Abstract
.Sequencing of the non-pathogenic Francisella tularensis sub-species novicida U112, and comparison with two pathogenic sub-species, provides insights into the evolution of pathogenicity in these species.
Highlights
Francisella tularensis subspecies tularensis and holarctica are pathogenic to humans, whereas the two other subspecies, novicida and mediasiatica, rarely cause disease
Few strain-specific regions were detected in this three-way comparison: the genome of U112 carries about 240 kb of sequences not found in the two other strains; the genome of Schu S4 carries 17.3 kb of strain-specific regions; and the genome of live vaccine strain (LVS) does not contain any specific regions
The average nucleotide identity (ANI) corroborates the proposition that novicida arose by diverging from an ancestor common to the subspecies tularensis and holarctica, and that the subspecies tularensis and holarctica subsequently diverged from a common ancestor [31,32]
Summary
Francisella tularensis subspecies tularensis and holarctica are pathogenic to humans, whereas the two other subspecies, novicida and mediasiatica, rarely cause disease. To uncover the factors that allow subspecies tularensis and holarctica to be pathogenic to humans, we compared their genome sequences with the genome sequence of Francisella tularensis subspecies novicida U112, which is nonpathogenic to humans. The acquisition of additional virulence factors or pathogenicity islands can alter a pathogen's virulence or host range, or both. The diseases caused by pathogenic Escherichia coli strains can take very diverse forms, depending on the virulence factors encoded in the locus of enterocyte effacement present in their genomes [3]. In addition to gain of function by gene acquisition, loss of function has been postulated to play a role in evolution toward greater pathogenicity and host adaptation. It is probable that many of the pseudogenes are merely the result of lack of selection for functions that are not needed in the host environment or of evolutionary bottlenecks [8,9,10,11]
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