Chemical Constituents And Anti-MRSA Bioactivity Of Morus Alba

Infection in-and out-hospital caused by methicillin-resistant Staphylococcus aureus（MRSA）has been a serious public health problem,which necessitates comprehensive measures in prevention and treatment.The root barks of Morus alba,a traditional Chinese medicine,was extensively used in folk for treating various diseases,such as vomit,fever,cough,edema,enteritis,nephritis,some of the diseases may be related to bacterial infection.Although DielsAlder adducts,flavonoids and stilbene compounds have been isolated from the root barks of M.alba,few compounds are reported their antimicrobial efficacy in vivo and the mechanism.For finding more antimicrobial ingredients,further chemical investigation was carried out according to the thin layer chromatography（TLC）of the active fraction MA-6.Chemical isolation resulted 32 known compounds,among them,seven stilbene compounds were identified as moracin M（1）,moracin C（2）,mulberrofuran B（3）,albafuran A（4）,moracin P（5）,moracin G（6）,and 3′,5′,2,4-tetrahydroxy-4（3-methyl-1-butenyl）stilbene（7）;thirteen flavonoids as kuwanon C（8）,5′-geranyl-5,7,2′,4′-tetrahydroxyflavone（9）,morusin（10）,morusignin L（11）,austraone A（12）,kuwanon B（13）,dihydromorin（14）,steppogenin（15）,kuwanon E（16）,kuwanon U（17）,benzokuwanon E（18）,dicyclokuwanon E（19）,and tetrapterol A（20）;nine Diels-Alder adducts as mulberrofuran G（21）,mulberrofuran K（22）,albanol B（23）,mulberrofuran Q（24）,kuwanol A（25）,kuwanon G（26）,kuwanon H（27）,kuwanon L（28）,kuwanon O（29）;two steroids as β-sitosterol（30）,daucosterol（31）;and one terpenoid ursolic acid（32）.The obtained compounds presented good antimicrobial activities against Gram-positive bacteria（S.aureus,Bacillus subtilis,Listeria monocytogenes）,and weak against Gram-negative ones（Escherichia coli,Salmonella typhimurium,Pseudomonas aeruginosa）.Most of compounds with one or more geranyl and prenyl units exhibited superior anti-MRSA bioactivity,MIC≤16 μg/m L.In addition,the mode of action indicated that compounds presented persistent antimicrobial effect,which also produced concentration-dependent and time-dependent killing.Preliminary mechanism showed that the compound kuwanon O（29）damaged the bacterial cell membranes,leading to the accumulation of antibiotics inside bacterial cells,moreover,MA-6and kuwanon O（29）inhibited the efflux of drugs by combining with methicillin or ethidium bromide,resulting in the MICs of ethidium bromide and methicillin were obviously decreased in three-fold.The anti-MRSA efficacy in vivo indicated that the active fraction MA-6 could reduce bacteria in spleen,liver,kidney and mortality of acutely infectious mice,which was better than the positive drug berberine chloride.