Antibacterial and efflux pump inhibitors of thymol and carvacrol against food-borne pathogens

Development and characterization of a carvacrol nanoemulsion and evaluation of its antimicrobial activity against selected food-related pathogens

Carvacrol has been recognized as an efficient growth inhibitor of food pathogens. However, carvacrol oil is poorly water-soluble and can be oxidized, decomposed or evaporated when exposed to the air, light, or heat. To overcome these limitations, a carvacrol nanoemulsion was developed and its antimicrobial activity against food pathogens evaluated in this study. The nanoemulsion containing 3% carvacrol oil, 9% surfactants (HLB 11) and 88% water, presented good stability over a period of 90 days. In general, the carvacrol nanoemulsion (MIC: 256 µg ml^-1 for E. coli and Salmonella spp., 128 µg ml^-1 for Staphylococcus aureus and Pseudomonas aeruginosa) exhibited improved antimicrobial activity compared to the free oil. The carvacrol nanoemulsion additionally displayed bactericidal activity against Escherichia coli, P. aeruginosa and Salmonella spp. Therefore, the results of this study indicated that carvacrol oil nanoemulsions can potentially be incorporated into food formulations, wherein their efficacy for the prevention and control of microbial growth could be evaluated.

Synergistic antimicrobial combination of carvacrol and thymol impairs single and mixed-species biofilms of Candida albicans and Staphylococcus epidermidis

Candida albicans and Staphylococcus epidermidis are important opportunistic human pathogens, which form mixed-species biofilms and cause recalcitrant device associated infections in clinical settings. Further to many reports suggesting the therapeutic potential of plant-derived monoterpenoids, this study investigated the interaction of the monoterpenoids carvacrol (C) and thymol (T) against mono- and mixed-species growth of C. albicans and S. epidermidis. C and T exhibited synergistic antimicrobial activity. The time-kill study and post-antimicrobial effect results revealed the effective microbicidal action of the C + T combination. Filamentation, surface coating assays and live-dead staining of biofilms determined the anti-hyphal, antiadhesion, and anti-biofilm activities of the C + T combination, respectively. Notably, this combination killed highly tolerant persister cells of mono-species and mixed-species biofilms and demonstrated less risk of resistance development. The collective data suggest that the C + T combination could act as an effective therapeutic agent against biofilm associated mono-species and mixed-species infections of C. albicans and S. epidermidis.

Atomistic insight and modeled elucidation of conessine towards Pseudomonas aeruginosa efflux pump

Drug-resistant Pseudomonas aeruginosa efflux pump extrudes antibiotics from cells for survival. Efflux pump inhibitor (EPI) thus becomes an interesting alternative to handle the drug-resistant bacteria. Conessine, a natural steroidal alkaloid from Holarrhena antidysenterica, previously exhibited efflux pump inhibitory potential. Our molecular docking and molecular dynamics (MD) studies provided atomistic information as well as the interaction of conessine with bacterial MexB efflux pump in phospholipid bilayer membrane to further the previous experimental report. Herein, the binding site and proposed mode of action of conessine were identified compared to known/commercial EPIs such as PAβN or designed-synthetic P9D. Our results explained conessine binding mode of action as an effective agent against the MexB efflux pump. The MD simulation also suggested that conessine was able to affect glycine loop (G-loop) flexibility, and the reduced G-loop flexibility due to conessine could hinder an antibiotics extrusion. In addition, our study suggested the conessine core structure buried in a hydrophobic region in the efflux pump similar to other known EPIs. Our finding could cope as a key for the design and development of the conessine derivative as novel EPI against P. aeruginosa.Communicated by Ramaswamy H. Sarma.

Multidrug and Pan-Antibiotic Resistance—The Role of Antimicrobial and Synergistic Essential Oils: A Review

Bacterial resistance to antibiotics continues to be a grave threat to human health. Because antibiotics are no longer a lucrative market for pharmaceutical companies, the development of new antibiotics has slowed to a crawl. The World Health Organization reported that the 8 new bacterial agents approved since July 2017 had limited clinical benefits. While a cohort of biopharmaceutical companies recently announced plans to develop 2-4 new antibiotics by 2030, we needn’t wait a decade to find innovative antibiotic candidates. Essential oils (EOs) have long been known as antibacterial agents with wide-ranging arsenals. Many are able to penetrate the bacterial membrane and may also be effective against bacterial defenses such as biofilms, efflux pumps, and quorum sensing. EOs have been documented to fight drug-resistant bacteria alone and/or combined with antibiotics. This review will summarize research showing the significant role of EOs as nonconventional regimens against the worldwide spread of antibiotic-resistant pathogens. The authors conducted a 4-year search of the US National Library of Medicine (PubMed) for relevant EO studies against methicillin-resistant Staphylococcus aureus, multidrug-resistant (MDR) Escherichia coli, EO combinations/synergy with antibiotics, against MDR fungal infections, showing the ability to permeate bacterial membranes, and against the bacterial defenses listed above. EOs are readily available and are a needed addition to the arsenal against resistant pathogens.

