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). Significance and impact of the study: Multidrug‐resistant bacteria associated with infections of the lower respiratory tract have become a significant public health concern. Therefore, known antimicrobial agents derived from plants (e.g. essential oils) and their synergistic combinations with conventional antibiotics may overcome the antimicrobial resistance. In this respect, the antibacterial effects of ajowan essential oil, thymol and their combinations with antibiotics were assessed against some standard strains and drug‐resistant clinical isolates of major respiratory bacteria (Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus pneumoniae).