Abstract
This study evaluated the effects of dietary Bacillus subtilis supplementation on the histomorphology, histomorphometry, and growth performance of the posterior kidney in common carp (Cyprinus carpiol). One hundred twenty juvenile fingerlings (15.2 ± 0.8 g) were randomly allocated into four groups for 60 days: a control group (no probiotic) and three treatment groups receiving 10⁶, 10⁷, and 10⁸ CFU/g feed (T1, T2, T3). Posterior kidneys were processed for paraffin embedding, sectioned at 4–5 µm, stained with H&E, and examined by light microscopy. Quantitative histomorphometry was performed using calibrated ImageJ software measuring eight renal parameters. Growth performances parameters including final body weight (FBW), weight gain, specific growth rate (SGR), and feed conversion ratio (FCR) were also assessed. All experimental groups exhibited intact renal architecture with preserved glomeruli, Bowman's capsules, and tubules, with no evidence of degeneration, necrosis, inflammation, or cast formation. Quantitative histomorphometry revealed no significant differences among groups for any parameter (P > 0.05). Glomerular diameter ranged from 68.4 ± 3.2 µm (control) to 71.2 ± 2.9 µm (T3) (P = 0.412). PCT epithelial cell height ranged from 14.6 ± 1.1 µm to 15.2 ± 1.2 µm (P = 0.498). Regarding growth performance, the T2 group (10⁷ CFU/g) showed the highest FBW (42.1 ± 1.6 g), weight gain (175.2%), SGR (2.31 ± 0.09 %/day), and the lowest FCR (1.72 ± 0.08) compared to control (P < 0.001). The T3 group showed slightly lower growth parameters than T2. Hepatosomatic index remained unchanged across all groups (2.99–3.00%; P = 0.998). Survival rates were high (96.7–100%) with no significant differences among groups. In conclusion, Bacillus subtilis supplementation up to 10⁸ CFU/g is structurally safe for renal tissues, with 10⁷ CFU/g recommended as the optimal dose for maximizing growth performance while maintaining histological safety.

Key words: Bacillus subtilis; Cyprinus carpio, kidney, nephron, probiotics