Antibacterial and antibiofilm activities of thyme oil against foodborne multiple antibiotics-resistant Enterococcus faecalis

The inhibitory and bactericidal activities of thyme oil against the foodborne multiple antibiotics-resistant Enterococcus faecalis biofilm were evaluated in this study. Gas chromatography-mass spectrometry revealed that more than 70% of the composition of thyme oil is thymol. Crystal violet staining assay showed that 128 and 256 μg/mL thyme oil significantly inhibited the biofilm formation of E. faecalis. The cell adherence of E. faecalis, as shown by its swimming and swarming motilities, was reduced by thyme oil. The exopolysaccharide (EPS) quantification assay showed that thyme oil inhibited the EPS synthesis in E. faecalis biofilms. The 3D-view observations through confocal laser scanning and scanning electron microscopy suggested that cell adherence and biofilm thickness were decreased in thyme oil–treated biofilms. Quantitative real-time analyses showed that the transcription of ebp and epa gene clusters, which were related to cell mobility and EPS production, was inhibited by thyme oil. Thus, thyme oil effectively inhibited the biofilm formation of E. faecalis by affecting cell adherence and EPS synthesis. Furthermore, 2,048 and 4,096 μg/mL thyme oil can effectively inactivate E. faecalis population in the mature E. faecalis biofilms by 5.75 and 7.20 log CFU/mL, respectively, after 30 min of treatment. Thus, thyme oil at different concentrations can be used as an effective antibiofilm or germicidal agent to control E. faecalis biofilms.

Essential Oils: An Impending Substitute of Synthetic Antimicrobial Agents to Overcome Antimicrobial Resistance

Antimicrobial resistance (AMR) is an emerging problem in the world that has a significant impact on our society. AMR made conventional drugs futile against microorganisms and diseases untreatable. Plant-derived medicines are considered to be safe alternatives as compared to synthetic drugs. Active ingredients and the mixtures of these natural medicines have been used for centuries, due to their easy availability, low cost, and negligible side effects. Essential oils (EOs) are the secondary metabolites that are produced by aromatic plants to protect them from microorganisms. However, these EOs and their constituents have shown good fighting potential against drug-resistant pathogens. These oils have been proved extremely effective antimicrobial agents in comparison to antibiotics. Also, the combination of synthetic drugs with EOs or their components improve their efficacy. So, EOs can be established as an alternative to synthetic antimicrobial agents to eradicate tough form of infectious microorganisms. EO's can interact with multiple target sites, like the destruction of cytoplasm membrane or inhibition of protein synthesis and efflux pump, etc. The purpose of this review is to provide information about the antimicrobial activity of EOs attained from different plants, their combination with synthetic antimicrobials. In addition, mechanism of antimicrobial activity of several EOs and their constituents was reported.

Combined Application of Bacteriophages and Carvacrol in the Control of Pseudomonas syringae pv. actinidiae Planktonic and Biofilm Forms

Pseudomonas syringae pv. actinidiae (Psa) is the causative agent of the bacterial canker of kiwifruit (Actinidia spp.). Phage therapy has been suggested as a viable alternative approach to controlling this disease, but its efficacy is limited by the emergence of phage-resistant mutants. Carvacrol is an essential oil that may be useful for the control of Psa. Combination therapies can be used to overcome resistance development. Here, the combination of phages (single phage suspensions of phages PN05 and PN09, and a cocktail of both phages) and carvacrol was investigated in controlling Psa planktonic and biofilm forms in vitro. The phage therapy alone (with phages PN05 and PN09), and the carvacrol alone (minimum inhibitory concentration 2.0 mg/mL), inhibited Psa growth, but the combined effect of both therapies was more effective. The phages alone effectively inhibited Psa growth for 24 h, but Psa regrowth was observed after this time. The carvacrol (2.0 mg/mL) alone prevented the biofilm formation for 48 h, but did not destroy the pre-formed biofilms. The combined treatment, phages and carvacrol (2.0 mg/mL), showed a higher efficacy, preventing Psa regrowth for more than 40 h. In conclusion, the combined treatment with phages and carvacrol may be a promising, environment-friendly and cost-effective approach to controlling Psa in the kiwifruit industry.

Evaluation of the Antibacterial Activity and Efflux Pump Reversal of Thymol and Carvacrol against Staphylococcus aureus and Their Toxicity in Drosophila melanogaster

The antibacterial activity and efflux pump reversal of thymol and carvacrol were investigated against the Staphylococcus aureus IS-58 strain in this study, as well as their toxicity against Drosophila melanogaster. The minimum inhibitory concentration (MIC) was determined using the broth microdilution method, while efflux pump inhibition was assessed by reduction of the antibiotic and ethidium bromide (EtBr) MICs. D. melanogaster toxicity was tested using the fumigation method. Both thymol and carvacrol presented antibacterial activities with MICs of 72 and 256 µg/mL, respectively. The association between thymol and tetracycline demonstrated synergism, while the association between carvacrol and tetracycline presented antagonism. The compound and EtBr combinations did not differ from controls. Thymol and carvacrol toxicity against D. melanogaster were evidenced with EC50 values of 17.96 and 16.97 µg/mL, respectively, with 48 h of exposure. In conclusion, the compounds presented promising antibacterial activity against the tested strain, although no efficacy was observed in terms of efflux pump inhibition.

