Genomic comparison of virulent Rickettsia rickettsii Sheila Smith and avirulent Rickettsia rickettsii Iowa

Infect Immun. 2008 Feb;76(2):542-50. doi: 10.1128/IAI.00952-07. Epub 2007 Nov 19.


Rickettsia rickettsii is an obligate intracellular pathogen that is the causative agent of Rocky Mountain spotted fever. To identify genes involved in the virulence of R. rickettsii, the genome of an avirulent strain, R. rickettsii Iowa, was sequenced and compared to the genome of the virulent strain R. rickettsii Sheila Smith. R. rickettsii Iowa is avirulent in a guinea pig model of infection and displays altered plaque morphology with decreased lysis of infected host cells. Comparison of the two genomes revealed that R. rickettsii Iowa and R. rickettsii Sheila Smith share a high degree of sequence identity. A whole-genome alignment comparing R. rickettsii Iowa to R. rickettsii Sheila Smith revealed a total of 143 deletions for the two strains. A subsequent single-nucleotide polymorphism (SNP) analysis comparing Iowa to Sheila Smith revealed 492 SNPs for the two genomes. One of the deletions in R. rickettsii Iowa truncates rompA, encoding a major surface antigen (rickettsial outer membrane protein A [rOmpA]) and member of the autotransporter family, 660 bp from the start of translation. Immunoblotting and immunofluorescence confirmed the absence of rOmpA from R. rickettsii Iowa. In addition, R. rickettsii Iowa is defective in the processing of rOmpB, an autotransporter and also a major surface antigen of spotted fever group rickettsiae. Disruption of rompA and the defect in rOmpB processing are most likely factors that contribute to the avirulence of R. rickettsii Iowa. Genomic differences between the two strains do not significantly alter gene expression as analysis of microarrays revealed only four differences in gene expression between R. rickettsii Iowa and R. rickettsii strain R. Although R. rickettsii Iowa does not cause apparent disease, infection of guinea pigs with this strain confers protection against subsequent challenge with the virulent strain R. rickettsii Sheila Smith.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Intramural

MeSH terms

  • Animals
  • Antibodies, Bacterial / blood
  • Antigens, Bacterial / analysis
  • Antigens, Bacterial / genetics
  • Bacterial Outer Membrane Proteins / analysis
  • Bacterial Outer Membrane Proteins / genetics
  • Chlorocebus aethiops
  • DNA, Bacterial / chemistry
  • DNA, Bacterial / genetics
  • Female
  • Gene Expression Profiling
  • Genome, Bacterial*
  • Genomics*
  • Guinea Pigs
  • Molecular Sequence Data
  • Oligonucleotide Array Sequence Analysis
  • Polymorphism, Single Nucleotide
  • Rickettsia rickettsii / chemistry
  • Rickettsia rickettsii / genetics*
  • Rickettsia rickettsii / pathogenicity*
  • Rocky Mountain Spotted Fever / immunology
  • Rocky Mountain Spotted Fever / physiopathology
  • Rocky Mountain Spotted Fever / prevention & control
  • Sequence Analysis, DNA
  • Sequence Deletion
  • Vero Cells
  • Virulence / genetics
  • Virulence Factors / genetics*


  • Antibodies, Bacterial
  • Antigens, Bacterial
  • Bacterial Outer Membrane Proteins
  • DNA, Bacterial
  • Virulence Factors
  • rOmpA, Rickettsia

Associated data

  • GENBANK/CP000766