Sabilla, Annisa Wina (2025) Effects of Dietary Bacillus subtilis var. natto on Growth Performance and Lysozyme Gene Expression in Nile Tilapia (Oreochromis niloticus) Infected with Flavobacterium (columnare) oreochromis. Undergraduate thesis, Fakultas Sains dan Matematika Undip.

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Abstract

PThe high demand for Nile tilapia worldwide has led to the development of
intensive aquaculture systems, which can increase stress on the fish, especially if
improper management is involved. These conditions can trigger outbreaks of
columnaris disease, caused by Flavobacterium oreochromis. This study evaluated
the effects of Bacillus subtilis var. natto (BSN1 and BSN2) supplementation (10⁵
or 10⁷ CFU/g feed) on growth, survival, and lysozyme gene expression in
uninfected and F. oreochromis–infected Nile tilapia. Fish (n = 15, 5.24 ± 0.50 g)
were fed unsupplemented or supplemented diets for 7 days before and 21 days after
immersion-challenge (10⁷ CFU/ml for the first and second infections; 10⁸ CFU/ml
for the third infection). BSN1 10⁷–supplemented feed resulted in the highest weight
gain (WG), average daily growth (ADG), specific growth rate (SGR), and the most
favorable feed conversion ratio (FCR), followed by the negative control, BSN1 10⁵,
BSN2 10⁷, BSN2 10⁵, and the positive control. No growth inhibition was observed
in any supplemented group, confirming feed safety. BSN1 10⁷ and BSN1 10⁵
significantly reduced mortality compared to the infected control, while the
remaining supplemented groups still showed higher survival rates, indicating
protective effects against F. oreochromis. Overall, BSN1 10⁷ performed best
numerically for improving growth performance and can significantly reduce
mortality after infection. Lysozyme gene expression in BSN1 10⁷–supplemented
feed during the second infection did not differ significantly from infected or
uninfected controls on days 1,2, and 3 post-infection. However, histopathology
findings confirm that the BSN1 107 group showed less gill damage compared to the
control group, suggesting the presence of immune protection during infection

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