The effects of naturally-derived plant compounds on pseudomonas aeruginosa pathogenicity

Pseudomonas aeruginosa has risen as one of the most resistant bacterial pathogens affecting people worldwide. Inefficacy of current antimicrobial treatments to relieve P. aeruginosa infections demands the need for novel antimicrobial therapies. Natural products such as the aqueous and alcoholic extracts of North American ginseng, as well as thymol and carvacrol, compounds purified from thyme and oregano, have previously displayed antibacterial properties against P. aeruginosa. In this study we examined the effect of these compounds on P. aeruginosa pathogenicity by measuring pathogenic markers (ie virulence factor secretion, biofilm formation) after PAO1 and Cystic Fibrosis clinical isolates were treated with the natural compounds. Thymol and carvacrol were able to attenuate the growth of P. aeruginosa clinical isolates in nutrient and minimal media however PAO1growth was only attenuated in the nutrient media. The alcoholic ginseng extract displayed no growth inhibitory action towards any strain tested; however the aqueous ginseng extract attenuated growth of all strains at concentrations of 8.0-7.0% w/v and 3.0-2.0% w/v in the minimal and nutrient media respectively. Sub-inhibitory concentrations of the aqueous ginseng extract increased growth rate and bacterial adherence, but decreased the production of many virulence factors, while the alcoholic ginseng treatment increases secretion of select virulence factors. Microarray analysis revealed decreases in virulence factors with sub-inhibitory aqueous ginseng extract exposure. Treatment with sub-inhibitory concentrations of thymol or carvacrol reduced pigment, lipase, and protease secretion after exposure to thymol or carvacrol in a strain specific manner; however bacterial motility was negatively affected in all strains with treatment of either compound. Microarray the thymol 6 h and carvacrol 24 h treatments displayed enhanced expression of pyoverdine and pyochelin synthesis genes. Enrichment of the ribosomal pathway was substantial after the 24 h carvacrol and aqueous extract exposures, findings which require further investigation. Overall reduction in pathogenesis was most evident with the aqueous ginseng extract however thymol and carvacrol were most effective at inhibiting bacterial growth. Further chemical analysis of the alcoholic and aqueous ginseng extracts may reveal specific extract constituents with potent antibacterial properties. Analysis of potential synergy between the tested plant components and current antibiotics of P. aeruginosa may provide a greater appreciation for their use in antibacterial therapies