In order to make chemical characterization of two Origanum vulgare L. subspecies in Montenegro, the essential oils of five wild growing populations were analyzed. Among 67 oil constituents, in O. vulgare subsp. hirtum dominant one was oxygenated monoterpene carvacrol (74.3%), while in O. vulgare subsp. vulgare prevailed sesquiterpene hydrocarbons: germacrene D (15.4–27.9%) and β-caryophyllene (7.7–14.6%), and among oxygenated monoterpenes: α- terpineol (4.8–17.8%), linalyl acetate (0.5–9.6%), linalool (3.0–8.8%), thymol (0.2–8.3), terpinene 4-ol (1.5–8.3%). Several of the main essential oil constituents appeared to be highly intercorrelated. Strong positive correlations (r > 0.70; P < 0.01) were observed between α-terpineol and linalyl acetate, α-terpineol and thymol, linalyl acetate and thymol, γ-terpenene and carvacrol, (E)-β-ocimene and β-bisabolene, while strong negative correlations (r < −0.70; P < 0.01) were evidenced between γ-terpenene and β-caryophyllene, γ-terpenene and germacrene D, p-cymene and germacrene D. Multivariate analyses allowed the grouping of the populations into three distinct chemotypes. Population P5 (Origanum vulgare subsp. hirtum) was distinguished from O. vulgare subsp. vulgare populations by predominance of carvacrol, while within the typical subspecies the population P1 (Boljevići) separated from P2 (Radovče), P3 (Lipovo) and P4 (Grahovo) by high oil levels of α-terpineol, linalyl-acetate and thymol.

Three Schiff bases were synthesized by reaction of different benzaldehydes with amino acids. The characterization of these compounds was performed using IR spectroscopy, molecular calculations, thin-layer chromatography, determining the melting point and other physical characteristics. IR spectra for imino groups (C=N), which are characteristic of Schiff bases, show stretching frequency from 1629 to 1654 cm-1. The obtained spectral results were confirmed by molecular calculations using the density functional theory (DFT) and were performed before experimental work. The DFT global chemical reactivity descriptors were calculated and used to predict their relative stability and reactivity of synthesized compounds. The antimicrobial assay of all compounds were screened for Grampositive bacteria species: Staphylococcus aureus ATCC 25923; Methicillin-resistant Staphylococcus aureus: MRSA ATCC 33591; Bacillus subtilis ATCC 6633; and Enterococcus faecalis ATCC 29212, Gram-negative: Salmonella enterica ATCC 31194; Pseudomonas aeruginosa ATCC 9027; Escherichia coli ATCC 25922; Extended Spectrum Beta-Lactamase producing E. coli: ESBL E. coli ATCC 35218, and one yeast Candida albicans ATCC 1023.The highest values of inhibition zones were recorded for compound1, followed by the compound 3, while compound 2 performed inhibitory effect just in case of MRSA. DFT calculations show that antimicrobial activity has a good correlation with chemical reactivity descriptors obtained Schiff bases.