This research confirmed that azalomycin F_5a can increase the membrane permeability of MRSA cell, and which had likely achieved by the synergy of its lactone ring binding to the polar head of phospholipid and its guanidyl side chain targeting to lipoteichoic acid (LTA). This would subsequently cause the release of cellular substance, and eventually lead to the autolysis of MRSA cells. Nevertheless, it is unknown how azalomycin F_5a interacts with MRSA cellular LTA, such as influencing the physiological function of LTA, inhibiting LTA synthase and polymerase, and/or accelerating the release of LTA?

Azalomycin F_5a was a polyhydroxy macrolide produced by streptomycete strains. Our preliminary researches indicated that it could kill methicillin-resistant Staphylococcus aureus (MRSA) likely by increasing the permeability of cell membrane, and that cell-membrane phospholipids were likely important targets. To confirm this, membrane permeability assay was performed and visualized by fluorescence staining, and then the detailed interactions between azalomycin F_5a and model membranes prepared with 1,2-dihexadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) were determined using attenuated total reflectance fourier transform infrared spectroscopy and ³¹P nuclear magnetic resonance techniques. The results indicated that there were strong interactions between azalomycin F_5a and model membranes, especially between azalomycin F_5a and the polar head of phospholipid. For further evidence and details, the molecular dynamics (MD) simulation of the interactions between azalomycin F_5a and DPPG or lysyl-DPPG were performed using Amber16 software package. A strong interaction between the lactone ring of azalomycin F_5a and the polar head of DPPG or lysyl-DPPG had been clearly observed. Moreover, a larger distribution probability out of phospholipid bilayer had been discovered for the guanidyl side chain of azalomycin F_5a, especially when probable anion molecules anchoring on the cytoplasmic membrane occurred. Therefore, lipoteichoic acid (LTA), a vital component of gram-positive bacterial envelope, was investigated for its probable interactions with azalomycin F_5a using broth microdilution method. The results showed that azalomycin F_5a-induced MRSA lysis could be prevented by LTA. This deduced that there were some interactions between azalomycin F_5a, more likely its guanidyl side chain, and LTA. Thereby, azalomycin F_5a increasing the cell-membrane permeability of MRSA had likely achieved by the synergy of its lactone ring binding to the polar head of phospholipid and its guanidyl side chain targeting to LTA, and which had eventually led to the autolysis of MRSA cells.