Probiotic Feed Additive from Indigenous <i>Bacillus subtilis</i> Enhances Growth and Health in Common Carp ( <i>Cyprinus carpio</i> )

Probiotics sourced from host-adapted microbes represent a sustainable innovation in aquaculture nutrition, aiming to replace antibiotics and enhance fish health. However, studies evaluating indigenous probiotic strains specifically adapted to the gut environment of target species remain limited. In this study, an indigenous <i>Bacillus subtilis</i> strain was isolated from common carp (<i>Cyprinus carpio</i>) intestine, PCR-authenticated via dual endoglucanase amplicons (545 and 1311 bp), formulated as a powdered feed additive, and tested at 1, 10, and 100 mg/kg against commercial Nutri-Fish®, Infloran Bio®, vitamin C, and a supplemented control in an 8-week feeding trial with (<i>n</i> = 126) carp. The 10 mg/kg dose significantly (<i>p</i> < 0.05) enhanced growth performance, producing the highest weight gain (133.27 ± 4.56 g), specific growth rate (2.59 ± 0.12% per day), and relative growth rate (43.09 ± 1.23%), along with improved feed conversion ratio (3.51 ± 0.09%), feed efficiency ratio (29.58 ± 1.45%), and protein efficiency ratio (475.95 ± 15.32%). Hematological analysis revealed neutrophilia (52%, <i>p</i> < 0.05), a moderated neutrophil-to-lymphocyte ratio, and stable erythron parameters, indicating primed innate immunity without systemic stress. Plasma alanine aminotransferase and aspartate aminotransferase levels decreased by 30-40% relative to other groups (<i>p</i> < 0.05), evidencing improved hepatic integrity and lipid metabolism. Intestinal histology showed moderate mucosal fold hypertrophy at 10 mg/kg, whereas the highest dose (100 mg/kg) caused epithelial sloughing and inflammation. These results demonstrate that a precision microdose (10 mg/kg) of host-adapted <i>B. subtilis</i> can outperform multicomponent commercial supplements by significantly enhancing growth, immunity, and organ health. This probiotic strategy provides a sustainable, eco-friendly alternative for warm-water carp aquaculture, supporting circular economy principles and reducing reliance on antibiotics.