This is the first time atraric acid found in Dirinaria applanata, a lichen species popular on Coconut tree in Vietnam.

Background

Lichen, Dirinaria applanata distributes very popular in Vietnam, however, there are not enough data for this species in terms of secondary metabolites and bioactivity. The aim of this study is isolating and evaluating antimicrobial activity of Dirinaria applanata sample collected from Ben Tre province, Viet Nam.

Materials and method

The nomenclature of this lichen sample was identified by morphology and some specific chemical reaction. Lichen was extracted with many solvents to choose the best antimicrobial activity using disc diffusion assay. The column vacuum chromatography, column chromatography, and recrystallization were applied to isolate compounds from methanol crude extracts. TLC-bioautography and diffusion method were used to indicate antimicrobial fractions. Chemical structures of the isolated compounds were interpreted by MS and NMR data. MIC and MBC values of the purity compounds were evaluated by broth microdilution method according to CLSI M07-A11 (2018). Time-kill study was also carried out on the active compound.

Results

The lichen sample was identified Dirinaria applanata (Fée) D. D. Awasthi. Methanol crude extract displays the strong antibacterial effects on Gram positive bacteria with MIC < 200 µg/ml; especially, on S. pyogenes (MIC ≤ 20 µg/ml) and MRSA (MIC = 78,1 µg/ml). Applying column chromatography, we collected 2 purity compounds which were proven to be atraric acid (the first report in this species) and divaricatic acid. Divaricatic acid possesses the good inhibitory capability on Gram positive bacteria, Candida albicans and dermatophytes, meanwhile, atraric acid does not show the activity at the testing concentrations, ≤ 512 µg/ml. In particular, MIC value of divaricatic acid on MRSA and S. pyogenes are 32 µg/ml and 4 µg/ml, respectively. Times kill kinetic assay demonstrated that divaricatic acid is a bactericidal and concentration-dependent compound on MRSA.

Conclusion

Divaricatic acid, isolated from Dirinaria applanata, is the potential anti-MRSA compound.

Keywords: Dirinaria applanata, antimicrobial, acid divaricatic.

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