In the later case, ice was the mode of transmission; however, how the ice was originally contaminated with the "F. novicida" is still unknown. 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 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.