Study of Intestinal Bacteria of Cirrhinus reba and Characterization of a New Probiotic Bacteria: an Initiative to Save the Threatened Species Of Cirrhinus

Cirrhinus reba (Hamilton, 1822) has gained high recognition in southern Asia for its nutritional and commercial benefits, although the carp is now facing productivity constraints. Probiotic-based captive breeding of the carp can be a promising technique to enhance the productivity and health status of the carp. The comprehensive study was performed on bacteria in the intestinal tract of C. reba. The total load of culturable bacteria in the gut of the carp was (8.34 ± 1.64) × 10⁷ CFU/g comprising 19 different types of colonies. Most (73.7%) of the isolates were Gram-positive and rod-shaped bacteria. Among them, two intestinal isolates (PKS9 and PKS10) had shown significant antagonism against common fish pathogens namely, Staphylococcus aureus, Vibrio harveyi, and Vibrio parahaemolyticus. 16S rDNA sequencing identified the isolates PKS9 and PKS10 as Bacillus paramycoides (OM038513) and Bacillus cereus (OM033468), respectively. The isolates were determined to be biofilm formers, could persist in both acidic and alkaline environments, had high resistance to bile salts, possess surface hydrophobicity and auto-aggregation efficacy. The tests on pathogenicity and bio-safety confirmed the non-pathogenic nature of the isolates. Moreover, the strains had liberated extracellular enzymes and exhibited antioxidative properties. Hence, the isolates might be used as potential candidate probiotics for the cultivation of C. reba to enhance the nutritional and health status of the species. To the best of our knowledge, this is the first report analyzing the gut microbiota of Reba carp and establishing B. paramycoides as aquaculture probiotics.