The emergence of drug resistant pathogens becomes a crucial problem for infectious diseases worldwide. Among these bacteria, Pseudomonas aeruginosa is one of which highly resists to many currently used drugs and becomes a major concern in public health. Up till now, the search for potential antimicrobial agents has been still a challenge for researchers.
Broth microdilution assay was used to determine minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of the essential oils and antibiotics against P. aeruginosa. Inhibition activity of the essential oils under vapor condition was examined to obtain the minimum inhibitory dose (MID). Time-kill assay included in this study was performed according to CLSI guideline. Bioautographic assay was used to detect active components of the essential oil. Synergistic effect with currently used antibiotics was further examined by checkerboard assay.
Results and Discussion
In this study, a variety of essential oils were examined for anti-multidrug resistant P. aeruginosa (MDR-PA) activity, of which cinnamon bark oil showed the strongest antimicrobial activity against all clinical-isolated MDR-PA strains with MIC of 0.0562–0.225 % v/v and MBC of 0.1125–1.8 % v/v. Bioautographic results demonstrated that the active compounds of cinnamon bark oil were cinnamaldehyde and eugenol which showed strong inhibitory effect against P. aeruginosa. Interestingly, cinnamaldehyde, a major constituent of cinnamon bark oil, possessed stronger antimicrobial effect to P. aeruginosa than eugenol. Under gaseous condition, cinnamon bark oil and cinnamaldehyde showed antibacterial activity against MDR-PA strains with MID of 0.5–1 mg/L. Moreover, combination of cinnamon bark oil or cinnamaldehyde with currently used antibiotics was further studied by checkerboard assay to examine synergistic interactions on clinically isolated MDR-PA strains. Cinnamon bark oil and cinnamaldehyde combined with colistin demonstrated synergistic rates at 16.7 and 10 %, respectively.
These results indicated that cinnamon bark oil and cinnamaldehyde might be active natural compounds which could be further examined as alternative treatment for multidrug-resistant P. aeruginosa infection.