Survival strategy of intracellular Rhodococcus equi : the role of virulence plasmid.

马红球菌的至少三个毒力水平已被确定：毒力，中间毒力和无毒。毒力强的马链球菌的特征是存在毒力相关的15- 17 kda抗原（VapA），毒力质粒DNA为85 - 90 kb，马驹出现化脓性肺炎（小鼠LD_<50>=10^6）。用毒力相关的20 kda抗原（VapB）和毒力质粒DNA （79 ~ 100 kb）鉴定了中等毒力的马瘟螨菌株，发现于猪的颌下淋巴结（小鼠LD_<50>=10^7）。为了分析马鼠毒力水平的差异，对马鼠中间毒力菌株A3的毒力质粒（pREA3）进行测序，并与毒力质粒（p33701）的DNA序列进行比较。质粒编码71个开放阅读帧（orf）。在71个orf中，发现52个与毒力质粒中的orf几乎相同（p33701）。pREA3还具有位于vapB上游的含有5个新的毒力相关蛋白（vap）基因[vapI、vapJ（2拷贝）、vapK和vapL]的致病性岛。p33701与pREA3除致病性岛外，在该地区存在广泛的同源性。这些结果提示，p33701和pREA3致病性岛的结构和功能的差异可能是导致马家鼠毒力水平差异的原因。

At least three virulence levels of Rhodococcus equi have been identified : virulence, intermediate virulence, and avirulence. Virulent R. equi is characterized by the presence of virulence-associated 15-to 17-kDa antigens (VapA), virulence plasmid DNA of 85 to 90 kb, and suppurative pneumonia in foals (mouse LD_<50>=10^6). R. equi strains of intermediate virulence are identified by a virulence associated 20-kDa antigen (VapB) and virulence plasmid DNA of 79 to 100 kb and are found in the submaxillary lymph nodes of pigs (mouse LD_<50>=10^7). To analyze the difference of virulence levels in R. equi, the virulence plasmid of the R. equi strain A3 of intermediate virulence (pREA3) was sequenced and compared with the DNA sequence of the virulence plasmid (p33701). pREA3 was 79,288 by and the plasmid encoded 71 open reading frames (ORFs). Of the 71 ORFs, 52 were found to be almost identical to those in the virulence plasmid (p33701). pREA3 also has a pathogenicity island containing 5 new virulence-associated protein (vap) genes [vapI, vapJ (2 copies), vapK, and vapL] located upstream of vapB. There was extensive homology in the region except the pathogenicity island between p33701 and pREA3. These results suggest that the difference in the structure and function of the pathogenicity islands between p33701 and pREA3 might be responsible for the difference in virulence levels of R. equi.