Abstract: Bletilla striata polysaccharide (BSP) was prepared by hot water extraction combined with alcohol precipitation.The molecular characteristics of BSP were analyzed by MCR702,GPC-MALS-RI,and SEM.On this basis,the combined antibacterial effect of BSP and daptomycin (DAP) was further investigated.The results showed that the yield of BSP was 53.3%,the number average molecular weight was 58.635 kDa,the weight average molecular weight was 83.319 kDa,the polydispersity index was 1.421,the average rotational radius was 18.3 nm,and the weight average radius of gyration was 22.5 nm.In salt solution,the BSP molecules presented a compact and quasi-spherical conformation.In water solution,BSP could spontaneously form multi-level aggregation structures with an average particle size of 410.7 nm.BSP (at a mass concentration of 1.0 g/L) could increase the antibacterial activity of DAP against Staphylococcus aureus by 2 times.Compared with DAP alone,the combination of DAP and BSP could enhance the permeability of bacterial cell membranes,leading to the leakage of nucleic acids,proteins,K+,and other contents,causing cell shrinkage and accelerating the decline of membrane potential.It is suggested that BSP can enhance the killing effect of DAP on Staphylococcus aureus by promoting the insertion of DAP into bacterial cell membrane phospholipids and the formation of ion channels in a synergistic manner.

Key words: Bletilla striata polysaccharide, daptomycin, antibacterial activity, synergy, Staphylococcus aureus