Isolation, Probiotic Characterization and Molecular Identification of Indigenous Lactic Acid Bacteria from Freshwater Fish Gut of Latur District, Maharashtra, India

Abstract

The gastrointestinal tract of freshwater fish contains a diverse microbial population that contributes significantly to digestion, nutrient utilization, immunity, and disease resistance. The present study investigated indigenous probiotic bacteria isolated from freshwater fish species collected from river ecosystems of Latur district, Maharashtra, India. Intestinal samples obtained from Catla catla, Labeo rohita, Cyprinus carpio, Pangasius pangasius, and Oreochromis niloticus were processed under sterile conditions for microbial isolation and characterization. A total of 110 bacterial isolates were recovered, among which PRB7 exhibited the highest antibacterial activity against Aeromonas hydrophila with an inhibition zone of 20.2 ± 0.40 mm. The selected isolates demonstrated common features associated with lactic acid bacteria, including Gram-positive reaction, rod-shaped morphology, absence of motility, and catalase negativity. Functional assessment showed tolerance to acidic conditions, bile salts, sodium chloride concentration, and elevated temperature. Extracellular enzymatic activities including protease, lipase, and amylase production were also observed. Antibacterial evaluation against Aeromonas hydrophila indicated variable inhibitory activity among isolates, with PRB7 showing the strongest antagonistic effect. Molecular identification through 16S rRNA sequencing identified PRB7 as Lacticaseibacillus rhamnosus with high sequence similarity to reference strains available in public databases. The sequence was submitted to the NCBI GenBank repository under accession number PZ246130.1. These findings indicate that freshwater fish of the Latur region serve as an important source of indigenous probiotic microorganisms suitable for sustainable aquaculture applications.