Solvent-dependent antibacterial activity of Nepeta nuda leaf extracts

. The medicinal plant Nepeta nuda L. (known as ‘naked catmint') is characterized by various biological activities due to the high content of terpenes, iridoids and phenolic compounds. In the present study, we analysed the antimicrobial activity of N. nuda leaf extracts against 16 human pathogenic microorganisms. The antimicrobial efficiency of N. nuda extracts was investigated depending on their preparation by applying primary extraction solvents (chloroform/Chl and methanol/Met), and the respective secondary solvents for dissolving the dry primary extracts (Chl, Met), as well as H2O and dimethyl sulfoxide/DMSO. The applied diskdiffusion method allowed evaporation of the solvent prior contact to the bacteria, and the effect of extracted N. nuda polar and non-polar secondary metabolites could be evaluated. The dry Met and Chl extracts showed good solubility in the respective secondary solvents Met, Chl and DMSO, while H2O appeared as inappropriate solvent. The leaf extracts showed activity to 6 of the tested 15 bacteria and no antifungal activity was detected. High inhibitory effect was established when using Met and Chl solvents, while the H2O extracts were inactive. The bacteria Staphylococcus aureus and Acinetobacter calcoaceticus showed the strongest sensitivity to Chl extracts. The most susceptible to exposure of Met extracts turned out to be Bacillus subtilis and A. calcoacetisus. We concluded that the use of solvents with different polarity succeeded to extract N. nuda antimicrobial substances. Therefore, the obtained extracts could be further explored for application as biocontrol agents, natural preservatives and for medicinal purposes.