Francisella Novicida

''Francisella novicida'' is a bacterium of the Francisellaceae family, which consist of Gram-negative pathogenic bacteria. These bacteria vary from small cocci to rod-shaped, and are most known for their intracellular parasitic capabilities. In this family, six species have been identified; however, the species ''F. novicida'' is under intense scrutiny. Though some believe it should be classified with its own species designation, others argue it should be reclassified as a subspecies under ''F. tularensis''. If it were to be classified as a subspecies, ''F. novicida'' would join the other known subspecies including ''F. t. tularensis'' (type A) and ''F. t. holarctica'' (type B). Biochemical assays for identifying ''F. tularensis'' subtypes and strains are not ideal because the results are often non-definitive and subject to variation, therefore these assays should only be considered as supplementary tests for identification of ''Francisella'' species and subspecies. Several strains of ''F. novicida'' or ''F. novicida''-like bacteria have been described, and these strains may be resolved by PCR-based methods.

Though ''F. novicida'' is considered a rare pathogen, its close relative ''F. tularensis'' is well known for causing tularemia. Unlike ''F. tularensis'', there have been no documented cases of ''F. novicida'' or F. ''novicida''-like strain transmission to humans through arthropod bites. The route of infection for the majority of human ''F. novicida'' or ''F. novicida''-'like' cases is unknown, although infected water or ice have been implicated. Some of the main symptoms associated with this infection include pneumonia, muscle pain, and fever, among many others. When reported, cases of ''F. novicida'' or ''F. novicida''-like infections are most commonly seen in immunocompromised people. Though ''F. novicida'' is rarely associated with human illness despite the close genetic relationship to ''F. tularensis'', cases have been reported in humans in the United States. For example, one case of ''F. novicida'' involved a woman in Arizona in 2009. Additionally, in 2011, three confirmed cases of ''F. novicida'' were found in a residential facility in Louisiana. In the later case, ice was the mode of transmission; however, how the ice was originally contaminated with the ''F. novicida'' is still unknown.

Classification arguments

The transfer of ''F. novicida'' to a ''Francisella tularensis'' subsp. ''novicida'' has been recommended, and many microbiologists already use this name. Results of DNA-DNA hybridization and genome sequencing experiments indicate ''F. novicida'' is genetically close to ''F. tularensis''. Also, the phenotypic differences observed are in agreement with the subspecies concept. From some scientists’ points of view, it is not consistent to have a species ''F. tularensis'' with three subspecies supported by the DNA-DNA hybridization data, but distinct by phenotypic traits and thus considered a separate species. The DNA-DNA relatedness was greater than 85%. Human or animal infections with ''F. t. novicida'' are very rare and few publications describe it, in part because it is infrequently isolated. This indicates most laboratories are not able to distinguish the two subspecies based on phenotype or metabolic requirements. For medically important organisms such as ''Francisella'', clear guidelines exist for differentiation of subspecies. According to EU guideline 2000/54/EC, which has been adopted in 27 European countries, ''F. t. tularensis'' 'F. tularensis'' (type A)is classified as a BSL 3 pathogen, while ''F. t. holarctica'' 'F. tularensis'' (type B)is classified as a BSL 2 pathogen. Eleven metabolic reactions have been found to differ in ''F. novicida'' and ''F. tularensis''. Many scientists argue this is not enough among the 98 traits to contradict ''F. novicida'' as a subspecies. It is common for bacterial species to be composed of individuals that are not identical in phenotypic traits. For example, the size of the ''Escherichia coli'' genome ranges from 4.6 to 5.7 Mb. This means the strains of a single species may differ in 20-25% of their genome without affecting their taxonomic status. Therefore, many believe more metabolic diversity is needed to distinguish the two as separate species. While all of this supports ''F. novicida'' to be classified as a subspecies, many still believe enough evidence exists to create a separate species.

When ''F. novicida'' and ''F. tularensis'' are grown, they appear to be morphologically very similar. They are both Gram-negative bacilli. Many tests have been done to try to distinguish if ''F. novicida'' and ''F. tularensis'' should be considered separate species. One of these tests involved growth on cysteine-glucose-blood agar (CGBA). ''F. tularensis'' took 2 to 7 days to appear on the CGBA, while ''F. novicida'' took only 24 hours to appear. ''F. novicida'' grows much more rapidly on CGBA than ''F. tularensis''. Another difference between the two is the virulence of ''F. novicida'' was lower. ''F. tularensis'' was highly virulent in the mice and cavies (guinea pigs) used in studies. It only took one to 10 cells of ''F. tularensis'' to kill the animal of either species, although ''F. novicida'' took 10 to 100 cells in cavies and up to a 1,000 cells in mice. The immunological differences, though, are the strongest evidence used to support the idea that ''F. novicida'' and ''F. tularensis'' are separate species. Nonliving vaccines provided no protection against the heterologous organism. However, these nonliving vaccines did provide protection against the homologous organism. The living vaccines provided protection and cross-protection. as well. No protection was demonstrated against ''F. tularensis'' when using an ''F. novicida'' vaccine in any experiment. Therefore, a fundamental difference appears to exist in the antigenic composition of the two organisms, which was also demonstrated by cross-absorption in passive cutaneous anaphylaxis test (PCAs). The ability of the given antigen to remove all reactivity from its homologous antiserum while leaving the heterologous antiserum intact indicates the lack of antigen identity. To many scientists, this is enough proof to consider ''F. novicida'' and ''F. tularensis'' as separate species. Much debate still occurs over how to classify the two organisms, and it is important for scientists to establish a species concept for this organism due to its medical relevance.

References

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Thiotrichales