Isolation of Lactic Acid Bacteria from Intestine of Freshwater Fishes and Elucidation of Probiotic Potential for Aquaculture Application.

Probiotics play significant roles in enhancing systemic immunity, improving intestinal balance and feed value, enhancing enzymatic digestion, and inhibiting pathogenic microorganisms of freshwater fish. Probiotics from an identical organism's gastrointestinal system promote effective colonization and provide greater benefits than other sources. This study aimed to evaluate the usefulness of probiotic bacteria isolated from the intestines of freshwater fishes for a dietary supplement of freshwater aquaculture. A total of 120 isolates were collected from freshwater fishes of Channa striata, Puntius filamentosus, Oreochromis mossambicus, Cirrhinus mrigala, and Rasbora daniconius. Seven of these isolates exhibited antagonistic activity against fish pathogens: Aeromonas hydrophila, Staphylococcus epidermidis, Staphylococcus aureus, Bacillus cereus, Escherichia coli, and Pseudomonas aeruginosa. Using 16S rRNA gene sequencing analysis, the isolates were identified as Enterococcus sp., Lactococcus lactis, Weissella cibaria, and Limosilactobacillus fermentum. Of these tolerates, L. fermentum URLP18 isolated from C. mrigala exhibited high tolerance to low acidic (pH 2.0) and high bile salt (2%) concentrations, exhibiting a significant hydrophobicity and extracellular enzyme secretions like amylase, protease, and lipase. In vitro evaluations on intestinal mucus indicate that L. fermentum URLP18 have strong adherence capacity, and its survival rate increased after being administered to Artemia nauplii. The results suggest that L. fermentum URLP18 has high probiotic potential and is an effective dietary supplement for freshwater aquaculture.

Identification and characterization of virulent Aeromonas hydrophila Ah17 from infected Channa striata in river Cauvery and in vitro evaluation of shrimp chitosan.

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 ß-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.