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Infection and Immunity, February 2008, p. 542-550, Vol. 76, No. 2
0019-9567/08/$08.00+0     doi:10.1128/IAI.00952-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Genomic Comparison of Virulent Rickettsia rickettsii Sheila Smith and Avirulent Rickettsia rickettsii Iowa{triangledown}

Damon W. Ellison,1 Tina R. Clark,1 Daniel E. Sturdevant,2 Kimmo Virtaneva,2 Stephen F. Porcella,2 and Ted Hackstadt1*

Laboratory of Intracellular Parasites, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 59840,1 Genomics Unit, Research Technology Section, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, Montana 598402

Received 12 July 2007/ Returned for modification 3 October 2007/ Accepted 5 November 2007

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.

* Corresponding author. Mailing address: Rocky Mountain Laboratories, 903 S. Fourth St., Hamilton, MT 59840. Phone: (406) 363-9308. Fax: (406) 363-9253. E-mail: thackstadt{at}niaid.nih.gov

{triangledown} Published ahead of print on 19 November 2007.

Editor: V. J. DiRita

Infection and Immunity, February 2008, p. 542-550, Vol. 76, No. 2
0019-9567/08/$08.00+0     doi:10.1128/IAI.00952-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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