Synergistic effects of unripe raspberry extracts (Rubus chingii) and antibiotics against three bacteria

Abstract

Dried unripe raspberries (Rubus chingii), a kind of Chinese herbal medicine, have been used for many years in China. They were prepared as unpurified raspberry extract (URE) by ethanol extraction, and purified raspberry extract (PRE) by macroporous resin. Synergistic effects between the URE (or the PRE) and ampicillin, erythromycin, tetracycline or streptomycin against Escherichia coli, Staphylococcus aureus and Salmonella enterica were carried out using checkerboard microdilution and time-kill curves, respectively. The results showed that all pairwise combinations revealed synergistic effects, with 0.10 ≤ fractional inhibitory concentration index ≤ 0.50 and more than 2 × log₁₀ reduction in colony-forming units/mL. In particular, the erythromycin concentration in the combinations was reduced to 1/50-1/100 of its dose alone while retaining its effectiveness against S. aureus and E. coli. The synergistic effects of raspberry extracts and four antibiotics against three bacteria were related to the abundance of polyphenols in the extracts, especially ellagic acid. The extracts may be a dietary supplement and/or natural antimicrobials.

Keywords:
antibiotics; ellagic acid; natural antimicrobials; raspberry extracts; synergistic effects