猪链球菌4型疫苗用菌株的筛选

【背景】猪链球菌4型（Streptococcus suis serotype 4，SS4）分离率日益升高，对养殖业和公共卫生安全造成严重危害，目前尚无有效的SS4疫苗。【目的】筛选致病力强、抗原性好、遗传性状稳定的SS4疫苗菌种。【方法】以7株SS4分离株（代号为A1—A7）为受试菌株，通过累积法测定菌株半数致死量（LD₅₀），ELISA测定免疫小鼠血清中IgG效价，攻毒保护试验测定免疫保护率，并采集小鼠脏器观察病理组织学变化。再连续传代培养受试菌株，分别对第10、20、30代菌株进行致病性和抗原性试验。【结果】A1—A7菌株对小鼠的LD₅₀分别为2.19×10⁸、1.76×10⁸、1.83×10⁸、1.01×10⁸、4.05×10⁸、1.19×10⁸和9.03×10⁷ CFU。二免7 d后，A1、A2、A3、A4、A6、A7免疫组IgG效价分别为1：1 600、1：1 600、1：3 200、1：6 400、1：3 200和1：6 400，免疫保护率分别为30%、30%、50%、70%、60%和80%，而且A4、A7免疫组小鼠组织病变较其余4组轻微。体外传至30代后，A4菌株的LD₅₀上升至3.81×10⁸ CFU，IgG效价下降至1：1 600，免疫保护率下降至40%，而A7菌株的LD₅₀上升至2.49×10⁸ CFU，IgG效价和免疫保护率稳定保持为1：6 400和80%，而且A7免疫组小鼠的组织病变较A4免疫组轻微。【结论】A7菌株（原始编号为HBgu18-4）具有强致病力和良好的抗原性，而且遗传性状均一、稳定，可作为SS4制苗候选菌株。

Screening of strains for inactivated vaccine against Streptococcus suis serotype 4

Background] The increasing isolation rate of Streptococcus suis serotype 4 (SS4) causes serious harm to animal farming and public health, and there has been no effective vaccine for SS4 so far. Objective] The purpose of this research was to screen out SS4 vaccine strains with strong pathogenicity and desirable antigenicity and genetic stability. Methods] Seven SS4 isolates (code A1-A7) were selected as the tested strains. They were determined for the half lethal dose (LD₅₀), the IgG titer in serum of immunized mice and the immune protection rate by Reed-Muench method, enzyme-linked immunosorbent assay (ELISA) and challenge assay, respectively. Meanwhile, the histopathological changes of the mice organs were observed, and after continuous subculture, the 10th, 20th, 30th generations of the tested strains were subjected to pathogenicity and antigenicity tests. Results] The LD₅₀ of A1-A7 for the mice were 2.19×10⁸, 1.76×10⁸, 1.83×10⁸, 1.01×10⁸, 4.05×10⁸, 1.19×10⁸, and 9.03×10⁷CFU, respectively. After 7 days of secondary immunization, the IgG titers of A1, A2, A3, A4, A6, and A7 immune group were 1:1 600, 1:1 600, 1:3 200, 1:6 400, 1:3 200 and 1:6 400, respectively, and the immune protection rates were 30%, 30%, 50%, 70%, 60% and 80%, respectively, with the pathological changes of A4 and A7 immune groups being milder than the other immune groups. After in vitro passing to the 30th generation, the LD₅₀ of A4 increased to 3.81×10⁸CFU, and the IgG titer and the immune protection rate decreased to 1:1 600 and 40%. By comparison the LD₅₀ of A7 elevated to 2.49×10⁸ CFU, and the IgG titer and the immune protection rate remained stable at 1:6 400 and 80%. Additionally, the pathological changes of A7 were milder compared with those of A4. Conclusion] A7 (original number HBgu18-4) had strong pathogenicity and good antigenicity, with uniform and stable genetic traits. It could be used as a candidate strain for SS4 vaccine.