中文摘要：

从鲫鱼肠道分离纯化获得一株细菌，编号为XA-2，对其进行形态观察，并对理化特性、16S rDNA克隆测序及系统发育进化树构建等研究．结果表明，XA-2菌株为革兰氏阴性短杆菌，可发酵葡萄糖产气；进一步采用 PCR方法克隆16S rDNA序列，测得长度为1508 bp ；对系统发育进化树分析，发现XA-2菌株与温和气单胞菌（Aero-monas sobria）模式菌株NCIMB 12065的亲缘关系最近，同源性达99.73％，从而鉴定XA-2菌株为温和气单胞菌．采用27种抗生素进行药敏试验，结果显示该菌株对头孢噻吩、头孢噻肟、头孢克肟、头孢哌酮、新霉素、诺氟沙星、复方新诺明等药物敏感，对先锋霉素Ⅳ、阿莫西林、庆大霉素、卡那霉素、红霉素等药物不敏感．

英文摘要：

A bacterial strain was isolated from the intestine of crucian carp and named XA-2 .Its morphological , physiological and biochemical characteristics ,16S rDNA sequencing and analysis of phylogenetic tree were carried out in this study .The results showed that XA-2 strain was a short ,gram-negative bacillus and it can produce gas from glucose .Furthermore ,PCR method was applied to amplify 16S rDNA gene of the isolated strain ,and a 1508 bp DNA fragment of XA-2 strain was obtained;the phylogenetic tree analysis showed that XA-2 strain was most closely related to Aeromonas sobria（NCIMB 12065） ,the sequence homology was 99.73% ,taken together ,the strain XA-2 was identified as Aeromonas sobria .The antibiotic sensitivity analysis showed that XA-2 was sensitive to ceftofur ,cefotaxime ,cefixime ,cefoperazone ,neomycin ,norfloxacin ,trimethoprim ,and resistant to cephaloth-in ,amoxicillin ,gentamicin ,kanamycin ,erythromycin .