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Probiotics utilization in aquaculture is getting great potential due to their functional attributes. The present study elucidates the effects of dietary supplementation of probiotic Lactobacillus fermentum URLP18 on growth performance, antioxidants, haematological indices, immune regulatory gene expression and disease resistance in juveniles of Cyprinus carpio. Fish were fed for 60 days with basal diet (BD) and diet supplemented with L. fermentum URLP18 at 2 × 108 CFU/g (DI) and 2 × 106 CFU/g (DII). Dietary supplementation of the selected probiotic showed significant increases in weight gain, specific growth rate, protein efficiency ratio, feed efficiency ratio (FER) and decreases in feed conversion ratio of C. carpio fed diet containing 2 × 108 CFU/g (DI). L. fermentum URLP18‐supplemented groups showed better haematological parameters such as red blood cell, white blood cell count, haematocrit (Hct) and haemoglobin (Hb) level relative to the BD group. A significant increase in the total serum protein and lower liver enzyme profile (ALT and AST) was noticed in probiotic‐supplemented groups. Pro‐inflammatory gene expression was significantly upregulated in L. fermentum URLP18 group. Likewise, supplementation of L. fermentum URLP18 at 2 × 108 CFU/g showed resistant against Aeromonas hydrophila infection and highest survival rate (88.09%) was observed. In conclusion, diet‐supplemented L. fermentum URLP18 at 2 × 108 CFU/g showed improved diet digestibility and resulted in better growth performance and FER in C. carpio. Haematological indices and total serum protein, antioxidant gene expression profile depicted the role of L. fermentum URLP18 through enhanced non‐specific immunity and health status of C. carpio.

Aeromonas hydrophila, an inhabitant in the aquatic ecosystem is considered as an important foodborne bacterial zoonotic pathogen in aquaculture. The present study aimed to identify virulent A. hydrophila from naturally infected Channa striata in river Cauvery and in vitro evaluation of shrimp chitosan. Rimler Shotts (RS) and blood agar medium identified the presence of pathogenic Aeromonas sp. from the infected C. striata. A. hydrophila Ah17 was identified using 16S rRNA nucleotide sequence. Extracellular enzymes such as amylase, lipase, and protease were screened in A. hydrophila Ah17. Antibiotic susceptibility tests showed A. hydrophila Ah17 was highly resistant against β‐lactam, glycopeptide, macrolides, phosphonic, fucidin, and oxazolidinone classes of antibiotics. Virulent genes such as hemolysin (aer and hly), heat‐labile enterotoxin (act), cytotonic heat‐stable enterotoxin (ast), elastase (ahyB), and lipase (lip) were identified. Growth and the viable cell population of virulent A. hydrophila Ah17 were significantly reduced in a dose‐dependent manner against shrimp chitosan (CHS) from Penaeus indicus (P. indicus). Thus, the present study isolated virulent A. hydrophila Ah17 from the environmental source and characterized in vitro with shrimp chitosan.

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