Soto Esteban, Fernandez Denise, Hawke John P
Department of Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
J Aquat Anim Health. 2009 Sep;21(3):140-9. doi: 10.1577/H08-056.1.
Fish francisellosis is an emergent disease caused by gram-negative facultative intracellular bacteria of the genus Francisella. Different strains of the bacterium have caused high mortalities in warmwater and coldwater fish species. Francisella sp. isolates from fish have been found to share more than 97% identity to the human pathogen Francisella tularensis upon 16S ribosomal RNA sequence comparison. Homologue genes of the F. tularensis intracellular growth locus (iglA*, iglB*, iglC*, and iglD*) were identified from LADL 07-285A, a clinical isolate obtained from diseased Nile tilapia Oreochromis niloticus. The iglABCD operon DNA sequence comparison revealed that Francisella LADL 07-285A had 94% identity with F. philomiragia subsp. philomiragia and 83% identity with F. tularensis subsp. novicida U112. The functions of the conserved proteins corresponding to the genes are elusive but appear to be essential for the ability of Francisella sp. to survive within macrophages and cause disease. An insertion mutation was made in the iglC* gene of LADL 07-285A by allelic exchange, and the iglC* mutant was found to be attenuated after intraperitoneal and immersion challenges in Nile tilapia. Laboratory challenge methods for inducing francisellosis in Nile tilapia were evaluated by intraperitoneal injection and immersion with serial dilutions of Francisella LADL 07-285A. The dose lethal to 50% of test fish at 40 d postchallenge was 10(-5.3) (about 1.2 X 10(3) colony-forming units/fish) by intraperitoneal injection and was 10(-1) (2.3 X 10(7) colony-forming units/mL of tank water) by immersion.
鱼类弗朗西斯菌病是一种由革兰氏阴性兼性胞内细菌弗朗西斯菌属引起的新发疾病。该细菌的不同菌株已在温水和冷水鱼类中导致高死亡率。经16S核糖体RNA序列比较发现，从鱼类分离出的弗朗西斯菌属菌株与人类病原体土拉弗朗西斯菌的同一性超过97%。从患病尼罗罗非鱼奥利亚罗非鱼分离得到的临床菌株LADL 07-285A中鉴定出土拉弗朗西斯菌细胞内生长位点（iglA*、iglB*、iglC和iglD）的同源基因。iglABCD操纵子DNA序列比较显示，弗朗西斯菌LADL 07-285A与嗜肺弗朗西斯菌亚种嗜肺弗朗西斯菌的同一性为94%，与土拉弗朗西斯菌亚种新凶手弗朗西斯菌U112的同一性为83%。与这些基因相对应的保守蛋白的功能尚不清楚，但似乎对弗朗西斯菌属在巨噬细胞内存活并致病的能力至关重要。通过等位基因交换在LADL 07-285A的iglC基因中产生了插入突变，发现iglC突变体在尼罗罗非鱼腹腔注射和浸泡攻击后毒力减弱。通过腹腔注射和用弗朗西斯菌LADL 07-285A的系列稀释液浸泡来评估在尼罗罗非鱼中诱导弗朗西斯菌病的实验室攻击方法。攻击后40天对50%试验鱼致死的剂量，腹腔注射为10(-5.3)（约1.2×10(3) 菌落形成单位/鱼），浸泡为10(-1)（2.3×10(7) 菌落形成单位/毫升养殖用水）。
