Vibrio has been widely recognized as a primary pathogen of many commercially cultured invertebrate species the world over.In order to characterize the dynamics of intestinal infection after Vibrio challenge,we investigated the pathological changes and transcriptional response in shrimp Litopenaeus vannamei midgut after bacteria immersion in multiple concentrations.The dynamic changes in intestinal morphology were explored under microscope.Changes in expression of immune genes including anti-lipopolysaccharide factor gene (ALF),penaeidin gene (Pen-4c),crustin gene (Cru),lysozyme gene (LZM) and fatty acid binding protein gene (Fabp) were quantified with qRT-PCR.In addition,the number of Vibrio in intestine and water was also monitored in time courses.To summarize the result,we found slight damage in intestinal tissue in low concentration group at 32 h,medium concentration group at 16 h and high concentration group at 8 h post infection.Severe damages were observed as dose of Vibrio increased and as infection time prolonged,such as the low and medium concentration groups after 40 h or high concentration group after 16 h.Expression level of Cru gene increased at 24 h in low and medium concentration and 16-24 h in high concentration.ALF gene increased at 24 h in low concentration,8-16 h in medium concentration and 16 h in high concentration.Pen-4c gene reached its peak value at 8 h in low and medium concentration and 8-16 h in high concentration.LZM gene was up-regulated at 24 h in low concentration and 8 h in medium and high concentration.Vibrio number in water in all three treatments decreased after infection.Cumulative bacteria in shrimp increased in intestine during 0-8 h and decreased at 32 h.To conclude,midgut damage became worse when bacteria concentration increased or the immersion time prolonged.Gut epithelial cells degraded and collapsed,mucosal folds shrank and ecclasis occurred,as well as the muscular layer was damaged.Within 40 h post infection,shrimp gut morphology deteriorated continuously,and the tissue remained unrepaired.Expression of antimicrobial peptide including ALF,Pen-4c,Cru and LZM increased and this might be used to fight against bacterial infection.However,the expression level of Fabp did not significantly increase.