Objective The current investigation is intended to ascertain therole of stn gene in the pathogenesis of Salmonellosis. Methods Cloned stn gene was inactivated by inserting kanamyc in resistant gene cassette at the SfiⅠ site. The mutant stn gene was subcloned into EcoRⅠ site of suicide vector pMW1823 and the recombinant plasmid was transformed to E.coli S17-1. Construction of double crossover mutant of S. typhimurium 2000 was made by homologous recombination. The integration of the recombinant suicide vector into the chromosome of S. typhimurium was confirmed by Southern blot analysis. The capacity of wild-type Salmonella and its stn mutant for fluid accumulation response in mouse intestinal loops and fifty percent lethal dose (LD50) for mice were examined. Results Salmonella mutant evoked significantly less fluid secretion in mouse intestinal loops compared to that seen with wild-type S. typhimurium (P<0.01). Upon oral challenge of mice, the LD50 of the Salmonella stn mutants was greater than that of wild-type strains (P<0.01). The fluid secretory potent as well the LD50 for these mutants were restored when the mutated stn gene was replaced by native stn gene sequence. Conclusion The role of Salmonella enterotoxin in causing a fluid secretory response in the ligated mouse ileal loops was demonstrated by developing stn deficient-mutants of Salmonella. The capacity to express the stn gene contributed significantly to the overall virulence of S. typhimurium.%目的 研究确定肠毒素基因（enterotoxingene,stn）在鼠伤寒沙门菌致病机理中的功能和毒力作用。方法 以鼠伤寒沙门菌株2000为研究对象，在克隆的stn基因SfiⅠ位点拼接入Km抗性标记基因以形成stn基因的插入失活，以此构建带有突变stn基因的重组自杀载体质粒，使重组自杀载体质粒与菌株2000染色体的同源基因发生交换，以突变stn基因置换其野生型拷贝，造成菌株2000stn基因位点的专一突变，筛选获得同源突变菌株。利用stn基因、Km抗性基因和自杀载体作探针进行菌株染色体Southernblot杂交，检测菌株对小鼠肠腔结扎攻毒实验和小鼠口服菌株半致死量的指标。结果 菌株2000染色体上野生型stn基因已被突变stn基因取代，同源突变菌株造成小鼠肠腔分泌能力比野生菌株明显减弱（P<0.01），小鼠口服突变菌株的致死剂量较野生菌株明显增高（P<0.01）。结论 stn基因产物能够诱发小鼠肠腔液体分泌反应，stn基因是鼠伤寒沙门菌感染机制中一个较为重要的毒力因子。
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