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Human infectious diseases once treatable with antibiotics are now becoming a serious life-threatening problem due to the emergence of multidrug resistant (MDR) micro-organisms. For this, essential oils (EOs) could constitute a viable source of bioactive compounds capable of inhibiting the mechanisms of this antimicrobial resistance. In this context, our goals were to determine for the first time the chemical composition of EO extracted from aerial parts of Rhus albida, its antibacterial and synergistic interactions with fluoroquinolones and finally to evaluate its ability to inhibit the Staphylococcus aureus NorA efflux pump. The chemical profile and antibacterial activity of R. albida EO were determined using Gas Chromatography coupled to Mass Spectrometry and microdilution assays, respectively. While its NorA efflux-pump-inhibiting activity was assessed using a fluorescent real-time efflux method (based on ethidium bromide (EtBr)) against two S. aureus strains: a NorA overproducing strain (SA1199B) and a wild type strain (SA1199). Results showed that the EO, which is characterized by the abundance sesquiterpenes (δ-cadinene (15.6%), α-copaene (6.3%) and caryophyllene (6.2%)), displayed moderate antimicrobial activities against both SA1199 and SA1199B (MIC = 2048 µg/mL). Nevertheless, its combination with fluoroquinolones contributed to the reduction of their intrinsic MICs only against SA1199B by 4- to 8-fold. Furthermore, the EO at 1/32 MIC, contributed to a large accumulation of EtBr and inhibition of its efflux in SA1199B. Our findings demonstrated the ability of R. albida EO to inhibit the NorA efflux pump. Thus, further studies should be performed in order to make the combination of this natural product with approved antimicrobials a viable option for future administrations in infections with S. aureus strains overproducing NorA EPs.

The present work was conducted to study the antioxidant activity and to determine for the first time the antimicrobial potential of Moroccan Cannabis sativa essential oil (EO) singly or in combination with two conventional antibiotics (fluconazol and ciprofloxacin) against some resistant and pathogenic microorganisms. The chemical composition analyzed by means of GC/MS showed that C. sativa EO was characterized by the dominance of (E)-caryophyllene (35.0%), α-humulene (12.8%) and caryophyllene oxide (10.6%). Results from antioxidant tests showed that C. sativa EO exhibit a moderate potency (IC50 = 1.6 ± 0.1 mg/m L for 2,2- diphenyl-1-picrylhydrazyl (DPPH) assay, IC50 = 1.8 ± 0.2 mg/mL for β-carotene/linoleic acid assay, and IC50 = 0.9 ± 0.1 mg/m L for ferric reducing power assay). Regarding antimicrobial assays, the results demonstrated that C. sativa EO displayed interesting antimicrobial activities with minimum inhibitory concentration (MIC) values in the range from (1.2 to 37.8) mg/m L for tested microorganisms. The combination of the studied EO with fluconazol and ciprofloxacin showed a significant decrease in their individual MICs. In fact, at sub-inhibitory concentrations, C. sativa EO contributed to the decrease of ciprofloxacin MICs of bacterial strains by 2- to 64-fold and by16-fold regarding fluconazol MICs against Candida spp. These findings suggest that C. sativa EO can be considered as a potential source of natural antioxidant and antimicrobials, and constitute in combinational treatments a promising strategy to overcome the intense use of antibiotics against some infectious diseases.

The aims of the present study were to evaluate for the first time the chemical composition and in vitro antimicrobial potential of essential oil (EO) obtained from endemic Lavandula maroccana (Murb.) singly or in combination with some conventional antimicrobials (ciprofloxacin, vancomycin, fluconazol, and amphotericin B) against a panel of pathogenic microorganisms. The gas chromatography/mass spectrometry (GC/MS) analysis of EO led to the identification of 20 components, representing 98.8% of the total oil. The main constituent was the phenolic monoterpene carvacrol (78.2%), followed by 2-Methoxy-4-vinylphenol (2.5%), and Spathulenol (2.2%). The results of the antimicrobial assays demonstrated that L. maroccana EO displayed moderate activity, with minimal inhibitory concentration (MIC) values of 4.604 mg/mL for yeasts and ranged from 2.302 to 18.416 mg/mL for bacteria. According to FIC index values, the combination between L. maroccana EO and conventional antimicrobials elicited very promising synergistic effects against all tested strains. In fact, at sub-inhibitory concentrations L. maroccana EO contributed to the decrease of fluconazol and amphotericin B MICs of Candida strains by 64- to 128-fold and by 2- to 4-fold, respectively. Regarding bacterial strains, L. maroccana EO has reduced the ciprofloxacin and vancomycin MICs by 4- to 33-fold and by 2- to 4-fold, respectively. From these results, it can be concluded that L. maroccana EO can be considered as a promising source of natural antimicrobials, and constitute a potential candidate to be used in combination antibiotic therapy in order to reduce the overuse of antibiotics, and to combat some human infectious diseases.

