New limonoids and the protective capacity of Vietnamese Azadirachta indica against Vibrio bacteria and white-spot syndrome virus.
The antibacterial and antiviral properties of Vietnamese Azadirachta indica were assessed against Vibrio species and white-spot syndrome virus (WSSV) in Litopenaeus vannamei shrimp. Vibrio parahaemolyticus was susceptible to most extracts, with the cold ethanolic extract showing the greatest efficacy (IZD = 15.53 ± 0.67 mm). Antibacterial activity against Vibrio alginolyticus was observed only with the hot aqueous extract, which also demonstrated the highest anti-WSSV activity, causing 50% mortality at 0.25 mg/mL. Ultra-performance -liquid chromatography tandem mass spectrometry analysis revealed significant compositional diversity and distinctive limonoid and -flavonoid profiles. Column chromatography of the ethyl acetate extract resulted in the isolation of four novel limonoids [azadirachtin G (1), 3-desacetyldeoxyazadirachtolid (2), 1-desenecioyl-7-O-methacryloyldeoxy-azadirachtolid (3) and azadirachtolid C (4)] and two previously reported compounds [23-deoxy-azadirachtolid (5) and azadirachtolid A (6)]. Structural elucidation was achieved using nuclear magnetic resonance spectroscopy and mass spectrometry.