Complete genome sequencing and comparative genomic analyses of a new spotted-fever Rickettsia heilongjiangensis strain B8

Rickettsia heilongjiangensis, a tick-borne obligate intracellular bacterium and causative agent of spotted fever in China, has attracted increasing concern regarding its capability in causing human rickettsiosis. Here, we conducted a genomic analysis of a new R. heilongjiangensis strain B8 (B8) isolated from the serum of a patient who had been bitten by a Haemaphysalis longicornis tick in Anhui Province, China. The present study sought to identify exclusive genes that might be associated with the pathogenicity of B8 using comparative genomics. Specifically, the sequences of B8 were assembled into one circular chromosome of 1,275,081 bp and predicted to contain 1447 genes. Comparative genome analyses were performed based on the genome of B8 and 28 spotted fever group (SFG) rickettsial genomes deposited in NCBI. Phylogenomic analyses indicated the B8 strain was clustered within the R. heilongjiangensis species; however, a sum of 112 and 119 B8-unique genes was identified when compared with R. heilongjiangensis and R. japonica strains, respectively. Functional annotation analyses revealed that these B8-unique genes were mainly annotated to defence mechanisms, lipid transport and metabolism, cell wall/membrane/envelope biogenesis. These data indicate B8 rather represents a previously undescribed human-pathogenic SFG rickettsia lineage, which may be an intermediate lineage of R. heilongjiangensis and R. japonica. Overall, this study isolated a new strain of R. heilongjiangensis in East-Central China for the first time, and provided potential B8-unique genetic loci that could be used for the discrimination of B8 from other R. heilongjiangensis and closely related SFG Rickettsial strains.

黑龙江立克次体（Rickettsia heilongjiangensis）是一种蜱传专性胞内菌，也是我国斑点热的致病菌，其引发人类立克次体病的能力日益受到学界关注。本研究针对一株从中国安徽省被长角血蜱（Haemaphysalis longicornis）叮咬的患者血清中分离得到的新型黑龙江立克次体菌株B8（B8）开展基因组分析。本研究旨在通过比较基因组学手段，鉴定可能与B8致病性相关的特有基因。具体而言，B8的序列被组装为一条长度1,275,081 bp的环状染色体，经预测包含1447个基因。基于B8基因组与NCBI中收录的28株斑点热群（spotted fever group, SFG）立克次体基因组，本研究开展了比较基因组分析。系统基因组学分析结果显示，B8菌株聚类于黑龙江立克次体种群分支中；但分别与黑龙江立克次体、日本立克次体（R. japonica）菌株进行比对后，共鉴定出112个和119个B8特有基因。功能注释分析表明，这些B8特有基因主要富集于防御机制、脂质转运与代谢、细胞壁/膜/包膜生物发生等功能类别。上述数据表明，B8代表了一个此前未被报道的人类致病型斑点热群立克次体谱系，可能是黑龙江立克次体与日本立克次体的中间过渡谱系。综上，本研究首次在中国中东部地区分离得到一株新型黑龙江立克次体菌株，并鉴定出可用于区分B8与其他黑龙江立克次体及近缘斑点热群立克次体菌株的潜在特有遗传位点。