Over the past years, the emergence of multidrug-resistant pathogens has become a pressing public health problem. It is an important cause of morbidity and mortality worldwide and puts a considerably financial burden on the health care system. The development of new antimicrobials is becoming increasingly difficult, time-consuming and expensive. Hence, new approaches are urgently needed to replenish our arsenal of antimicrobial agents. Plant tissues contain a wild variety of antimicrobial constituents discovered and used to reverse drugs resistance and restore their efficiency. For this, essential oil (EO) extracted from aerial parts of Senecio anteuphorbium (L) Haw, a Moroccan endemic plant, was studied for the first time for its: (i) Qualitative and quantitative chemical profile, (ii) antimicrobial activities, (iii) and synergistic interactions with two conventional antifungals and ciprofloxacin. The data of the chemical composition showed that S. anteuphorbium EO was characterized by the abundance of γ-selinene (27.2%), cyperene (21.7%), γ-cadinene (11.4%) and α-cyperone (8.1%). Regarding antimicrobial assays, results demonstrated that S. anteuphorbium EO displayed moderate antimicrobial activities with MIC values in the range of 0.51 mg/mL and 1.02 mg/mL for yeasts and between 2.3 mg/mL and 4.6 mg/mL for bacteria. However, the combination of the studied EO with fluconazole and ciprofloxacin showed a significant decrease in their individual MICs. In fact, at sub-inhibitory concentrations S. anteuophorbium EO contributed to the decrease of fluconazole MICs of Candida strains by 4- to 64- fold and by 2- to 16- fold regarding ciprofloxacin MICs. The overall data suggest that the topical or systemic use of S. anteuphorbium EO in combinational treatments may be a promising strategy to combat infectious diseases and further studies should be performed in order to well determine the main mechanism of these synergistic associations.

Introduction Combinations of plant essential oils (EOs) provide a novel approach to increasing effectiveness against resistant bacteria. In this context, the combined antibacterial effects of EOs obtained from Lavandula maroccana (Murb.), Thymus pallidus Batt., and Rosmarinus officinalis L. against three multi-drug resistant Gram-negative bacteria were evaluated. Methods The antibacterial effect of each EO alone was evaluated by agar diffusion and broth microdilution methods, while the effects of EO binary combinations were evaluated through the determination of the fractional inhibitory concentration index (FICI) using the checkerboard method. Results GC-MS analysis of EOs showed that T. pallidus was dominated by γ-terpinene, thymol and p-cymene, while the main constituents of R. officinalis were 1,8-cineole, camphor, and α-pinene. L. maroccana EO was characterized by a high percentage of carvacrol. EOs tested individually demonstrated moderate antibacterial activity with MIC and MBC values from 1.3 mg/mL to72 mg/mL, whereas the FICI of their binary combinations revealed interesting interactions. The combination of EOs obtained from T. pallidus and L. maroccanas showed the highest number of effective combinations, with synergistic effects against P. aeruginosa and K. pneumoniae, and an additive effect against E. coli. The mixture of L. maroccana and R. officinalis EOs showed synergistic effects against E. coli and P. aeruginosa with an additive effect against K. pneumoniae. The combination of R. officinalis and T. pallidus EOs showed a synergistic effect against K. pneumoniae only. Conclusion These findings suggest that combining L. maroccana with T. pallidus and/or R. officinalis EOs may constitute a promising option for controlling the emergence of multidrug-resistant bacteria.

Inhibition of bacterial efflux mechanisms has been investigated as a promising target to combat the bacterial resistance to antibiotics. The present work investigated the ability of Artemisia herba-alba and Cymbopogon citratus essential oils (EOs) to inhibit efflux pumps of some selected antibiotic-resistant Gram-negative bacteria. The chemical profiles of tested EOs were evaluated by GC–MS. The major constituents of the oils were chrysanthenone (47.0%), camphor (24.0%), and verbenone (7.2%) in A. herba-alba EO and geranial (46.3%), neral (35.2%), and geraniol (5.4%) in C. citratus EO. Antibacterial tests showed that these EOs had high antibacterial activity. This activity was significantly enhanced in the presence of an efflux pump inhibitor, phenylalanine arginyl s-naphthylamide (PAsN). When combined with PAsN, the MIC values against EA27 and AG102 (strains overexpressing efflux pumps) were decreased 32-fold and >32-fold, respectively, for A. herba-alba , and 512-fold and 1024-fold, respectively, for C. citratus EO. The involvement of a lipopolysaccharide (LPS) structure in the resistance to the active compounds of the studied EOs was also demonstrated in Salmonella LPS deep-rough mutants. Moreover, the combination of A. herba-alba and C. citratus EOs, at a low concentration, reduced significantly chloramphenicol MIC 2–4-fold, 4-fold and 8–16-fold, respectively, for EAEP294 (deleted of AcrAB), EA27 and AG102. For Salmonella strains, all tested combinations recorded a gain greater than or equal to 64-fold. It was concluded that these EOs can alter efflux pump activity and may be good candidates for the development of new chemosensitizer drugs able to restore antibiotic activity of some drug-resistant Gram-negative bacteria.

