| Due to the high quality and nutrient-rich caviar,sturgeon is regarded as one of the most important economic fish in the world,and the artificial breeding industry has also developed rapidly.China is the largest sturgeon farming country all over the world,however,sturgeon diseases caused by Streptococcus iniae(S.iniae)has frequently broken out in recent years,causing serious loss.Intestine is an important digestive organ of fish and also is an important component of mucosal immune system.However,the systematic research on intestinal barrier damage and flora structure after S.iniae infection is still lack.In this study,Siberian sturgeon(Acipenser baerii)was taken as the experimental object.The digestive tract structure of Siberian sturgeon was oberved through physiological anatomy.It was found that the digestive tract mainly consists of the oropharyngeal cavity,oesophagus,cardiac stomach,pyloric stomach,pyloric caeca,duodenum,valvula intestine,rectum and cloaca.Then,a pathogenic strain(B1805014)S.iniae which was isolated from intestinal inflammation sturgeon was anal perfusion in different groups(1.0×10~6CFU/m L,1.0×10~7 CFU/m L,1.0×10~8 CFU/m L S.iniae and 0.01M PBS)to observe intestinal anatomical changes and survival rate.Then,anal perfusion infection of 1.0×10~7～1.0×10~8CFU/m L S.iniae was found to cause intestinal inflammatory symptoms of Siberian sturgeon.Further,2.0×10~7CFU/m L S.iniae was used to infect Siberian sturgeon,the intestinal inflammatory symptoms was evaluated by Disease activity index(DAI)for swimming,feeding rate and intestinal damage(red inflammation,intestinal inflammation,abdominal dropsy and percentage of intestinal mucosal lesions)at different times.Then,the DAI index increased significantly at 3-10 day,and the death of Siberian sturgeon was concentrated at 4-6 day.Combined with typical changes of intestinal inflammation and development of the disease,the infection was divided into three periods:pre-death stage(1-3 d),death stage(4-6 d),and post-death stage(7-10 d).Based on the intestinal infection model constructed by S.iniae,Siberian sturgeons were divided into S.iniae infection group and PBS control group.Intestinal pathological damage of Siberian sturgeon was studied through the detection of intestinal Myeloperoxidase(MPO),observation of histopathology and ultrapathology at different times after S.iniae infection.The results displayed that Myeloperoxidase(MPO)activity of valvula intestine was significantly increased during the death stage(4-6 d),and this of duodenal was not obvious,illustrating that the neutrophil response mainly occurred in the valve intestine.Histopathology showed that the intestinal submucosa was edematous,the inflammatory cell was infiltrative,the mucus cells of intestinal wall increased,the mucin was secreted and microvilli epitheliumpartially was covered with mucin at the 1-3 day.Then,the submucosa was edema and exposed,epithelial cell was shedding and necrotis,mucus cellsmorphology were lost,mucin covered the intestinal villi surface and microvilli ruptured and fell off which showing a honeycomb shape at the 4-6 day.Subsequently,pithelial cells were hyperplastic,morphology of mucus cells gradually recovered and microvilli partly became longer at the 7-10 day.The results indicate that the intestinal pathological changes mainly manifested inflammation,which the exudative inflammation,metamorphic inflammation and proliferative inflammation were the predominant symptoms at the pre-death stage(1-3d),death stage(4-6 d)and post-death stage(7-10 d),respectively.Based on the above research,Siberian sturgeons were divided into control group(0day before infection)and S.iniae infection group(2,4,7 day after infection).Intestinal microbiota change of Siberian sturgeon was studied through the detection of intestinal microbiota of valvula intestinal content and isolation of S.iniae from each fish.The results displayed that,after S.iniae challenge,the proportion of Clostridium-sensu-stricto-1,Cetobacterium and their attached Firmicutes,Fusobacterium increased compared with control group at the pre-death stage,then decreased at the death and post-death stage compared with the pre-death stage.Moreover,compared with control group,the proportion of Plesiomonas,Bacteroides and their attached Proteobacteria,Bacteroides decreased at the pre-death stage,among them,the proportion of Proteobacteria,Plesiomonas recovered to the level of control group at the death stage,but the Bacteroides,Bacteroides did not recover.It was suggested that S.iniae could cause changes in intestinal flora of Siberian sturgeon.Futhermore,the family level of Streptococcaceae and the genus level of Lactococcusare is rare in healthy fish.But the proportion increased to 20%after the outbreak of disease,indicating that Lactococcus may play an important role in the process of homeostasis and defense against intestinal inflammation.In additional,the prevalence of S.iniae could not be detected in intestinal microbiota but could be isolated in liver and blood in various degrees.It indicated that S.iniae is not a resident of Siberian sturgeon,and the inflammatory damage of intestine is invade enteritis. |
