Aijaz Ahmad

Fungicidal activity of thymol and carvacrol by disrupting ergosterol biosynthesis and membrane integrity against Candida

Natural isopropyl cresols have been reported to have antifungal activity. This work is an attempt to examine thymol and carvacrol against 111 fluconazole-sensitive and -resistant Candida isolates. Insight into the mechanism of action was elucidated by flow cytometric analysis, confocal imaging and ergosterol biosynthesis studies. The susceptibility tests for the test compounds were carried out in terms of minimum inhibitory concentrations (MICs), disc diffusion assays and time–kill curves against all Candida isolates by employing standard protocols. Propidium iodide (PI) cell sorting has been investigated by flow cytometric analysis and confocal imaging. Haemolytic activity on human erythrocytes was studied to exclude the possibility of further associated cytotoxicity. Both compounds were found to be effective to varying extents against all isolates, including the resistant strains. In contrast to the fungistatic nature of fluconazole, our compounds were found to exhibit fungicidal nature. Significant impairment of ergosterol biosynthesis was pronouncedly induced by the test entities. Negligible cytoxicity was observed for the same compounds. Furthermore, it was observed that the positional difference of the hydroxyl group in carvacrol slightly changes its antifungal activity. Carvacrol and thymol show strong fungicidal effect against all of the Candida isolates. The mechanisms of action of these natural isopropyl cresols appear to originate from the inhibition of ergosterol biosynthesis and the disruption of membrane integrity.

Ocimum sanctum essential oil and its active principles exert their antifungal activity by disrupting ergosterol biosynthesis and membrane integrity

The increasing incidence of drug-resistant pathogens and host toxicity of existing antifungals attracts attention toward the efficacy of natural products as antifungals in mucocutaneous infections and combinational therapies. The composition and antifungal activity of the essential oil obtained from Ocimum sanctum (OSEO) was studied. On GC-MS analysis, OSEO showed a high content of methyl chavicol (44.63%) and linalool (21.84%). Antifungal activity of OSEO and its two main constituents was determined against sixty clinical and five standard laboratory isolates of Candida. OSEO, methyl chavicol and linalool showed inhibitory activity toward all tested strains. The mechanism of their fungicidal action was assessed by studying their effect on the plasma membrane using flow cytometry, confocal imaging and determination of the levels of ergosterol, a fungal-specific sterol. Propidium iodide rapidly penetrated a majority of yeast cells when they were treated with OSEO concentrations just above MIC, implying that fungicidal activity resulted from extensive lesions of the plasma membrane. OSEO and its components also caused a considerable reduction in the amount of ergosterol. The present study indicates that OSEO, methyl chavicol and linalool have significant antifungal activity against Candida, including azole-resistant strains, advocating further investigation for clinical applications in the treatment of fungal infections.

Evolution of ergosterol biosynthesis inhibitors as fungicidal against Candida.

Azoles target the ergosterol synthesizing enzyme lanosterol 14alpha-demethylase and are a widely applied class of antifungal agents. Unfortunately azoles are generally fungistatic, and resistance to fluconazole is emerging in several fungal pathogens. In contrast to the increasing number of agents for the treatment of invasive fungal infections, discoveries of new antifungal agents with therapeutic value in dermatomycoses are reported only rare. Attention has been drawn to the antimicrobial activity of plants and their active principles due to the challenge of growing incidences of drug-resistant pathogens. Eugenol and methyl eugenol were reported to possess antimycotic properties. To further explore the antifungal activity of these compounds, in vitro studies were conducted on various Candida isolates. Insight studies to mechanism suggested that both eugenol and methyl eugenol exerts their antifungal activity by targeting sterol biosynthesis. Furthermore, it was also observed that additional methyl group to eugenol increases its antifungal activity. The observed fungicidal characteristics of both eugenol and methyl eugenol indicate that both the compounds might be promising antifungal agents defining a new class of antimycotics.

Proton translocating ATPase mediated fungicidal activity of eugenol and thymol

Eugenol (1) and thymol (2) exhibit excellent fungicidal activity against pathogenic yeasts, including isolates resistant to azoles. The rapid irreversible action of compound-1 and compound 2 on fungal cells suggested a membrane-located target for their action. We investigated their effect on H(+)-ATPase mediated H(+)-pumping by various Candida species. Both compounds inhibit H(+)-ATPase activity at their respective MIC values--500 and 100 μg/ml. Glucose stimulated H(+)-extrusion was also inhibited significantly by compound 1 and compound 2. Inhibition of H(+)-ATPase leads to intracellular acidification and cell death. Inhibition of cell growth and H(+)-efflux by test compounds suggests that their antifungal properties are related to their inhibitory effects on H(+)-ATPase.