Antimicrobial activity and inhibition of the NorA efflux pump of Staphylococcus aureus by extract and isolated compounds from Arrabidaea brachypoda

Arrabidaea brachypoda is a native shrub of the Brazilian Cerrado widely used in the folk medicine for treatment of renal diseases and articular pains. This study aimed to, first, evaluate the antimicrobial activity of both extracts and isolated molecules Brachydins BR-A and BR-B obtained from the flowers of A. brachypoda against Staphylococcus aureus, Escherchia coli and Candida albicans species. A second objective was to investigate if these natural products were able to potentiate the Norfloxacin activity against the strain Staphylococcus aureus SA1199-B that overexpress the norA gene encoding the NorA efflux pump. Extracts and isolated compounds were analyzed by HPLC-PDA and LC-ESI-MS respectively. Minimal inhibitory concentrations of Norfloxacin or Ethidium Bromide (EtBr) were determined in the presence or absence of ethanolic extract, dichloromethane fraction, as well as BR-A or BR-B by microdilution method. Only BR-B showed activity against Candida albicans. Addition of ethanolic extract, dichloromethane fraction or BR-B to the growth media at sub-inhibitory concentrations enhanced the activity of both Norfloxacin and EtBr against S. aureus SA1199-B, indicating that these natural products and its isolated compound BR-B were able to modulate the fluoroquinolone-resistance possibly by inhibition of NorA. Moreover, BR-B inhibited the EtBr efflux in the SA1199-B strain confirming that it is a NorA inhibitor. Isolated BR-B was able to inhibit an important mechanism of multidrug-resistance very prevalent in S. aureus strains, thus its use in combination with Norfloxacin could be considered as an alternative for the treatment of infections caused by S. aureus strains overexpressing norA.

Determination of sensitivity of some food pathogens to spice extracts

Spices are primarily used as flavor enhancers and have attracted attention as natural food preservatives since their antimicrobial effects were determined. In the present study, the antimicrobial effects, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values on 5 important food-borne pathogenic bacteria were investigated in 20 different types of spices that are not commonly used. The results indicated that Hibiscus (Hibiscus sabdariffa) was the most effective against Listeria monocytogenes (26.37 mm zone diameter) and Staphylococcus aureus (24.15 mm zone diameter) (P < 0.05) followed by the chebulic myrobalan (Terminalia chebula) (21.34 ± 0.35 and 23.85 ± 1.69 mm diameter zone respectively) (P < 0.05). Likewise, Hibiscus (H. sabdariffa) showed the lowest MICs and MBCs concentration values on five important food-borne pathogens (L. monocytogenes) MIC; 0.187 mg/L, MBC; 0. Thus, this study determined that spices with antimicrobial activities can be used as natural preservatives.

MexAB-OprM Efflux Pump of Pseudomonas aeruginosa Offers Resistance to Carvacrol: A Herbal Antimicrobial Agent

Carvacrol is a herbal antimicrobial agent with in vitro activity against several bacterial pathogens. However, multidrug resistant strains of Pseudomonas aeruginosa are resistant to herbal antimicrobial compounds including carvacrol. Resistance of P. aeruginosa to carvacrol is not well studied. This study was aimed to identify the gene(s) associated with carvacrol resistance, thus to understand its mechanisms in P. aeruginosa. A herbal drug resistant strain was isolated from a hospital environment. Carvacrol sensitive mutant was generated using transposon mutagenesis. The inactivated gene in the mutant was identified as mexA, which is part of the mexAB-oprM operon. Inactivation of the mexA gene resulted in a >31-fold reduction in MIC of carvacrol, whereas a >80-fold reduction was observed in the presence of drug efflux inhibitor phenylalanine-arginine β-naphthylamide (PAβN). The parental herbal-resistant strain was completely killed within 3 h of incubation in the presence of carvacrol and PAβN. The mexA inactivation did not affect the resistance to other herbal compounds used. The results demonstrate that resistance to carvacrol in P. aeruginosa is mediated by the MexAB-OprM efflux pump.

Plant-derived secondary metabolites as the main source of efflux pump inhibitors and methods for identification

The upsurge of multiple drug resistance (MDR) bacteria substantially diminishes the effectiveness of antibiotic arsenal and therefore intensifies the rate of therapeutic failure. The major factor in MDR is efflux pump-mediated resistance. A unique pump can make bacteria withstand a wide range of structurally diverse compounds. Therefore, their inhibition is a promising route to eliminate resistance phenomenon in bacteria. Phytochemicals are excellent alternatives as resistance-modifying agents. They can directly kill bacteria or interact with the crucial events of pathogenicity, thereby decreasing the ability of bacteria to develop resistance. Numerous botanicals display noteworthy efflux pumps inhibitory activities. Edible plants are of growing interest. Likewise, some plant families would be excellent sources of efflux pump inhibitors (EPIs) including Apocynaceae, Berberidaceae, Convolvulaceae, Cucurbitaceae, Fabaceae, Lamiaceae, and Zingiberaceae. Easily applicable methods for screening plant-derived EPIs include checkerboard synergy test, berberine uptake assay and ethidium bromide test. In silico high-throughput virtual detection can be evaluated as a criterion of excluding compounds with efflux substrate-like characteristics, thereby improving the selection process and extending the identification of EPIs. To ascertain the efflux activity inhibition, real-time PCR and quantitative mass spectrometry can be applied. This review emphasizes on efflux pumps and their roles in transmitting bacterial resistance and an update plant-derived EPIs and strategies for identification.

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Active efflux as the main resistance strategy in bacteria.

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Phytochemicals as promising alternatives against efflux pumps-mediated MDR.

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Herbals-based efflux pump inhibitors screening, the methods.

The antibacterial properties of phenolic isomers, carvacrol and thymol

Most of the antibacterial activities of essential oils from the Lamiaceae herbaceous plant family thyme and oregano are attributed to their bioactive isomeric monoterpenoid constituents, carvacrol and thymol. Commercially available antibiotics of thymol or carvacrol have not yet been developed but health products have incorporated thymol into their formulations for their antimicrobial properties. Carvacrol and thymol are generally considered safe for consumption and they have been used in dental applications, approved as food flavorings and have been considered as antibacterial additives in food and feed. Many studies have demonstrated that carvacrol and thymol are potent antibacterial agents against both Gram-positive and Gram-negative bacteria. The most frequently reported mechanism of antibacterial action of both isomers involves the disruption of bacterial membrane leading to bacterial lysis and leakage of intracellular contents resulting in death. Other proposed mechanisms of antibacterial action include the inhibition of efflux pumps, prevention in the formation and disruption of preformed biofilms, inhibition of bacterial motility, and inhibition of membrane ATPases. In addition, both isomers have been found to act additively or synergistically with conventional antibiotics important in overcoming the problem of bacteria resistance in food and disease.

