Sonneratia caseolaris-Mediated Silver Nanoparticles: Characterisation and Antibacterial Activity Against Edwardsiella tarda from Diseased Freshwater Fish

Edwardsiella tarda infection can cause significant economic losses to the aquaculture industry. It causes haemorrhagic septicaemia in freshwater and marine fish, and serious health issues in humans and other animals. The abuse of antibiotics in aquaculture has been linked to the emergence of multidrug-resistant bacteria, and silver nanoparticles (AgNPs) are regarded as a potential therapeutic alternative. This study biosynthesised AgNPs using the methanolic leaf extract of Sonneratia caseolaris. A 0.1 mg/ml (H2) extract solution formed AgNPs successfully as indicated by brownish colour change after 24 h incubation and evidenced by UV-Vis spectroscopy. Energy-dispersive X-ray spectroscopy (EDS) revealed silver as the main constituent (84.49%). Transmission electron microscopy (TEM) displayed polydisperse, near-spherical nanoparticles with an average size of 22.3 nm. Fourier transform infrared (FTIR) analysis suggested that several functional groups were involved in SC-AgNPs formation. SC-AgNPs exhibited strong antibacterial activity against 12 E. tarda isolates from diseased African catfish and Asian swamp eels, including a β-haemolytic strain (ET10). The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined to be 2.5 μg/mL. This study suggests that SC-AgNPs has the potential as an alternative to antibiotics to reduce the risk of antibioticresistance in aquaculture.