Chemical composition and antimicrobial activity relationship of essential oils from certain Myrtaceous plants using spectroscopic techniques coupled to chemometrics

Antimicrobial resistance represents an increasingly serious global threat to public health as new resistance mechanisms emerge and spread all over the world. Special concerns are given to the emerging resistance rates to last-line antibiotics, especially in developing countries. In this context the evaluation of plant essential oils may serve as an alternative antimicrobial approach. Therefore, the essential oils from 15 species of three Myrtaceae genera (Syzygium, Eucalyptus, Melaleuca) harvested from botanical gardens (Egypt), were investigated regarding their individual chemical composition and antimicrobial activity. Their chemical profiles were analyzed using GC/FID, GC/MS, and ATR-IR and applying various chemometric algorithms (PCA and HCA). Eucalyptus camaldulensis and Corymbia citriodora showed the highest antimicrobial activity against Staphylococcus aureus ATCC 1578 (MIC 6 mg/ml), Staphylococcus aureus USA200 (12 & 6 mg/ml), MDR Enterobacter clocacae isolate (12 mg/ml), Klebsiella pneumoniae isolate (24 & 12 mg/ml), Bacillus subtilis ATCC CC33 (12, 1.5 mg/ml) and E. coli ATCC 87 (24 mg/ml). Analysis by GC/FID and GC/MS allowed the identification of 46 compounds, representing 89 to 99.5% of the total oil composition. The main components were methyleugenol (0.24 – 95%), citronellal (75 – 87.2%), 1,8-cineole (0.19 – 77.7%), α-pinene (0.21 – 64.8%), limonene (0.18 – 44.1%), caryophyllene oxide (0.23 – 42.8%), p-cymene (0.19 – 34.9%) and crypton (17.9 – 21.04%). E. camaldulensis was the richest species in p-cymene and crypton, while C. citriodora was the richest in citronellal. Therefore, citronellal, p-cymene and crypton could serve as potential candidates for developing novel antibiotics. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) separated the 15 species essential oils into seven groups, each constituting a chemotype. ATR-IR analyses showed characteristic key bands that discriminated different chemotypes and correlated with those obtained by GC analysis. So, combination of vibrational spectroscopy and HCA could provide a fast and reliable method for chemotaxonomic characterization and control the purifying and redistilling processes of essential oils in the flavor and fragrance industry [1]

Fig. 1: Score 2 vs. Score 1

[1] Vibrational spectroscopy 2006; 42: 341 – 345