中文摘要：

【目的】除了猪链球菌2型外,猪链球菌9型（SS9）也是目前流行血清型,同时也是人畜共患病原菌。前期研究发现,DNA核酸酶（Ssn A）存在于SS9毒力株中,在SS9无毒株中不存在。为明确Ssn A对SS9毒力的影响,本研究构建ssn A缺失株Δssn A,并研究其生物学功能。【方法】用穿梭质粒p SET-4s构建Δssn A,并通过斑马鱼毒力试验、HEp-2细胞黏附、猪全血存活和酶活检测等试验,评价Ssn A对SS9毒力的影响。【结果】斑马鱼毒力试验显示,Δssn A对斑马鱼毒力显著降低,半数致死量是野生株的11.2倍;Δssn A对HEp-2细胞的黏附率为野生株的60.61%;Δssn A在猪全血中的存活率为野生株的71.88%;酶活试验表明,Ssn A可降解线性和环状DNA。【结论】本研究表明SS9 Ssn A具有降解线性和环状DNA能力,该基因缺失后细菌对斑马鱼毒力、黏附HEp-2细胞能力、在猪全血中存活及分解DNA能力都显著降低,证实Ssn A是SS9的一个毒力因子。

英文摘要：

[Objective] In addition to Streptococcus suis serotype 2, Streptococcus suis serotype 9（SS9） is also a currently prevalent serotype and a zoonotic pathogen. In our previous study, SS9 DNA nuclease（SsnA） was considered as a candidate virulence factor. To clarify the impact of Ssn A on SS9 virulence, we constructed ssnA mutant（ΔssnA） and studied its biological functions. [Methods] We evaluated the virulence of wild type strain and ΔssnA in a zebrafish infection model and compared the adherence rate to HEp-2 cells, the survival rate in pig blood, and enzymatic activity between wild type stain and ΔssnA. [Results] In a zebrafish infection experiment, the 50% lethal dose value of ΔssnA was 11.2-fold higher than that of wild type strain. The adherence rate of ΔssnA to HEp-2 cells was only 60.61% of the wild strain level. The survival rate of ΔssnA in pig blood was declined to 71.88% of wild strain level. The enzymatic activity assay showed that SsnA can degrade both linear and circular DNA. [Conclusion] SsnA contributes to SS9 virulence in a zebrafish infection model, the adherence to HEp-2 cells, and the survival in pig blood. Ssn A is indeed an essential virulence factor for SS9.