Antibacterial activity of traditional spices against lower respiratory tract pathogens: combinatorial effects of Trachyspermum ammi essential oil with conventional antibiotics

The aim of this study was to investigate the effects of ajowan essential oil (AjEO)/thymol and antibiotics combinations against three standard strains and six resistant clinical isolates of major respiratory bacteria ( Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus pneumoniae ). The broth microdilution method was conducted to determine the minimum inhibitory concentrations ( MIC ) of essential oil/thymol and antibiotics. The checkerboard method was used to investigate the interactions between the essential oil/thymol and antibiotics by means of the fractional inhibitory concentration index ( FICI ). The chemical composition of essential oil was also analysed by GC – MS and GC ‐ FID . Thymol (50·75%), γ ‐terpinene (25·94%) and p ‐cymene (18·31%) were identified as major constituents of the oil. The most sensitive organisms to ajowan volatile oil were Strep. pneumoniae bacteria ( MIC  = 0·125–0·5 mg ml −1 ). Synergistic effects were observed with AjEO/thymol and amoxicillin combinations on methicillin‐resistant Staph. aureus clinical isolates ( FICI  = 0·37–0·50) and with essential oil and ciprofloxacin combinations against P. aeruginosa ATCC 27853 , Staph. aureus ATCC 25923 and penicillin (P)‐resistant Strep. pneumoniae bacteria ( FICI  = 0·37–0·50). Combination of thymol and ciprofloxacin produces synergistic effects only against P. aeruginosa ATCC 27853 and P‐resistant Strep. pneumoniae clinical isolate ( FICI  = 0·46–0·49).