Fish farms can raise production, but crowded conditions also make fish more vulnerable to infection. This study tested whether helpful bacteria mixed into feed could protect fish and help them grow. Fish demand has been growing, and aquaculture has developed alternatives to improve production.

Intensive systems can increase fish production, but they also increase fish susceptibility to bacterial infections because of stress on the animals. That creates a difficult tradeoff: producing more fish can also make the animals more vulnerable to disease.

The study tested three Bacillus species in Nile tilapia fed with high levels of soybean meal and challenged with Aeromonas hydrophila, a bacterium that can infect fish. The infection itself was dangerous: the pathogen killed half of the control fish in thirteen days.

The three strains were selected because they could grow in soy-based media and inhibit Aeromonas hydrophila development on agar plates. In everyday terms, the researchers were looking for helpful bacteria that could live on the planned feed ingredient while slowing the harmful bacterium—like choosing a guard that can survive in the same neighborhood as the threat.

The three strains produced antimicrobial activity against Aeromonas hydrophila, but the sp2 strain produced the highest inhibition activity against the pathogen. Its inhibition activity was threefold higher than sp1 and twofold higher than sp3.

After forty days of feeding, the fish were measured. Most growth parameters were significantly improved in the Bacillus sp2 group compared with the control, sp1, and sp3 groups. After infection, the fish were followed for an additional fifteen days.

The control group reached the point where half had died after thirteen days of initial infection. On that same day, cumulative mortality was twenty-five percent with sp1, five percent with sp2, and fifteen percent with sp3. The growth results suggest that Bacillus strains, especially Bacillus sp2, could lessen harmful effects produced by soybean products.

A similar growth rate has been reported with more soybean meal, but that comparison used almost three times more fishmeal than this experiment. The formulated feed contained soybean meal and Bacillus sp2, which was capable of increasing fish health by inhibiting Aeromonas hydrophila development and production of factors linked to tissue damage.

The feed also enhanced growth performance, while Bacillus sp2 could grow on many plant-based substrates, including soybean meal and other leguminous plants. For fish farmers, that points toward a feed that may help fish survive infection and grow well while relying more on plant-based ingredients.

A feed containing one of the tested Bacillus strains was linked to both lower death rates after infection and better growth, suggesting a possible way to make tilapia farming more resilient.