Synergistic interaction between antibiotics and natural product extracts to inhibit methicillin-resistant staphylococcus aureus (MRSA)

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the Staphylococcus aureus strains that resists to a variety of antibiotics, especially β–lactam antibiotics. The treatment of MRSA infections are very difficult because very few antibiotics are effective against MRSA. Therefore, the objectives of this research were to search for potential antimicrobial agents from native Thai plant extracts and to study the synergism between these plant extracts and antibiotics against MRSA. Eleven native Thai plants, i.e., Acanthus ebracteatus Vahl., Cissus quadrangularis Lin, Kaempferia parviflora, Barleria lupulina Lindl, Orthosiphon aristatus (Blume) Miq, Glycyrrhiza glabra L., Stevia rebaudiana Bertoni, Clitoria ternatea Linn., Curcuma longa L.,Curcuma zedoaria (Berg.) Rosc. and Zingiber cassumunar Roxb were extracted with water and 95% ethanol. These extracts were determined for antimicrobial activities against S.aureus by agar-well diffusion method. Minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were also determined. The synergistic effect between crude plant extracts and antibiotics was assayed using disc diffusion technique. In addition, the interaction between crude plant extracts and antimicrobial agents was estimated by calculating the fractional inhibitory concentration (FIC index) of the combination. The results showed that Kaempferia parviflora ethanol extract possessed antibacterial activity against only MRSA with inhibition zones of 15+3 mm. The MIC and MBC of Kaempferia parviflora ethanol extract against MRSA was 46.87 μg/mL and 96.75 μg/mL, respectively. Study of synergism between plant extracts and antibiotics revealed that when Kaempferia parviflora ethanol extract was combined with cefoxitin, the MIC and MBC of cefoxitin against MRSA reduced from 30.47±11.32 μg/mL to 1.76±0.62 μg/mL and from 89.06±14.82 μg/mL to 3.74±1.21 μg/mL, respectively. In addition, the FIC of this combination indicated that Kaempferia parviflora ethanol extract and cefoxitin could be synergistic in antibacterial effect against MRSA.