Molecular Characterization of Host-associated Bacteria in The Gut of Cyprinus carpio and in vitro Analysis of Their Probiotic Properties

Common carp (Cyrinus carpio) is a globally important aquaculture species valued for its nutritional quality. This study aimed to isolate intestinal bacteria from the gut of C. carpio, evaluate their probiotic potential, phenotypic characteristics (biochemical tests) and characterize them using molecular techniques. While only one isolate showed the highest pH tolerance, numerous isolates also exhibited strong bile tolerance. Lysozyme tolerance, phenol tolerance, and high autoaggregation ability were recorded in four different isolates. Notably, isolates three isolates showed high cell surface hydrophobicity. Antagonistic activity was demonstrated against Bacillus subtilis and Micrococcus luteus in two isolates, and against Staphylococcus aureus and M. luteus in one isolate. Strong antioxidant activity was observed in three isolates. All isolates were susceptible to neomycin and ofloxacin, but resistant to ampicillin, bacitracin, and carbenicillin. Hemolysis assays revealed α-hemolytic activity in some isolates, while others showed γ-hemolysis. 16S rRNA gene sequence analysis identified that isolate C1-1 as closely related to Klebsiella pneumoniae subsp. rhinoscleromatis, while isolates C4-1, C4-3, and C4-4 were closely related to Klebsiella quasivariicola. MALDI-TOF MS revealed that isolates belonged mainly to the genera Klebsiella, Bacillus, and Enterobacter. The results show that the different isolates exhibit probiotic potential and are therefore promising candidates for in vivo studies and use as feed additives in carp farming.