Immunomodulation with Probiotics Against Aeromonas veronii in Grass Carp (Ctenopharyngodon idellus)

Abstract

Aeromonas comprisea wide range of opportunistic bacteria that cause septicemia (blood poisoning)in fish.Withexpandingaquaculture, myriad methods have been used to combat these pathologiesincludingprobiotics and plant extracts which canpotentiallyaddress these diseases without harmingthe environment. Inthis study, four strains of opportunistic pathogens were isolated from diseased fish, thenmolecular identification of 16SrDNA was performed. Results identifiedthe four strains asAeromonas veronii, Shewanella sp., Bacillus cereus, and Aeromonas salmonicida. Experimental trialsrevealed that A.veroniiwas the primary strain. Multiple virulence-related factors were discovered, includingaerolysin (Aer), cytotoxic enterotoxin (Act), and the outer membrane protein (OmpAII).Thesewere foundonly in A.veronii. LC50 in grass carp was 3.6×104CFU/mLover 96 h, whereas the safe concentration was 9.11×103CFU/mL.Grass carp were challenged with 3 differentconcentrations of A.veronii. Following a 48h incubation period they were treated for 24 h, 48 h, and 72 h with 8.1×106CFU/mLconcentrations.The expression ofInterleukin-8(IL-8)andTumor Necrosis Factor-α(TNF-α)was up-regulated in the kidney, liver, spleen, and heart using qRT-PCR. Simultaneously, enzyme activity involving Pyruvate Kinase(PK),Alkaline Phosphatase(AKP), Alanine aminotransferase (ALT),Succinate Dehydrogenase (SDH), and Lysozyme (Lsz)inserum and liver tissues was significantly up-regulated.We found that probiotics enhancedgrowth of grass carp, improvedhealth status,and was beneficial in reducing the prevalence of this disease. Subsequent to feeding mixed probiotics,IL-8 and TNF-α geneexpression was significantly lower (p>0.05)than inthe infected group,at 16 and 31 days. Expression of these genes in the liver, spleen, and intestines of healthy grass carp fed probiotics wassignificantly up-regulated in contrast to grass carp fed a basal diet (p<0.05).Our results suggestthat use of probiotics as immune stimulator may improve the control of fish diseases in aquaculture.