Susceptibility Screening of Thymol, Quercetin, Neem, Cinnamaldehyde and Eugenol against Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa exhibits intrinsic and acquired resistance to antibiotics. An under‐utilized source of antimicrobic activity is phytochemicals. Their use against P. aeruginosa may serve as an alternative to conventional antibiotics in this age of ever‐increasing resistance. The purpose of this study was to investigate susceptibility patterns of P. aeruginosa in the presence of phytochemicals selected from literature citations reporting antibacterial activity. The hypothesis was that phytochemicals thymol, quercetin, neem bark extract, cinnamaldehyde and eugenol inhibit growth of P. aeruginosa. Susceptibility testing of phytochemicals was performed by the Kirby Bauer disk diffusion method and phytochemicals were tested at different concentrations (0–100%) after 2‐fold serial dilutions of the agents into tryptic soy broth. Results showed zones of growth inhibition of P. aeruginosa (ATCC strain 27853) around cinnamaldehyde‐ and eugenol‐impregnated disks when the phytochemicals were presented singly or in combination with each other. Specific results indicated that significant zones of inhibition were evident at 100%, 50% and 25% concentrations of cinnamaldehyde. Zones of inhibition were evident at 100% and 50% eugenol. A combination study of cinnamaldehyde and eugenol, each at 12.5% concentration, produced a notable zone of inhibition (measured at 16 mm diameter): neither cinnamaldehyde nor eugenol displayed zones of inhibition independently at that concentration (of 12.5%). Thymol, quercetin and neem bark extract did not yield visible zones of inhibition at any concentration tested. To conclude, cinnamaldehyde and eugenol each individually possess antimicrobial activity against P. aeruginosa (strain ATCC 27853) as indicated by disk diffusion susceptibility testing. Additionally, the two phytochemicals act synergistically in combination to effectively inhibit bacterial growth. Evidence of synergistic activity at low concentrations not effective individually by either cinnamaldehyde or eugenol offers potential therapeutic value. Other phytochemicals (thymol, quercetin, neem bark extract) failed to inhibit P. aeruginosa growth at any concentration tested. Future studies aim to screen more phytochemicals independently and in combination against other strains of P. aeruginosa to allow for statistical analysis and determination of susceptibility patterns.Support or Funding InformationFinancial support for JK was received from the DeNardo Education and Research Foundation.