Restraining the multidrug efflux transporter STY4874 of Salmonella Typhi by reserpine and plant extracts

Efflux-mediated multidrug resistance is a well-known phenomenon facilitated by multidrug resistant (MDR) transporters. One of the approaches to counteract efflux-mediated resistance is the use of MDR pump inhibitors, and thus be used in combination with the conventional antibiotics to treat deadly diseases like typhoid fever. We have previously reported that STY4874, an efflux transporter of Salmonella serotype Typhi, exhibited promising characteristics as MDR pump. In this study, we aimed to get an insight into possible STY4874 inhibitors of plant origin. STY4874 was overexpressed in Escherichia coli and extracts from pomegranate peel, milk thistle seeds and reserpine, a synthetic plant alkaloid, were screened for inhibition of ciprofloxacin efflux. The extracts of milk thistle seeds and reserpine when incubated with ciprofloxacin showed statistically significant STY4874-mediated inhibitory activity, rendering the efflux pump inactive and hence early growth inhibition of host cells compared with cells expressing efflux pump and incubated only with ciprofloxacin. This efflux pump inhibitory activity was further confirmed by time-kill experiments. This study is the first to report on efflux pump inhibition of S. Typhi STY4874 and results can be extended towards its close homologues such as MdfA and MdtM from E. coli. SIGNIFICANCE AND IMPACT OF THE STUDY: Understanding and combating resistance governed by multidrug efflux transporters is an ongoing research intensive area, affecting treatment of various nosocomial and endemic/epidemic infections. Confronting drug resistance requires that inhibitors debilitating the underlying mechanisms should be included in combination therapy. One such example is the prescription of clavulanic acid as combination therapy with amoxicillin, collectively called as co-amoxiclav to combat β-lactamase-mediated resistance. However, research related to finding the inhibitors of efflux transporters, the resistance mechanism distinct from β-lactamase mediated resistance is at an early stage. The current study finds that plant-derived inhibitors can be an option towards restraining efflux-mediated resistance.

Antimicrobial Electrospun Polycaprolactone-Based Wound Dressings: An In Vitro Study About the Importance of the Direct Contact to Elicit Bactericidal Activity

Objective: To prepare efficient antibacterial carvacrol (CAR) and thymol (THY)-loaded electrospun polycaprolactone (PCL)-based wound dressings. Approach: Using electrospinning we were able to prepare wound dressings with antimicrobial action thanks to their large surface per volume ratio, which allows their loading with therapeutic amounts of active principles. By nuclear magnetic resonance we demonstrated that the antimicrobial compounds are donors of hydrogen bonds to the ester functional group in PCL, which acts as acceptor and that intermolecular interaction is responsible for the high drug loading achieved. Results: Those mats loaded with CAR and THY without the use of solubilizing agents were able to completely eradicate both Gram-positive (Staphylococcus aureus ATCC 25923) and Gram-negative (Escherichia coli S17 strain) bacteria at doses inferior to the ones needed when using the free nonsupported compounds. A superior antimicrobial action was observed for THY and CAR against Gram-negative bacteria than against Gram-positive bacteria, despite the higher hydrophilicity of the outer layer of Gram-negative bacteria. Innovation: We demonstrate that a direct contact between the bacteria and the dressing is required to elicit antimicrobial action. We also evaluated drug loadings by gas chromatography coupled with mass spectrometry and nuclear magnetic resonance validating a new analytical approach. Finally we were able to visualize the pathogenic bacteria on the dressings by confocal microscopy. Conclusion: The interaction between the PCL-based mat and the pathogenic bacteria is a key issue to achieve complete pathogen eradication. Under no-contact conditions, released CAR or THY from the electrospun mats did not exert any antimicrobial action at the doses tested.

Efficacy of oregano and rosemary essential oils to affect morphology and membrane functions of noncultivable sessile cells of Salmonella Enteritidis 86 in biofilms formed on stainless steel

AIMS

This study evaluated the efficacy of essential oil from Origanum vulgare L. (oregano; OVEO) and Rosmarinus officinalis L. (rosemary; ROEO) to inactivate sessile cells of Salmonella enterica serovar Enteritidis 86 (SE86) in young and mature biofilms formed on stainless steel.

METHODS AND RESULTS

Ultrastructural alterations and damage in different physiological functions caused by OVEO and ROEO in noncultivable sessile cells of SE86 were investigated using scanning electron microscopy and flow cytometry. OVEO (2·5 μl ml^-1 ) and ROEO (40 μl ml^-1 ) were effective to eradicate young and mature biofilms formed by SE86 sessile cells on stainless steel surfaces; however, the efficacy varied with exposure time. OVEO and ROEO caused alterations in morphology of SE86 sessile cells, inducing the occurrence of bubbles or spots on cell surface. OVEO and ROEO compromised membrane polarization, permeability and efflux activity in noncultivable SE86 sessile cells. These findings show that OVEO and ROEO act by a multitarget mechanism on SE86 membrane functions.

