Antimicrobial Activity and Mechanism of Limonoids from Lemon Peel against Rhizopus

The antimicrobial activity of limonoids from lemon peel on Rhizopus was investigated by determining the minimal inhibitory concentration(MIC),the minimal bactericidal concentration(MBC),spore germination inhibition rate and mycelium germination inhibition rate.The stability of the antimicrobial activity was determined according to the influence of various factors.The antibacterial mechanism of limonoidsfrom lemon peel on Rhizopus was analyzed by observing scanning electron microscopy(SEM)and determining the contents of extracellular alkaline phosphatase(AKP),protein and total sugar in Rhizopus mycelium.The results showed that the minimum inhibitory concentration(MIC)and minimum bactericidal concentration(MBC)of limonoids on Rhizopus were 1250 and 5000 μg/mL respectively,and the EC₅₀ values of limonoids on spore germination and mycelium growth were 918.34 and 1707.31 μg/mL respectively. Antimicrobial activity was strongest when pH was 4. The antibacterial activity of limonoids on Rhizopus was not influenced by temperature and gelatin,significantly(p<0.05)improved by Fe³⁺ and Fe²⁺. Limonoids could exudate intracellular substances in Rhizopus such as sugar,protein and AKP.The observation of scanning electron microscope(SEM)showed that the spores of Rhizopus treated with limonoids were depressed and shrunk seriously,and the dissolution was obvious.The mycelium of Rhizopus was distorted,deformed and thinned. Therefore,limonoids could destroy the integrity of the cell wall and membrane of Rhizopus,and the substance exudate out of the cell,anddeform mycelium and spore,thus affecting the growth and metabolism of Rhizopus.