularensis " have been classified . The biovar " tularensis " ( or type A ) is found predominantly in North America and is the most virulent of the four known subspecies and is associated with lethal pulmonary infections . Biovar " palearctica " ( also known as biovar " holarctica " or type B ) is found predominantly in Europe and Asia but rarely leads to fatal disease . An attenuated live vaccine strain of subspecies " palearctica " has been described , though it is not yet fully licensed by the FDA as a vaccine . Subspecies " novicida " ( previously classified as " F . novicida " ) was characterized as a relatively nonvirulent strain ; only two tularemia cases in North America have been attributed to " novicida " and these were only in severely immunocompromised individuals . The fourth , biovar " mediasiatica " , is found primarily in central Asia ; little is currently known about this subspecies or its ability to infect humans . Pathogenesis . " F . tularensis " is capable of infecting a number of small mammals such as voles , rabbits , and muskrats , as well as humans . Despite this , no case of tularemia has been shown to be initiated by human - to - human transmission . Rather , tularemia is caused by contact with infected animals or vectors such as tick s , mosquito s , and deer flies . Infection with " F . tularensis " can occur via several routes . The most common occurs via skin contact , yielding an " ulceroglandular " form of the disease . Inhalation of bacteria - particularly biovar " tularensis " , leads to the potentially lethal tularemia . While the pulmonary and ulceroglandular forms of tularemia are more common , other routes of inoculation have been described and include " oropharyngeal " infection due to consumption of contaminated food and conjunctival infection due to inoculation at the eye . " F . tularensis " is capable of surviving outside of a mammalian host for weeks at a time and has been found in water , grassland , and haystacks . Aerosols containing the bacteria may be generated by disturbing carcasses due to brushcutting or lawn mowing ; as a result , tularemia has been referred to as " lawnmower disease " . Recent epidemiological studies have shown a positive correlation between occupations involving the above activities and infection with " F . tularensis " . Life cycle . " F . tularensis " is a facultative intracellular bacterium that is capable of infecting most cell types but primarily infects macrophages in the host organism . " F . tularensis " entry into the macrophage occurs via phagocytosis and the bacterium is sequestered from the interior of the infected cell by a phagosome . " F . tularensis " then breaks out of this phagosome into the cytosol and rapidly proliferates . Eventually the infected cell undergoes apoptosis , and the progeny bacteria are released to initiate new rounds of infection . Virulence factors . The virulence mechanisms for " F . tularensis " have not been well characterized . Like other intracellular bacteria that break out of phagosomal compartments to replicate in the cytosol , " F . tularensis " strains produce different hemolytic agents , which may facilitate degradation of the phagosome . A hemolysin activity , named NlyA , with immunological reactivity to " Escherichia coli " anti - HlyA antibody was identified in biovar " novicida " . Acid phosphatase AcpA has been found in other bacteria to act as a hemolysin , whereas in Francisella its role as a virulence factor is under vigorous debate . While " F . tularensis " does not contain virulence secretion systems typical of some better - characterized pathogenic bacteria , it does contain a number of ATP binding cassette ( ABC ) proteins that may be linked to the secretion of virulence factors . In addition , " F . tularensis " uses type IV pili to bind to the exterior of a host cell and thus become phagocytosed . Mutant strains lacking pili show severely attenuated pathogenicity . The expression of a 23 - kD protein known as IglC is required for " F . tularensis " phagosomal breakout and intracellular replication ; in its absence mutant " F . tularensis " die and are degraded by the macrophage . This protein is located in a putative pathogenicity island regulated by the transcription factor MglA . " F . tularensis " , " in vitro " , downregulates the immune response of infected cells , a tactic used by a significant number of pathogenic organisms to ensure that replication is ( albeit briefly ) unhindered by the host immune system by blocking the warning signals from the infected cells . This downmodulation of the immune response requires the IglC protein , though again it is not clear what the contributions of IglC and other genes are . Several other putative virulence genes exist but have yet to be characterized for function in " F . tularensis " pathogenicity . Genetics . Like many other bacteria , " F . tularensis " undergoes asexual replication . Bacteria will divide into two daughter cells , each of which contains identical genetic information . Genetic variation may be introduced via mutation or horizontal gene transfer . The genome of " F . tularensis " biovar tularensis strain SCHU4 has been sequenced . The studies resulting from the sequencing suggest that a number of gene coding regions in the " F . tularensis " genome are disrupted by mutations and thus create blocks in a number of metabolic and synthetic pathways that are required for survival . This indicates that " F . tularensis " has evolved to depend on the host organism for certain nutrients and other processes ordinarily taken care of by these disrupted genes . The " F . tularensis " genome contains unusual transposon - like elements resembling counterparts that normally are found in eukaryotic organisms . Use as a biological weapon . When the U . S . biological warfare program ended in 1969 " F . tularensis " was one of seven standardized biological weapons it had developed .