CONCLUSIONS

ROEO and OVEO showed efficacy to eradicate SE86 sessile cells in preformed biofilms on stainless steel, displaying a time-dependent effect and multitarget action mode on bacterial cell membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study provides for the first time the effects of OVEO and ROEO on morphology and physiological functions of noncultivable sessile cells of S. Enteritidis biofilms preformed on stainless steel surfaces.

Thymol Mediates Bactericidal Activity against Staphylococcus aureus by Targeting an Aldo-Keto Reductase and Consequent Depletion of NADPH

Staphylococcus aureus is a common pathogen that can cause life-threatening infections. Treatment of antibiotic-resistant S. aureus infection needs effective antibacterial agents. Thymol, a generally recognized safe natural compound, has potential as an alternative to treat S. aureus infections. However, the targets and mechanisms of action of thymol were not fully understood. Bioinformatics analysis showed that IolS, a predicted aldo-keto reductase (AKR) in S. aureus, could be a potential target of thymol. Isothermal titration calorimetry (ITC) analysis demonstrated that thymol directly binds IolS and amino acid residues (Y30 and L33) are essential for such binding. Deletion of IolS or mutation of Y30A and L33A reduced the bactericidal activity of thymol at the concentration of 200 μg/mL, suggesting that thymol mediates bactericidal activity via binding with IolS. Biochemical analysis showed that addition of thymol significantly increased AKR activity of IolS from 1.6 ± 0.1 to 2.4 ± 0.2 U (p < 0.05). The content of NADPH within S. aureus cells decreased significantly from 105 ± 5 to 72 ± 3 pmol/10⁸ cells (p < 0.05) following thymol treatment at the concentration of 200 μg/mL. Importantly, addition of NADPH could alleviate the bactericidal effect of thymol on S. aureus, indicating that the depletion of NADPH is responsible for thymol-mediated bactericidal activity. Overall, these results demonstrated that thymol could directly bind IolS and increase its AKR activity, leading to the depletion of NADPH and bactericidal effect. AKR activity of IolS could be a promising target for the development of new antimicrobials.

Review on plant antimicrobials: a mechanistic viewpoint

Microbial resistance to classical antibiotics and its rapid progression have raised serious concern in the treatment of infectious diseases. Recently, many studies have been directed towards finding promising solutions to overcome these problems. Phytochemicals have exerted potential antibacterial activities against sensitive and resistant pathogens via different mechanisms of action. In this review, we have summarized the main antibiotic resistance mechanisms of bacteria and also discussed how phytochemicals belonging to different chemical classes could reverse the antibiotic resistance. Next to containing direct antimicrobial activities, some of them have exerted in vitro synergistic effects when being combined with conventional antibiotics. Considering these facts, it could be stated that phytochemicals represent a valuable source of bioactive compounds with potent antimicrobial activities.

Vanillin mediated green synthesis and application of gold nanoparticles for reversal of antimicrobial resistance in Pseudomonas aeruginosa clinical isolates

Antimicrobial resistance (AMR) is a serious concern in pathogenic bacteria. As a new approach to addressing AMR, we report here the green synthesis of vanillin capped gold nanoparticles (VAuNPs) using the popular flavouring molecule vanillin (C₈H₈O₃) as a reducing and capping agent. Physicochemical characterization revealed that the synthesised VAuNPs were stable and crystalline in nature. VAuNPs were non-bactericidal even at high concentration (>2000 μg/ml). The antibiotic potentiation activity was studied in combination with seven widely used antibiotics against extremely drug resistant (XDR) Pseudomonas aeruginosa. Major reductions in minimum inhibitory concentrations (MIC, 10–14-folds) of the antibiotics meropenem (10 fold) and trimethoprim (14 fold) were observed in the presence of VAuNPs (50 μg/ml). Furthermore, it was found that VAuNPs in combination with meropenem or trimethoprim provided 1.5–3-fold better potentiation effects than that of vanillin alone. Use of an ethidium bromide agar cart wheel assay indicated that VAuNPs can block the activity of efflux pumps. High reduction in the MIC of antibiotics was therefore attributed to the efflux pump repression activity of VAuNPs. Further, RT-qPCR of clinically relevant MexAB-OprM efflux pump components showed down-regulation in mexB and OprM transcripts in VAuNPs treated P. aeruginosa clinical isolates. Our results reveal that VAuNPs impart susceptibility to the last line antibiotics meropenem, trimethoprim and few widely used antibiotics in XDR P. aeruginosa clinical isolates that display resistance to these antibiotics. Therefore, this study indicate the ability of VAuNPs and vanillin to be used as antibiotic adjuvants for inhibiting bacterial efflux pumps to potentiate antibiotics for addressing AMR problem affecting human health and environment.

Antimicrobial activity of Phyllanthus amarus Schumach. & Thonn and inhibition of the NorA efflux pump of Staphylococcus aureus by Phyllanthin

The aim of this study was to evaluate the antimicrobial activity of ethanoic extract of P. amarus (PAEE) and its compound Phyllanthin, as well as, investigate if these natural products could modulate the fluoroquinolone-resistance in S. aureus SA1199-B by way of overexpression of the NorA efflux pump. Microdilution tests were carried out to determine the minimal inhibitory concentration (MIC) of the PAEE or Phyllanthin against several bacterial and yeast strains. To evaluate if PAEE or Phyllanthin were able to act as modulators of the fluoroquinolone-resistance, MICs for Norfloxacin and ethidium bromide were determined in the presence or absence of PAEE or Phyllanthin against S. aureus SA1199-B. PAEE showed antimicrobial activity against Gram-negative strains, meanwhile Phyllanthin was inactive against all strains tested. Addition of PAEE or Phyllanthin, to the growth media at sub-inhibitory concentrations enhanced the activity of the Norfloxacin as well as, Ethidium Bromide, against S. aureus SA1199-B. These results indicate that Phyllanthin is able to modulate the fluoroquinolone-resistance possibly by inhibition of NorA. This hypothesis was supported by in silico docking analysis which confirmed that Phyllantin is a NorA ligand. Thus, this compound could be used as a potentiating agent of the Norfloxacin activity in the treatment of infections caused by fluoroquinolone-resistant S. aureus.

