Synergistic Activity of Three Iranian Medicinal Plants in Combination with Ceftazidime and Neomycin against Bacterial Strains Causing Nosocomial Infections

This study aimed to investigate the antimicrobial properties of Salvia limbata, Centella asiatica, and Bacopa Monnieri extracts against Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii and methicillin-resistant Staphylococcus aureus (MRSA). We also examined the synergistic effect of these extracts with ceftazidime and neomycin. The antimicrobial effects of these plants had been reported before but synergistic effect with broad spectrum antibiotics such as ceftazidime and neomycin was an important issue that we tried to determine. Methanol extracts were prepared by percolation method and phenolics content was determined by Folin–Ciocalteu method. The minimum inhibitory concentrations (MIC) of the extracts were determined by the Clinical and Laboratory Standards Institute (CLSI) broth microdilution method. Checkerboard assay was used to evaluate the synergistic effect of the extracts with ceftazidime and neomycin. Salvia limbata methanolic extract with MIC of 25, 100, and 150 mg/mL could inhibit the growth of S. aureus, P. aeruginosa, and MRSA, respectively. Bacopa monnieri with MIC of 50 mg/mL and 100 mg/mL inhibited the growth of S. aureus and P. aeruginosa, respectively. Centella asiatica did not affect the studied strains. The fractional inhibitory concentration (FIC) results showed partial synergistic activity between S. limbata with ceftazidime and neomycin against P. aeruginosa and MRSA. The MIC of ceftazidime and neomycin in combination with S. limbata was reduced fourfold for each antibiotic. Salvia limbata is a potentially rich source of bioactive compounds with antimicrobial properties that can be used with ceftazidime and neomycin to provide a synergistic effect.