In vitro synergy of eugenol and methyleugenol with fluconazole against clinical Candida isolates.

The species Candida is a group of opportunistic pathogenic commensals in immune-compromised patients. Treatment of Candida infections is becoming increasingly difficult due to antifungal drug resistance, especially with fluconazole (FLC), which is a commonly used azole. In the present study the in vitro antifungal activity of eugenol (EUG) and methyleugenol (MEUG) alone and in combination against 64 FLC-sensitive and 34 FLC-resistant clinical Candida isolates is highlighted. All the strains were susceptible to both the naturally occurring phenyl propanoids. The nature of the interaction was studied from fractional inhibitory concentration indices (FICIs) for both EUG plus FLC, and MEUG plus FLC combinations calculated from chequerboard microdilution assays. FICI values depicted a high synergism of FLC with both compounds, which was greatest with MEUG. FLC-resistant Candida isolates showed high sensitivity to both compounds. No antagonistic activity was seen in the strains tested in the present study. From these results we suggest that EUG and MEUG have great potential as antifungals, and that FLC can be supplemented with EUG and MEUG to treat FLC-resistant Candida infections.

Reversal of efflux mediated antifungal resistance underlies synergistic activity of two monoterpenes with fluconazole

Thymol (THY) and carvacrol (CARV), the principal chemical components of thyme oil have long been known for their wide use in medicine due to antimicrobial and disinfectant properties. This study, however, draws attention to a possible synergistic antifungal effect of these monoterpenes with azole antimycotic-fluconazole. Resistance to azoles in Candida albicans involves over-expression of efflux-pump genes MDR1, CDR1, CDR2 or mutations and over-expression of target gene ERG11. The inhibition of drug efflux pumps is considered a feasible strategy to overcome clinical antifungal resistance. To put forward this approach, we investigated the combination effects of these monoterpenes and FLC against 38 clinically obtained FLC-sensitive, and eleven FLC-resistant Candida isolates. Synergism was observed with combinations of THY-FLC and CARV-FLC evaluated by checkerboard microdilution method and nature of the interactions was calculated by FICI. In addition, antifungal activity was assessed using agar-diffusion and time-kill curves. The drug efflux activity was determined using two dyes, Rhodamine6G (R6G) and fluorescent Hoechst 33342. No significant differences were observed in dye uptakes between FLC-susceptible and resistant isolates, incubated in glucose free buffer. However, a significantly higher efflux was recorded in FLC-resistant isolates when glucose was added. Both monoterpenes inhibited efflux by 70-90%, showing their high potency to block drug transporter pumps. Significant differences, in the expression levels of CDR1 and MDR1, induced by monoterpenes revealed reversal of FLC-resistance. The selectively fungicidal characteristics and ability to restore FLC susceptibility in resistant isolates signify a promising candidature of THY and CARV as antifungal agents in combinational treatments for candidiasis.

Induction of oxidative stress as a possible mechanism of the antifungal action of three phenylpropanoids

The increasing incidence of hospital-acquired infections caused by drug-resistant pathogens, host toxicity, the poor efficacy of drugs and high treatment costs has drawn attention to the potential of natural products as antifungals in mucocutaneous infections and combinational therapies. Moreover, cellular and subcellular targets for these compounds may provide better options for the development of novel antifungal therapies. Eugenol, methyl eugenol and estragole are phenylpropanoids found in essential oil. They are known to possess pharmacological properties including antimicrobial activity. Induction of oxidative stress characterized by elevated levels of free radicals and an impaired antioxidant defence system is implicated as a possible mechanism of cell death. An insight into the mechanism of action was gained by propidium iodide cell sorting and oxidative stress response to test compounds in Candida albicans. The extent of lipid peroxidation (LPO) of cytoplasmic membranes was estimated to confirm a state of oxidative stress. Activity levels of primary defence enzymes and glutathione were thus further determined. Whereas these compounds cause fungal cell death by disrupting membrane integrity at minimum inhibitory concentrations (MIC), sub-MIC doses of these compounds significantly impair the defence system in C. albicans. The study has implications for understanding microbial cell death caused by essential oil components eliciting oxidative stress in Candida. The formation of membrane lesions by these phenylpropanoids thus appears to be the result of free radical cascade-mediated LPO.