Essential oils of Origanum compactum increase membrane permeability, disturb cell membrane integrity, and suppress quorum-sensing phenotype in bacteria

The aim of this study was to investigate antibacterial activity of Origanum compactum essential oils collected at three phenological stages on Escherichia coli and Bacillus subtilis. The antibacterial activity was evaluated using the agar-well diffusion assay. The MIC and MBC values were determined using the micro-dilution assay. The investigation of the antibacterial action was carried out by the evaluation of the effect of O. compactum essential oils on the antibacterial kinetic growth, the integrity of cell membrane and permeability of the cell membrane. The anti-quorum sensing activity was tested by the inhibition of the biofilm formation. The findings of this study showed that O. compactum essential oil has potent antibacterial activities against E. coli and B. subtilis. The lowest inhibition value against B. subtilis was obtained with O. compactum essential oil at the post-flowering stage (MIC = MBC = 0.0312% (v/v)). The antibacterial mechanisms of O. compactum essential oils are related to the disturbing of the cell membrane integrity and the increasing of the membrane permeability, which leads to the leakage of genetic materials (DNA and RNA). Moreover, O. compactum essential oils inhibited the formation of the biofilms, a phenotype that has been known to be quorum sensing regulated.

Chemical composition, antioxidant activity and antibacterial mechanism of action from Marsilea minuta leaf hexane: methanol extract

BACKGROUND

In the present study, hexane: methanol (50:50) leaf extract of Marisela minuta has been evaluated for its chemical composition, antioxidant effect and the antimicrobial mechanism of action against food borne pathogenic bacteria.

RESULTS

The phytochemical evaluation of extract by GC/MS revealed the major abundance of benzoic acid-4-ethoxyethyl ester (43.39%) and farnesol acetate (18.42%). The extract exhibited potential antioxidant and free radical scavenging properties with promising antibacterial activities against the test pathogens with Pseudomonas aeruginosa being the most susceptible with maximum inhibition zone (17 mm) and IC50 value of 125 µg, respectively. The significant (p < 0.05) increase in intracellular super oxide dismutase (SOD), protein leakage, extracellular alkaline phosphatase and lactate dehydrogenase in treated test pathogens suggested an increase in oxidative stress reveling the mechanism of action of phytochemicals. Scanning electron microscopy analysis of treated pathogens also showed swollen and distorted cells. The bioactive molecules in the extract were efficiently docked with virulent enzymes and farnesol acetate showed best energy value of - 5.19 and - 4.27 kcal/mol towards Topoisomerase IV and SHV-2 respectively. Benzoic acid-4-ethoxyethyl ester showed best binding against TEM-72 with low binding energy value of - 4.35 kcal/mol.

CONCLUSION

Due to its antioxidant and antibacterial properties, the leaf extract of M. minuta may act as promising natural additives to prevent food spoilage bacteria.

Preparation and biological evaluation of metronidazole derivatives with monoterpenes and eugenol

Two series of metronidazole derivatives (ester derivatives and ether derivatives) were prepared reacting metronidazole and its acetic acid oxidized form with menthol, thymol, carvacrol, and eugenol. Both series of compounds were tested in vitro against two strains of Helicobacter pylori (the ATCC 26695 and P12), and one strain of Clostridium (Clostridium perfringens). Most of the prepared compounds showed biological activity against the targeted bacteria. Compound 11 was highly active against all tested bacterial strains, especially against P12 with IC₅₀ 0.0011 μM/ml. Compound 6 was highly active against C. perfringens with MIC 0.0094 nM/ml. Viability test was conducted for compound 11 to test its selectivity for normal human fetal lung fibroblasts (MRC5), and it was found to be non-toxic with IC₅₀ more than 50 μM/ml.

Pharmacokinetic and antimicrobial activity of a new carvacrol-based product against a human pathogen, Campylobacter jejuni

AIM

In vitro and in vivo studies were conducted to test a new carvacrol-based product designed to delay the carvacrol release so that it could reach the caeca of broiler chickens in order to control Campylobacter jejuni.

METHODS AND RESULTS

Antimicrobial activity of carvacrol, a constituent of oregano and thyme essential oil, has been demonstrated against C. jejuni in vitro, and this compound was found beneficial for broiler growth. Here, we tested a new liquid formulation that did not change the antibacterial efficacy of carvacrol against C. jejuni in vitro, as assessed by broth microdilution. The mode of action of carvacrol also remained unchanged as illustrated by electronic microscopy. A pharmacokinetic assay monitored carvacrol of the solid galenic formulation in the avian digestive tract and this showed that this compound was mainly found in the last part (caeca, large intestine) and in the droppings. Extremely low concentrations of free carvacrol were present in blood plasma, with larger amounts of carvacrol metabolites: carvacrol glucuronide and sulphate. A qPCR analysis showed that the solid galenic form of carvacrol added at 5 kg per tonne of food (i.e. 9·5 mg of carvacrol per kg of bodyweight per day) significantly decreased the C. jejuni caecal load by 1·5 log.

