Bioactivity of Amaranthus spinosus L. leaf extracts and meals against Aeromonas hydrophila

Plant-based protein is being sought after as a substitute for fish meals (powdered fish) in tilapia feeds. This is to promote sustainable aquaculture, as fish meals contribute to the dwindling marine fish catch. Amaranthus spinosus is an edible weed that shows potential to improve the growth and immunity of Nile tilapia. However, most studies only consider the survivability of fish to evaluate the benefit of using plant-based feeds and do not necessarily elucidate whether a pathogen is affected in vivo. A. spinosus leaf meals (ASLMs) were used to determine effectiveness against Aeromonas hydrophila (BIOTECH 10089) injected intraperitoneally into Nile tilapia. Formulated feeds with fish meals substituted with 50 % (ASLM(50)) and 75 % (ASLM(75)) A. spinosus leaves were fed to Nile tilapia challenged with A. hydrophila . Then spleen and kidney tissue were collected and analysed 10 days post-injection for total plate count. The fish fed with ASLM(50) appeared healthier than those fed with ASLM(75) and those fed with control feeds. Fish fed with ASLMs had lower A. hydrophila counts (P=0.03). Phytochemical screening and antimicrobial activity determination for crude methanolic A. spinosus leaf (ASL) and ASLMs were also conducted to enhance the in vivo results. The metabolites present in the extracts were carbohydrates, amino acids and proteins, cardiac glycosides, saponins and terpenoids. The ASL and ASLM extracts had antimicrobial activity (MIC=115 mg ml(−1)). Overall, the study showed that ASLMs can make tilapia more resilient against A. hydrophila infections. Fish meal substitution was best at 50 %. Higher substitution had unwanted effects (more bacterial counts), possibly due to antinutritional factors.