The in vitro antibacterial activity of Thymus riatarum essential oil was evaluated in the present study using different assays toward human pathogenic isolates collected during serious infections. The phytochemical analysis of T. riatarum essential oil (EO) indicated the presence of 20 components, including mainly borneol (41.67%), terpinen-4-ol (8.65%), and trans -caryophyllene (7.59%). Determination of antibacterial activity showed that the EO possesses a noticeable potential of inhibiting the growth of tested strains with Minimum Inhibitory Concentrations (MICs) ranging from 3.57 mL/L to 7.5 mL/L. However, Pseudomonas aeruginosa showed the least sensitivity and was only inhibited with high concentrations (MIC > 7.5 mL/L). The release of cytoplasmic material absorbing at 260 nm, measured in the presence of T. riatarum EO, increased in response to oil concentration. The ability of T. riatarum EO to block efflux pumps systems of resistant Gram-negative bacteria was also studied. The results indicated that the EO activity was significantly enhanced in the presence of an efflux pump inhibitor such as phenylalanine arginyl s-naphthylamide (PAsN). In addition, the tested EO, used at MIC/4, was able to increase chloramphenicol susceptibility of several resistant isolates. These results demonstrated that T. riatarum EO could permeabilize bacterial membrane impairing efflux pump activity and may be used in combination with conventional antibiotics against multidrug-resistant bacteria. To our knowledge, this is the first report about the antibacterial activity and the mode of action of T. riatarum EO.

The aim of this study was to evaluate the antibacterial effect of the association between conventional antibiotics and essential oils (EOs) of endemic Moroccan thyme species, Thymus maroccanus and T. broussonetii, on antibiotic-resistant bacteria involved in nosocomial infections. Synergistic interactions between antibiotics (ciprofloxacin, gentamicin, pristinamycin, and cefixime) and EOs, and between T. maroccanus and T. Broussonetii EOs were determined by the checkerboard test. Serial dilutions of two antimicrobial agents were mixed together so that each row (and column) contained a fixed amount of the first agent and increasing amounts of the second one. The results indicate that the oils had a high inhibitory activity against tested bacteria, except for Pseudomonas aeruginosa. In parallel with the increase of cellular killing, the release of 260nm-absorbing materials from bacterial cells, treated with EOs, increased in response to oil concentration. Out of 80 combinations tested between EOs and antibiotics, 71% showed total synergism, 20% had partial synergistic interaction and 9% showed no effect. Combination with carvacrol, the major constituent of T. maroccanus and T. broussonetii, showed also an interesting synergistic effect in combination with ciprofloxacin. The effect on Gram-positive bacteria was more important than on Gram-negative bacteria. These findings are very promising since the use of these combinations for nosocomial infections treatment is likely to reduce the minimum effective dose of the antibiotics, thus minimizing their possible toxic side effects and treatment cost. However, further investigations are needed to assess the potential for therapeutic application.

Bacterial drug resistance is a worrying public health problem. Antibiotic efflux is a major non-specific resistance mechanism used by bacteria, and efflux pumps are involved in the low-level susceptibility of various important Gram-negative pathogens. Use of molecules that can block bacterial pumps is an attractive strategy, but several studies report only partial efficacy owing to limits of these molecules (stability, selectivity, bioavailability, toxicity, etc.). The objective of this study was to search for natural sources of molecules able to inhibit efflux pump systems of resistant Gram-negative bacteria (Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, Salmonella enterica serotype Typhimurium and Pseudomonas aeruginosa). The results indicate that the studied essential oils exhibit interesting activity against the tested bacteria. This activity was significantly enhanced in the presence of an efflux pump inhibitor such as phenylalanine arginyl β-naphthylamide (PAβN). The role of lipopolysaccharide (LPS) structure in the effect of essential oils was also reported in Salmonella LPS deep-rough mutants. In addition, essential oils of Thymus maroccanus and Thymus broussonetii, used at a low concentration (a fraction of the minimum inhibitory concentration), are able to significantly increase chloramphenicol susceptibility of several resistant isolates. These results demonstrate that these essential oils can alter efflux pump activity and may be attractive candidates to develop new drugs for chemosensitising multidrug-resistant strains to clinically used antibiotics.