Project description: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.
Project description:Rocky Mountain spotted fever (RMSF) is a life-threatening tick-borne disease caused by Rickettsia rickettsii, which is widely distributed throughout the Americas. Over 4000 cases of RMSF are recorded annually in the United States, while only around 100 cases are reported in Brazil. Conversely, while case fatality rates in the United States oscillate around 5%, in Brazil they can surpass 70%, suggesting that differences in tick vectoring capacity, population sensitivity, and/or variability in virulence of the rickettsial strains may exist. In this study, we compared the susceptibility of C3H/HeN mice to two highly virulent strains of R. rickettsii, one from the United States (Sheila Smith) and the other from Brazil (Taiaçu). Animals inoculated with the Taiaçu strain succumbed to infection earlier and exhibited severe histological lesions in both liver and spleen sooner than mice infected with the Sheila Smith strain. These differences in survival and signs of the disease are not related to a greater proliferation of the Taiaçu strain, as there were no significant differences in the rickettsial load in mice tissues inoculated with either strain. The present study is the first step to experimentally assess differences in fatality rates of RMSF in two different regions of the American continent.
Project description:Comparison of gene expression between the virulent Rickettsia rickettsii R strain and avirulent Rickettsia rickettsii Iowa. Keywords: virulent vs avirulent
Project description:Comparison of gene expression between the virulent Rickettsia rickettsii R strain and avirulent Rickettsia rickettsii Iowa. Keywords: virulent vs avirulent Virulent Rickettsia rickettsii R strain in triplicate was compared to avirulent Rickettsia rickettsii Iowa in triplicate