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Extremely Low Genomic Diversity of Rickettsia japonica Distributed in Japan.

ABSTRACT: Rickettsiae are obligate intracellular bacteria that have small genomes as a result of reductive evolution. Many Rickettsia species of the spotted fever group (SFG) cause tick-borne diseases known as "spotted fevers". The life cycle of SFG rickettsiae is closely associated with that of the tick, which is generally thought to act as a bacterial vector and reservoir that maintains the bacterium through transstadial and transovarial transmission. Each SFG member is thought to have adapted to a specific tick species, thus restricting the bacterial distribution to a relatively limited geographic region. These unique features of SFG rickettsiae allow investigation of how the genomes of such biologically and ecologically specialized bacteria evolve after genome reduction and the types of population structures that are generated. Here, we performed a nationwide, high-resolution phylogenetic analysis of Rickettsia japonica, an etiological agent of Japanese spotted fever that is distributed in Japan and Korea. The comparison of complete or nearly complete sequences obtained from 31 R. japonica strains isolated from various sources in Japan over the past 30 years demonstrated an extremely low level of genomic diversity. In particular, only 34 single nucleotide polymorphisms were identified among the 27 strains of the major lineage containing all clinical isolates and tick isolates from the three tick species. Our data provide novel insights into the biology and genome evolution of R. japonica, including the possibilities of recent clonal expansion and a long generation time in nature due to the long dormant phase associated with tick life cycles.

SUBMITTER: Akter A

PROVIDER: S-EPMC5381555 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Rickettsia heilongjiangensis is the causative agent of Far-Eastern spotted fever (FESF). In Japan, a human case of FESF was identified in Sendai in Miyagi Prefecture in 2008, and R. heilongjiangensis bacteria were isolated from Haemaphysalis concinna ticks collected in the suspected geographical area of infection. Although the intraspecies genome diversity of Rickettsia has been poorly investigated, our recent analysis revealed extremely low genomic diversity of R. japonica, the agent of Japanese spotted fever, which is a close relative of R. heilongjiangensis. In this study, to investigate the genomic diversity of R. heilongjiangensis and understand the genetic relationship between Japanese and Chinese isolates, we sequenced three isolates from H. concinna ticks collected in Sendai and one isolate from a H. concinna tick collected in Inner Mongolia, China, and performed genomic comparisons between these isolates and strain 054, the type strain isolated from a Dermacentor silvarum tick in Heilongjiang Province, China. Although the three Japanese strains were isolated in 2008, 2009, and 2012, their genome sequences were identical, indicating that H. concinna ticks carrying a single R. heilongjiangensis clone have been distributed in Sendai, Japan. Among the five R. heilongjiangensis isolates, only 81 SNPs and 13 insertion/deletion sites were identified, despite the significant differences in these isolates both geographically and temporally. A significant portion of the 81 SNPs (16/81) were found to be recombinogenic. These results indicate low genomic diversity of R. heilongjiangensis, as observed in R. japonica. We further performed a detailed genomic comparison of R. heilongjiangensis and R. japonica to accurately define conserved and species-specific genes. This analysis revealed that although notable variations were found in the genomic loci encoding RelA/SpoT family proteins and tandem repeats in major surface proteins, there was only a small difference in the gene repertoire between the two species, suggesting that SNPs and small InDels are responsible for the functional or physiological differences between the two species, if present. Through this analysis, several species-specific genomic regions that can serve as ideal PCR targets for distinguishing R. heilongjiangensis and R. japonica were also identified.

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Extremely Low Genomic Diversity of Rickettsia japonica Distributed in Japan.

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