Correlation between plasmid content and infectivity in Borrelia burgdorferi

Infectivity-associated plasmids were identified in Borrelia burgdorferi B31 by using PCR to detect each of the plasmids in a panel of 19 clonal isolates. The clones exhibited high-, low-, and intermediate-infectivity phenotypes based on their frequency of isolation from needle-inoculated C3H/HeN mice. Presence or absence of 21 of the 22 plasmids was determined in each of the clones by using PCR primers specific for regions unique to each plasmid, as identified in the recently available genome sequence. Southern blot hybridization results were used to confirm the PCR results in some cases. Plasmid lp25 exhibited a direct correlation with infectivity in that it was consistently present in all clones of high or intermediate infectivity and was absent in all low-infectivity clones. lp28–1, containing the vmp-like sequence locus, also correlated with infectivity; all clones that lacked lp28–1 but contained lp25 had an intermediate infectivity phenotype, in which infection was primarily restricted to the joints. Plasmids cp9, cp32–3, lp21, lp28–2, lp28–4, and lp56 apparently are not required for infection in this model, because clones lacking these plasmids exhibited a high-infectivity phenotype. Plasmids cp26, cp32–1, cp32–2 and/or cp32–7, cp32–4, cp32–6, cp32–8, cp32–9, lp17, lp28–3, lp36, lp38, and lp54 were consistently present in all clones examined. On the basis of these results, lp25 and lp28–1 appear to encode virulence factors important in the pathogenesis of B. burgdorferi B31.

本研究借助聚合酶链式反应（PCR）检测19株克隆分离株的质粒携带情况，在伯氏疏螺旋体（Borrelia burgdorferi）B31中鉴定得到与感染性相关的质粒。研究人员根据这些克隆从针刺接种的C3H/HeN小鼠中的分离频率，将其划分为高、低、中等感染性三种表型。利用新近公开的基因组序列中各质粒的特有区域设计特异性PCR引物，检测了所有克隆中22种质粒的存在与否，最终确认其中21种的携带状态。部分样本通过Southern印迹杂交验证了PCR检测结果的可靠性。质粒lp25与感染性呈直接正相关：所有高或中等感染性克隆均稳定携带lp25，而所有低感染性克隆均缺失该质粒。携带vmp样序列位点的lp28–1同样与感染性相关：所有缺失lp28–1但携带lp25的克隆均表现为中等感染性表型，其感染主要局限于关节组织。质粒cp9、cp32–3、lp21、lp28–2、lp28–4及lp56似乎并非该感染模型中的必需质粒，因为缺失这些质粒的克隆仍可表现出高感染性表型。质粒cp26、cp32–1、cp32–2和/或cp32–7、cp32–4、cp32–6、cp32–8、cp32–9、lp17、lp28–3、lp36、lp38及lp54在所有受检克隆中均稳定存在。基于上述结果，lp25与lp28–1似乎编码了伯氏疏螺旋体B31致病过程中关键的毒力因子。