Antimicrobial, Multidrug Resistance Reversal and Biofilm Formation Inhibitory Effect of Origanum majorana Extracts, Essential Oil and Monoterpenes

Abstract

<i>Origanum majorana</i> L. is a widely used medicinal plant; its distilled oil and preparations are extensively utilised in the phytotherapy and food industries. The objective of this study is to evaluate the extracts and the essential oil (EO) of <i>Origanum majorana</i> L, and its monoterpenes for antimicrobial, bacterial multidrug resistance reversing, and biofilm formation inhibitory potency. The composition of EO and <i>n</i>-hexane extract was characterized by GC-MS. In the essential oil terpinen-4-ol (24.92%), <i>trans</i>-sabinene hydrate (25.18%), γ-terpinene (6.48%), <i>cis</i>-sabinene hydrate (5.44%), <i>p</i>-cymene (4.72%), sabinene (4.53%), α-terpineol (4.43%), and α-terpinene (3.00%) were found as the main constituents while <i>trans</i>-sabinene hydrate (1.43%), and terpinen-4-ol (0.19%) were detected in the <i>n</i>-hexane extract besides a series of hydrocarbons. The antibacterial activity of EO and terpinen-4-ol, α-terpinene, and linalool was also assessed against sensitive and drug-resistant <i>S. aureus</i>, and <i>E. coli</i> strains with MIC values of 0.125-0.250% and 30-61 µM, respectively. In the efflux pump (EP) inhibitory assay, made by the ethidium bromide accumulation method in <i>E. coli</i> ATCC 25922, and AG100 and <i>S. aureus</i> ATCC 25923, and MRSA ATCC 43300 strains, EO exhibited substantial activity, especially in the <i>E. coli</i> ATCC 25922 strain. Among the EO constituents, only sabinene was an EP inhibitor in sensitive <i>Escherichia</i> strain. In the case of <i>S. aureus</i> strains, EO and sabinene hydrate exhibited moderate potency on the drug-resistant phenotype. The antibiofilm effects of the samples were tested by crystal violet staining at sub-MIC concentration. γ-Terpinene, terpinen-4-ol, sabinene, sabinene hydrate and linalool were found to be effective inhibitors of biofilm formation (inhibition 36-86%) on <i>E. coli</i> ATCC 25922 and <i>S. aureus</i> MRSA ATCC 43300, while EO was ineffective on these strains. In contrast to this, biofilms formed by <i>E. coli</i> AG100 and <i>S. aureus</i> ATCC 25923 were significantly inhibited by the EO; however, it was not affected by any of the monoterpenes. This observation suggests that the antibiofilm effect might be altered by the synergism between the components of the essential oil.