CONCLUSIONS

The new liquid formulation was as effective as unformulated carvacrol in vitro. In vivo the solid galenic form seems to delay the carvacrol release into the caeca and presented interesting results on C. jejuni load after 35 days.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results suggested that this product could be promising to control Campylobacter contamination of broilers.

Synthesis and antibacterial properties of thymol and carvacrol grafted onto lignocellulosic kraft fibers

Bacterial infections and surface contaminations are worrying public health issues. It becomes urgent to find solutions. One of the ways to limit bacterial proliferation is to develop new antimicrobial materials. The phenolic compounds of essential oils like thymol and carvacrol, are attractive antibacterial candidates, which have gained great popularity in the food, cosmetic, and pharmaceutical industries. This work describes the elaboration of bioinspired antibacterial materials. Thymol and carvacrol are linked to kraft pulp fibers, via triazine link. This novel material has been investigated for its antibacterial properties against Escherichia coli and Staphylococcus aureus. The developed materials show very interesting antibacterial activity. The grafting of thymol and carvacrol by covalent bond allows to avoid the problem of their release and, thus, could maintain the antibacterial properties of support.

Carvacrol activity & morphological changes in Mycobacterium tuberculosis

Aim: Evaluating carvacrol, derivatives and carvacrol plus anti-TB (anti-tuberculous) drug combination activities in Mycobacterium tuberculosis as well as carvacrol cytotoxicity, efflux pump inhibitor activity and morphological changes in M. tuberculosis H37Rv. Methods: Carvacrol (CAR) and derivatives’ activities were determined by resazurin microtiter assay and drug interaction by resazurin drug combination microtiter. Carvacrol cytotoxicity in VERO cells and efflux pumps inhibitor activity by ethidium bromide assay were determined and scanning electron microscopy performed. Results: Carvacrol MIC ranged from 19 to 156 μg/ml and carvacrol plus rifampicin combination showed synergistic effect in clinical isolates. No anti-M. tuberculosis activity improvement was observed with carvacrol derivatives. Carvacrol showed to be selective for M. tuberculosis, to have efflux pumps activity and to induce rough bacillary and agglomerates. Conclusion: Carvacrol shows good anti-M. tuberculosis activity and synergism with rifampicin.

Synchronous application of antibiotics and essential oils: dual mechanisms of action as a potential solution to antibiotic resistance

Antibiotic resistance has increased dramatically in recent years, yet the antibiotic pipeline has stalled. New therapies are therefore needed to continue treating antibiotic resistant infections. One potential strategy currently being explored is the use of non-antibiotic compounds to potentiate the activity of currently employed antibiotics. Many natural products including Essential Oils (EOs) possess broad spectrum antibacterial activity and so have been investigated for this purpose. This article aims to review recent literature concerning the antibacterial activity of EOs and their interactions with antibiotics, with consideration of dual mechanisms of action of EOs and antibiotics as a potential solution to antibiotic resistance. Synergistic interactions between EOs and their components with antibiotics have been reported, including several instances of antibiotic resensitization in resistant isolates, in support of this strategy to control antibiotic resistance. However, a lack of consistency in methods and interpretation criteria makes drawing conclusions of efficacy of studied combinations difficult. Synergistic effects are often not explored beyond preliminary identification of antibacterial interactions and mechanism of action is rarely defined, despite many hypotheses and recommendations for future study. Much work is needed to fully understand EO-antibiotic associations before they can be further developed into novel antibacterial formulations.

Chemical analysis and in vitro antimicrobial effects and mechanism of action of Trachyspermum copticum essential oil against Escherichia coli

OBJECTIVE

To find a natural plant essential oil (EO) with excellent antimicrobial effects on food-borne bacteria and to explore the mechanism of its antimicrobial function against Escherichia coli (E. coli).

METHODS

The antimicrobial activity of seven EOs against Gram-negative E. coli ATCC 8739 and Gram-positive Staphylococcus aureus ATCC 6538 was investigated using agar disk diffusion method, and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of each EO was determined using the broth dilution method. The chemical composition of the Trachyspermum copticum (T. copticum) EO was analyzed using gas chromatography-mass spectrometry (GC/MS). In order to explore the mechanism of the antimicrobial action, 1 MIC and 2 MIC of T. copticum EO was added to a suspension of E. coli, the growth curve and the scanning electron microscopy (SEM) analysis of E. coli, and the release of cell constituents and protein and potassium ions from the bacterial cell were measured.

RESULTS

The T. copticum EO had the best antimicrobial activity against the test bacteria, and 10 compounds accounting for 94.57% of the total oil were identified, with the major components being thymol (46.22%), p-cymene (19.03%), and γ-terpinene (22.41%). The addition of 1 MIC that T. copticum EO significantly inhibited the growth of E. coli and increased the release of cell constituents and protein and potassium ions from the bacterial cells. Scanning electron micrographs showed that T. copticum EO caused most of the E. coli cell membranes to collapse and rupture, leading to cell death.

CONCLUSIONS

These results indicate that T. copticum EO is a good natural antimicrobial agent for food-borne pathogens.

