Characterization of the Bactericidal Actions of a High-Purity Sophoroselipid-Copper Complex

In this study, a high-purity sophoroselipid-copper (SL-Cu) complex was synthesized using an improved method employing copper (II) acetate. Fourier transform infrared analysis of the purified SL-Cu complex revealed a distinctive absorption peak near 1,600 cm⁻¹. The bactericidal activity of the SL-Cu complex, at a copper (II) ion concentration of 1,000 µM and after incubation at 20°C for 30 min, exhibited a reduction greater than 4 log units against Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. In contrast, its bactericidal activity against Enterococcus faecalis was comparatively lower, with a reduction of 0.90 log units. Furthermore, the SL-Cu complex demonstrated a statistically higher antibacterial efficacy against E. coli and S. aureus than CuSO₄ at the same copper (II) ion concentration. The bactericidal effect of the SL-Cu complex against E. coli was enhanced with increasing treatment temperatures between 15 and 40°C, consistently surpassing that of CuSO₄ across all tested temperatures. The inhibitory effects of bovine serum albumin, sodium chloride, EDTA-2Na, and 2,2 bipyridyl on the SL-Cu complex were restricted compared to those on CuSO₄, and in some experiments, these chemicals even enhanced its activity. These characteristics render the SL-Cu complex a promising antibacterial reagent. Flow cytometry analyses using fluorescent dyes suggested that the SL-Cu complex disrupted bacterial cell membranes and induced the generation of reactive oxygen species.