The Study On Pathogenicity And Vaccine Of Aeromonas From Fish

Large yellow croaker Pseudosciaena crocea is a marine fish with high commercial value in southern China. Bacterial pathogens associated with this species such as vibriosis, edwardsiellosis, and pasteurellosis have been well documented, but reports of aeromonas bacterial infections were limited. The present study examined motile aeromonad septicemia in large yellow croaker and revealed two causative species Aeromonas hydrophila and A. caviae with various strains isolated from different tissues and organs. Specifically, five distinct colonies were isolated from liver (2 strains), brain (2 strains), and intestine (1 strain) of the sick fish, and their biological properties were characterized. Behaviorally, affected fish showed signs of loss appetite or stop feeding, slow movement, and swimming at the surface. Examination of moribund (alive but near the point of death) and freshly dead fish also showed petechial haemorrhages (pinpoint red spots) in the belly region, rotten muscles at the base of dorsal fin, swollen anus, and anemic liver. Artificial re-infection of health fish with bacterial live culture confirmed that these symptoms were directly associated with A. hydrophila and A. caviae.To evaluate the toxicities of A. hydrophila, A. caviae, and their extracellular products (ECPs), healthy fish were re-infected with live bacterial culture and their ECPs. Healthy larger yellow croakers and mullets (Mugil cephalus) were used as experimental animals. Injection of live bacteria included A. hydrophila and A. caviae alone, and mixture of these two bacteria, and each group was tested with three concentrations. ECPs of A. hydrophila and A. caviae were collected at day 1, 2, 3 from two culture media: basic nutritious broth (NB) and modified nutritious broth(mNB). Control fish were injected with PBS and NB or mNB. All fish were monitored for mortality over a period of 5 days. Major findings of these toxicity studies include: 1) for live bacterial infection, fish injected with the mixture of A. hydrophila & A. caviae yielded the highest average mortality (85.7%), which is significantly higher than that of A. hydrophila (P<0.000001) or ,4. caviae (P<0.005) individually, and between these two A hydrophila yielded significantly higher (P<0.005) mortality than A. caviae; 2) between live bacteria and ECPs infections, fish injected with live bacterial cultures yielded higher mortality than those injected with ECPs except for A. caviae, in which no significant difference was found between live bacteria and ECPs; 3) for ECPs infections, fish injected with the mixture of ECPs from A.