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Analysis of the Rickettsia africae genome reveals that virulence acquisition in Rickettsia species may be explained by genome reduction

Pierre-Edouard Fournier1, Khalid El Karkouri1, Quentin Leroy1, Catherine Robert1, Bernadette Giumelli1, Patricia Renesto1, Cristina Socolovschi1, Philippe Parola1, Stéphane Audic2 and Didier Raoult1*

Author affiliations

1 Unité des rickettsies, IFR 48 CNRS UMR 6020, Faculté de médecine, Université de la Méditerranée, 27 Boulevard Jean Moulin, 13385 Marseille cedex 05, France

2 Information Génomique et Structurale, CNRS UPR2589, Institut de Biologie structurale et Microbiologie, Marseille, France

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Citation and License

BMC Genomics 2009, 10:166  doi:10.1186/1471-2164-10-166

Published: 20 April 2009



The 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.


We 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.


Based 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].