Expression and purification of HgbA from Haemophilus ducreyi and its partial fragment for development of a Sandwich ELIS杜克雷嗜血杆菌血红蛋白受体和其部分蛋白片段的表达纯化及其多克隆抗体建立ELISA检测杜克雷嗜血杆菌的初步研究