Synthesis and synergistic antifungal activities of a pyrazoline based ligand and its copper(II) and nickel(II) complexes with conventional antifungals

A pyrazoline based ligand; (5-(4-chlorophenyl)-3-phenyl-4, 5-dihydro-1H-pyrazole-1-carbothioamide) has been synthesized by Claisen-Schmidt condensation of acetophenone with p-chlorobenzaldehyde, followed by sodium hydroxide assisted cyclization of the resulting chalcone with thiosemicarbazide. Metal ion complexes of the synthesized ligand were prepared with Cu(II) and Ni(II) metal ions, separately and respectively. Ligand and the metal complexes were characterized by elemental analysis, FT-IR, UV-Vis, (1)HNMR, ESI-MS and (13)CNMR spectroscopic techniques. Molar conductance measurements in DMSO suggested non-electrolytic nature of the complexes. Tetragonally distorted octahedral geometry for copper and octahedral geometry for the nickel complexes was proposed on the basis of UV-Vis spectroscopic studies and magnetic moment measurements. The complexes were investigated for their ability to kill human fungal pathogen Candida by determining MICs (Minimum inhibitory concentrations), inhibition in solid media and ability to produce a possible synergism with conventional most clinically practiced antifungals by disc diffusion assay and FICI (fractional inhibitory concentration index).

The in vitro antimicrobial activity of Cymbopogon essential oil (lemon grass) and its interaction with silver ions

BACKGROUND It is well known that Cymbopogon (lemon grass) essential oil exhibits antimicrobial activity while the efficacy of silver ions as a disinfectant is equally well reported. HYPOTHESIS The antimicrobial activity of CEO and Ag(+) and their synergistic combinations will be useful in improving the current treatment strategies for various infections. STUDY DESIGN In the present study, we determined the chemical composition and in vitro antimicrobial activity of six different Cymbopogon essential oils (CEOs) alone and in combination with silver ions (Ag(+)) against two Gram-positive (Staphylococcus aureus and Enterococcus faecalis), two Gram-negative (Escherichia coli and Moraxella catarrhalis) and two yeast species (Candida albicans and Candida tropicalis). The nature of potential interactions was determined by fractional inhibitory concentration indices (FICIs) for CEOs and Ag(+) calculated from microdilution assays and time-kill curves. RESULTS Gas chromatography-mass spectrometry results confirmed the presence of nerol, geranial and geraniol as major volatile compounds. Minimum inhibitory concentration (MIC) values confirmed that all the tested pathogens are variably susceptible to both CEOs as well as Ag(+). The MIC of CEOs and Ag(+) against all the tested pathogens ranged from 0.032 mg/ml to 1 mg/ml and 0.004 and 0.064 mg/ml respectively, whereas when assayed in combination the FICI values were drastically reduced to range between 0.258 and 2.186, indicating synergy, additive and indifferent interactions. The most prominent interaction was observed between Cymbopogon flexuosus essential oil and Ag(+) against C. albicans with ∑FIC = 0.254. The synergistic interactions were further confirmed through the construction of isobolograms and time-kill plots. Transmission electron microscopy showed disturbance in the cell envelope upon the concomitant treatment of CEOs and Ag(+), which ultimately leads to cell death. CONCLUSION Results suggest that CEOs and Ag(+) when used in combination offers an opportunity to the formulation scientist to produce novel combinations acting synergistically in the continued quest to control important infectious pathogens.

Unravelling the complex antimicrobial interactions of essential oils--the case of Thymus vulgaris (thyme).

Thymus vulgaris has gained tremendous popularity as an ornamental, culinary herb and its use in phytotherapy is well established and supported in the literature. The objective of this study was to explore possible interactions between selected molecules within Thymus vulgaris essential oil (TvEO) to gain a better understanding of how this complex essential oil exerts its antimicrobial activity. Evaluation of the antimicrobial efficacy and interactions were assessed on the essential oil and volatile constituents against various pathogens. Interactions between molecules at various ratios were graphically observed through the construction of isobolograms. Gas chromatography-mass spectrometry (GC-MS) analysis revealed 22 compounds which collectively represent >95% of the oil composition. Based on their minimum inhibitory concentration (MIC) values, they were categorised into weak (≥4 mg mL−1), moderate (2–4 mg mL−1) and noteworthy active (≤2 mg mL−1) compounds. For the combination study, 21% synergistic, 42% additive, 36% indifferent and 1% antagonistic interactions were observed. Most of the interactions were observed between the weak and highly active molecules, and interestingly, no synergistic interaction was observed between the highly active compounds. Synergistic and additive interactions between the strong and weaker antimicrobial constituents present in TvEO enhance the antimicrobial efficacy of this commercially important essential oil.