Antibacterial spectrum of silibinin and isomers and its antibacterial effect with antibiotics / 中草药

ABSTRACT

Objective To investigate the in vitro antibacterial spectrum of silibinin and its isomers and the combined antibacterial effect of silibinin and commonly-used clinical antibiotics. Methods Micro-broth dilution method was used to determine the minimum inhibitory concentration (MIC) of silibinin and its isomers against six standard strains and six clinical isolates (74 strains) of common bacteria infections. Plate count method was used to measure the growth inhibition curves of different concentrations of silibinin against six standard strains. Checkerboard micro-broth dilution method was used to carry out the combined susceptibility test of silibinin and clinical antibiotics. The fractional inhibitory concentration (FIC) was calculated to determine the combined antibacterial effect of silibinin and antibiotics. Results The MICs of silibinin against standard strains of Staphylococcus epidermidis, Staphylococcus aureus, Enterococcus faecalis, and Enterococcus faecium were between 50 μg/mL and 400 μg/mL, and those of silibinin for standard strains of Escherichia coli and Pseudomonas aeruginosa were both greater than 400 μg/mL. The MICs of silibinin for clinical strains, Staphylococcus epidermidis, Staphylococcus aureus, Enterococcus faecalis, and Enterococcus faecium were all within the range of 100 μg/mL to 400 μg/mL, and MICs of silibinin against clinical strains of Escherichia coli and Pseudomonas aeruginosa were greater than 400 μg/mL. The MICs of other isomers of silibinin against six standard strains were greater than or equal to 400 μg/mL. Silibinin had a significant inhibitory effect on the growth curve of standard strains of Staphylococcus epidermidis, Staphylococcus aureus, Enterococcus faecalis, and Enterococcus faecium, which increased with increasing drug concentration, and it had no effect on that of Escherichia coli and Pseudomonas aeruginosa. The FIC of silibinin combined with penicillin/erythromycin for gram-positive test bacteria was in the range of 0.5 2 or 1 < FIC ≤ 2. Conclusion Silibinin has a notable antibacterial activity to gram-positive bacteria with the best inhibitory effect on Staphylococcus epidermidis. The antibacterial activities of silibinin is higher than those of other isomers. The combined antibacterial effect of silibinin and penicillin/erythromycin is mainly additive and irrelevant, while that of silibinin and ciprofloxacin or gentamicin is mainly antagonistic or irrelevant.