Screening of the antibacterial effect of Juniperus sibirica and Juniperus sabina essential oils in a microtitre platebased MIC assay

Abstract

© 2016 Institute of Botany and Botanical Garden Jevremovac, Belgrade. The antibacterial effect of wild-growing Juniperus sibirica Burgsdorf and Juniperus sabina L. var.sabina essential oils was studied in a microtitre plate-based MIC assay. Bacterial growth wasmonitored by measuring turbidity of the sample (OD600), as well as by following the colorimetricresazurin reaction. Essential oils were prepared from the needles of female plant samples andanalysed by GC-MS. Hydrocarbon monoterpenes were determined as the dominant constituents;the compounds detected in the highest amounts were a-pinene (74.5%) and sabinene (54.3%) in J.sibirica and J. sabina oil, respectively. As indicator strains in the MIC assay, we used selected Grampositive(Enterococcus faecalis ATCC29212, Staphylococcus aureus ATCC25923, Bacillus subtilisATCC6633 and Listeria innocua ATCC33090) and Gram-negative (Escherichia coli ATCC25922,Salmonella typhimurium ATCC14028, Salmonella enteritidis ATCC13076, Aeromonas hydrophilaATCC49140) bacteria. Bacterial inocula used in the MIC assay were adjusted to a 0.5 McFarlandstandard, corresponding to approximately 108 CFU/mL. The obtained results indicated thatdetermination of turbidity decrease cannot be used to precisely quantify MIC values of the oils. Theresazurin-incorporated MIC assay showed that the most susceptible strains were A. hydrophila andB. subtilis, with MIC values of 12.5 mg/mL and 6.25 mg/mL, respectively, for J. sibirica, and 6.25mg/mL for both bacteria for J. sabina. The remaining bacteria were far less sensitive to Juniperusoils. In the range of tested concentrations, the effect of both oils was predominantly bactericidal, butJ. sibirica oil showed a bacteriostatic effect against some Gram-negative bacteria.