Content area
Full Text
Significance of the Study
• The study revealed the synergistic effect of combinations of R-limonene, S-limonene, myrcene, sabinene, and β-elemene with first-line tuberculostatic antibiotics against isolated Mycobacterium tuberculosis. Hence, such combinations might be promising for treating resistant M. tuberculosis.
Introduction
The World Health Organization reported that there was a 3.3% rise in new cases of tuberculosis (TB) in 2015 [1]. It was also estimated that 20% of previously treated cases involved multidrug-resistant (MDR)-TB, and that, in this group, 9.7% were extensive drug-resistant (XDR)-TB [1]. Drug resistance in Mycobacterium tuberculosis (Mtb) is caused by the sequential accumulation of mutations in the genes encoding the targets of tuberculostatic antibiotics [2]. More importantly, the active transmission of genotypes of several strains circulating worldwide is the cause of increased resistance [2]. Molecular fingerprinting methods enable the identification of MDR-TB and XDR-TB, which constitute 0.5% of the circulating strains in Japan and 40% in Russia, respectively [2]. XDR-TB is the biggest threat because it is a form of the disease that is nearly untreatable [1] and may develop as a multisystem disease [3].
The natural terpenes are known for their antimicrobial properties, and their detrimental effects on the structure and function of the microbial membranes and cell walls are thought to be evidence of antimicrobial action [4]. Combinations of essential oils and their constituents have been reported to have synergistic, additive, or inhibiting activity with antimicrobial interactions against several microorganisms [5]. A high antimycobacterial activity of thymol and carvacrol has been described against Mtb and Mycobacterium bovis [4]. Synergistic in vitro interactions between oleanolic acid and isoniazid, rifampicin, or ethambutol against Mtb have also been described [6].
In this study, we aimed to describe the influence of R-limonene, S-limonene, myrcene, sabinene, α-pinene, and β-elemene on the antimycobacterial activity of first-line tuberculostatic drugs, i.e. isoniazid (an inhibitor of fatty acid synthesis), rifampicin (an inhibitor of RNA polymerase), and ethambutol (an inhibitor of arabinose transferases involved in cell wall biosynthesis).
Materials and Methods
Standards and Media
The standard antibiotics, rifampicin, isoniazid, and ethambutol, and also the terpenes (with the exception of sabinene) were purchased from Sigma-Aldrich (Munich, Germany). Gas chromatography-mass spectrometry (GC-MS) was used to verify the purity of the terpenes: 98% for α-pinene, 97% for