Aims: To evaluate the antifungal activity and to analyse the structure-activity relationship of eleven natural phenolic compounds against four Candida species which are resistant to fluconazole. Methods and Results: Four different species of Candida isolates were used: Candida albicans, Candida krusei, Candida tropicalis and Candida dubliniensis. The phenolic compound carvacrol showed the highest anti-Candida bioactivity, followed by thymol and isoeugenol. The obtained minimum inhibitory concentration (MIC) values obtained were used in a quantitative structureactivity relationship (QSAR) analysis where the electronic, steric, thermodynamic and topological descriptors served as dependent variables. According to the descriptors obtained in this QSAR study, the antifungal activity of phenols has a first action specific character which is based on their interaction with plasma or mitochondrial membranes. The second action is based on a steric descriptorthe maximal and minimal projection of the area-which could explain the inability of some phenolic compounds to be biotransformed to quinones methylene by Candida species. Conclusions: According to the descriptors obtained in this QSAR study, the anti-Candida activity of ortho-substituted phenols is due to more than one action mechanism. The anti-Candida activity of phenolic compounds can be predicted by their molecular properties and structural characteristics. Significance and Impact of the Study: These results could be employed to predict the anti-Candida activity of new phenolic compounds in the search for new alternatives or complementary therapies to combat against candidiasis.

Pseudomonas syringae is a phytopathogenic bacterium that causes lesions in leaves during the colonisation process. The damage is associated with production of many virulence factors, such as biofilm and phytotoxins. The essential oils of Thymus vulgaris (thyme) and Origanum vulgare (oregano) have been demonstrated to inhibit P. syringae. The aim of this study was to investigate the effects of T. vulgaris and O. vulgare essential oils on production of virulence factors of phytopathogenic P. syringae strains, including anti-biofilm and anti-toxins activities. The broth microdilution method was used for determination of MIC and biofilm inhibition assays. Coronatine, syringomycin and tabtoxin were pheno- and genotypically evaluated. Both oils showed good inhibitory activity against P. syringae, with MIC values from 1.43 to 11.5 mg·ml for thyme and 5.8 to 11.6 mg·ml for oregano. Biofilm formation, production of coronatine, syringomycin and tabtoxin were inhibited by thyme and oregano essential oil in most strains. The results presented here are promising, demonstrating the bactericidal activity and reduction of virulence factor production after treatment with thyme and oregano oil, providing insight into how they exert their antibacterial activity. These natural products could be considered in the future for the control of diseases caused by P. syringae.

The aim of this work was to study the antimicrobial activity of essential oils obtained from Thymus vulgaris (thyme) and Origanum vulgare (oregano) on phytopathogenic Pseudomonas species isolated from soybean. Strains with characteristics of P. syringae were isolated from leaves of soybean plants with blight symptoms. Ten of these could be identified in Group Ia of LOPAT as P. syringae. Six of these were confirmed as P. syringae using 16S rRNA, indicating the presence of these phytopathogenic bacteria in east and central Argentina. All the phytopathogenic bacteria were re-isolated and identified from the infected plants. MIC values for thyme were 11.5 and 5.7 mg·ml(-1) on P. syringae strains, while oregano showed variability in the inhibitory activity. Both essential oils inhibited all P. syringae strains, with better inhibitory activity than the antibiotic streptomycin. The oils were not bactericidal for all pseudomonads. Both oils contained high carvacrol (29.5% and 19.7%, respectively) and low thymol (1.5%). Natural products obtained from aromatic plants represent potential sources of molecules with biological activity that could be used as new alternatives for the treatment of phytopathogenic bacteria infections.

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