Antibacterial and Antifungal Activities of Spices

Infectious diseases caused by pathogens and food poisoning caused by spoilage microorganisms are threatening human health all over the world. The efficacies of some antimicrobial agents, which are currently used to extend shelf-life and increase the safety of food products in food industry and to inhibit disease-causing microorganisms in medicine, have been weakened by microbial resistance. Therefore, new antimicrobial agents that could overcome this resistance need to be discovered. Many spices-such as clove, oregano, thyme, cinnamon, and cumin-possessed significant antibacterial and antifungal activities against food spoilage bacteria like Bacillus subtilis and Pseudomonas fluorescens, pathogens like Staphylococcus aureus and Vibrio parahaemolyticus, harmful fungi like Aspergillus flavus, even antibiotic resistant microorganisms such as methicillin resistant Staphylococcus aureus. Therefore, spices have a great potential to be developed as new and safe antimicrobial agents. This review summarizes scientific studies on the antibacterial and antifungal activities of several spices and their derivatives.

Thymol improves high-fat diet-induced cognitive deficits in mice via ameliorating brain insulin resistance and upregulating NRF2/HO-1 pathway

The impaired insulin signaling has been recognized as a common pathogenetic mechanism between diabetes and Alzheimer's disease (AD). In the progression of AD, brain is characterized by defective insulin receptor substrate-1 (IRS-1) and increased oxidative stress. Thymol, a monoterpene phenol isolated from medicinal herbs, has exhibited robust neuroprotective effects. The present study was designed to investigate the protective effect of thymol on HFD-induced cognitive deficits, and explore the possible mechanisms. C57BL/6 J mice were fed for 12 weeks with either HFD or normal diet. The mice fed with HFD were dosed with metformin (200 mg/kg) or thymol (20, 40 mg/kg) daily. It was observed that thymol treatment significantly reversed the gain of body weight and peripheral insulin resistance induced by HFD. Meanwhile, thymol improved the cognitive impairments in the Morris Water Maze (MWM) test and decreased HFD-induced Aβ deposition and tau hyperphosphorylation in the hippocampus, which may be correlated with the inhibition of hippocampal oxidative stress and inflammation. In addition, thymol down-regulated the level of P-Ser307 IRS-1, and hence enhancing the expression of P-Ser473 AKT and P-Ser9 GSK3β. We further found that the protective effects of thymol on cognitive impairments were associated with the up-regulation of nuclear respiratory factor (Nrf2)/heme oxygenase-1(HO-1) pathway. In conclusion, thymol exhibited beneficial effects on HFD-induced cognitive deficits through improving hippocampal insulin resistance, and activating Nrf2/HO-1 signaling.

Evaluation of antibacterial potential and toxicity of plant volatile compounds using new broth microdilution volatilization method and modified MTT assay

With aim to develop effective proof-of-concept approach which can be used in a development of new preparations for the inhalation therapy, we designed a new screening method for simple and rapid simultaneous determination of antibacterial potential of plant volatiles in the liquid and the vapour phase at different concentrations. In addition, EVA (ethylene vinyl acetate) capmat™ as vapour barrier cover was used as reliable modification of thiazolyl blue tetrazolium bromide (MTT) assay for cytotoxicity testing of volatiles on microtiter plates. Antibacterial activity of carvacrol, cinnamaldehyde, eugenol, 8-hydroxyquinoline, thymol and thymoquinone was determined against Haemophilus influenzae, Staphylococcus aureus, and Streptococcus pneumoniae using new broth microdilution volatilization method. The cytotoxicity of these compounds was evaluated using MTT test in lung fibroblast cells MRC-5. The most effective antibacterial agents were 8-hydroxyquinoline and thymoquinone with the lowest minimum inhibitory concentrations (MICs) ranging from 2 to 128μg/mL, but they also possessed the highest toxicity in lung cell lines with half maximal inhibitory concentration (IC₅₀) values 0.86-2.95μg/mL. The lowest cytotoxicity effect was identified for eugenol with IC₅₀ 295.71μg/mL, however this compound produced only weak antibacterial potency with MICs 512-1024μg/mL. The results demonstrate validity of our novel broth microdilution volatilization method, which allows cost and labour effective high-throughput antimicrobial screening of volatile agents without need of special apparatus. In our opinion, this assay can also potentially be used for development of various medicinal, agricultural, and food applications that are based on volatile antimicrobials.

Use of carvacrol, thymol, and eugenol for biofilm eradication and resistance modifying susceptibility of Salmonella enterica serovar Typhimurium strains to nalidixic acid

The Aims of the study was to evaluate the antibacterial susceptibility and the biofilm eradication of three natural compounds carvacrol (CAR), thymol (TH) and eugenol (EUG), alone or in combination with nalidixic acid (NA) against twelve Salmonella Typhimurium strains. The minimum inhibitory concentration (MIC) and the minimum biofilm eradication concentration (BEC₅₀) of the tested compounds (CAR, TH and EUG) and their combinations with NA were evaluated. In order to assess whether these bacteria had active efflux pumps, ethidium bromide (EtBr) accumulation assays was achieved using spectrophotometric accumulation assays. Moreover, scanning electron microscopy was used to visualize the bacterial biofilm formation on stainless steel surfaces after exposed to NA, CAR, TH and EUG alone and in combination. TH was the most effective essential oil, with the lowest MICs values ranging from 32 to 128 μg/mL followed by EUG and CAR. In addition, the combination of NA with the different compounds enhances antibiotic susceptibility of the tested bacterial strains. These results were confirmed by EtBr accumulation assays. A pronounced effect in decreasing biofilm mass was also noticed. Moreover, SEM revealed that bacterial membrane was disrupted and a complete loss of membrane integrity was also evident. The combination of natural compounds with antibiotic enhances bacterial susceptibility to NA. This combination ameliorates eradication of biofilm formed by S. Typhimurium on polystyrene microtitre plates. Additionally, this synergy induces an alteration of the bacterial cell surface visualized by SEM.