Abstract
BackgroundThe Rickettsia genus includes 25 validated species, 17 of which are proven human pathogens. Among these, the pathogenicity varies greatly, from the highly virulent R. prowazekii, which causes epidemic typhus and kills its arthropod host, to the mild pathogen R. africae, the agent of African tick-bite fever, which does not affect the fitness of its tick vector.ResultsWe evaluated the clonality of R. africae in 70 patients and 155 ticks, and determined its genome sequence, which comprises a circular chromosome of 1,278,540 bp including a tra operon and an unstable 12,377-bp plasmid. To study the genetic characteristics associated with virulence, we compared this species to R. prowazekii, R. rickettsii and R. conorii. R. africae and R. prowazekii have, respectively, the less and most decayed genomes. Eighteen genes are present only in R. africae including one with a putative protease domain upregulated at 37°C.ConclusionBased on these data, we speculate that a loss of regulatory genes causes an increase of virulence of rickettsial species in ticks and mammals. We also speculate that in Rickettsia species virulence is mostly associated with gene loss.The genome sequence was deposited in GenBank under accession number [GenBank: NZ_AAUY01000001].
Highlights
The Rickettsia genus includes 25 validated species, 17 of which are proven human pathogens
Rickettsiae are obligate intracellular Gram-negative bacteria mostly associated to arthropods, some of which causing mild to severe diseases in humans
General Features of the Genome The genome of R. africae consists of two replicons: a circular chromosome of 1,278,540 base pairs (Figure 1) and a 12,377 bp circular plasmid (Table 1, Figure 2[25,26])
Summary
The Rickettsia genus includes 25 validated species, 17 of which are proven human pathogens. The pathogenicity varies greatly, from the highly virulent R. prowazekii, which causes epidemic typhus and kills its arthropod host, to the mild pathogen R. africae, the agent of African tick-bite fever, which does not affect the fitness of its tick vector. Rickettsiae are obligate intracellular Gram-negative bacteria mostly associated to arthropods, some of which causing mild to severe diseases in humans. The typhus group (TG) includes two Rickettsia (page number not for citation purposes). BMC Genomics 2009, 10:166 http://www.biomedcentral.com/1471-2164/10/166 prowazekii (R. prowazekii) and R. typhi, and the spotted fever group (SFG) includes 15 pathogenic species and numerous species of unknown pathogenicity [2,3]. Genome reduction [8] paradoxically results in higher virulence in R. prowazekii
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