Use of natural antimicrobials to increase antibiotic susceptibility of drug resistant bacteria

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.

Combination of essential oil compounds and phenolic acids against Escherichia coli O157:H7 in vitro and in dry-fermented sausage production

Escherichia coli O157:H7 is a foodborne pathogen that causes hemorrhagic colitis and hemolytic uremic syndrome. The low dose of infection and severity of the disease represent a concern to public health. Natural compounds have been widely applied as food additives to replace synthetic preservatives. The aim of this study was to determine the efficiency of essential oil compounds (EOCs) in combination with phenolic acids (PA) in vitro and in dry-fermented sausage production. Minimum Inhibitory Concentration (MIC) and Fractional Inhibitory Concentration index (FIC_index) were determined for a 5-strain mixture of E. coli O157:H7. Batches of sausage tainted with E. coli O157:H7 were produced using Pediococcus pentosaceus UM 116P and Staphylococcus carnosus UM 123M as starter cultures. The best combination of EOCs and PAs found in vitro was used as an additive. Chemical-physical and microbiological analyses were evaluated weekly from day 0 to 35 after production. Sensory evaluation (texture, odor, flavor, appearance and overall evaluation) of E. coli-free sausages was conducted using a 9-point hedonic scale with 56 untrained volunteers. The MIC values of allyl isothiocyanate (AITC), carvacrol (CAR), ferulic acid (FA), o-coumaric acid (CA) and p-hydroxybenzoic acid (AHB) were, respectively, 0.25; 1.3; 5.12; 18.27; and 37mM. AITC combined with CA had a synergistic effect (FIC_index=0.25) and together they were applied in the production of dry fermented sausage at concentrations of 10× FIC and 20× FIC. A_w had no significant difference among treatments, whereas the pH of 10× FIC and 20× FIC were higher than the control. E. coli O157:H7 was reduced by >5logCFU/g with 20× FIC after 21d, and by 2.8logCFU/g with 10× FIC after 35d. Sensory analysis showed that the combination of AITC and ο-coumaric acid in both treatments presented lower scores in the 5 categories when compared to the control, but none of the parameters received a negative score. This study demonstrated that the combination of AITC and ο-coumaric acid at 20× FIC reduced E. coli O157:H7 in compliance with the North American legislation, but adjustments in the dose are necessary to improve the sensory characteristics of the final product.

Antimicrobial Activity of Some Essential Oils-Present Status and Future Perspectives

Extensive documentation on the antimicrobial properties of essential oils and their constituents has been carried out by several workers. Although the mechanism of action of a few essential oil components has been elucidated in many pioneering works in the past, detailed knowledge of most of the compounds and their mechanism of action is still lacking. This knowledge is particularly important for the determination of the effect of essential oils on different microorganisms, how they work in combination with other antimicrobial compounds, and their interaction with food matrix components. Also, recent studies have demonstrated that nanoparticles (NPs) functionalized with essential oils have significant antimicrobial potential against multidrug- resistant pathogens due to an increase in chemical stability and solubility, decreased rapid evaporation and minimized degradation of active essential oil components. The application of encapsulated essential oils also supports their controlled and sustained release, which enhances their bioavailability and efficacy against multidrug-resistant pathogens. In the recent years, due to increasingly negative consumer perceptions of synthetic preservatives, interest in essential oils and their application in food preservation has been amplified. Moreover, the development of resistance to different antimicrobial agents by bacteria, fungi, viruses, parasites, etc. is a great challenge to the medical field for treating the infections caused by them, and hence, there is a pressing need to look for new and novel antimicrobials. To overcome these problems, nano-encapsulation of essential oils and exploiting the synergies between essential oils, constituents of essential oils, and antibiotics along with essential oils have been recommended as an answer to this problem. However, less is known about the interactions that lead to additive, synergistic, or antagonistic effects. A contributing role of this knowledge could be the design of new and more potent antimicrobial blends, and understanding of the interplay between the components of crude essential oils. This review is written with the purpose of giving an overview of current knowledge about the antimicrobial properties of essential oils and their mechanisms of action, components of essential oils, nano-encapsulated essential oils, and synergistic combinations of essential oils so as to find research areas that can facilitate applications of essential oils to overcome the problem of multidrug-resistant micro-organisms.

Pharmacological Properties and Molecular Mechanisms of Thymol: Prospects for Its Therapeutic Potential and Pharmaceutical Development

Thymol, chemically known as 2-isopropyl-5-methylphenol is a colorless crystalline monoterpene phenol. It is one of the most important dietary constituents in thyme species. For centuries, it has been used in traditional medicine and has been shown to possess various pharmacological properties including antioxidant, free radical scavenging, anti-inflammatory, analgesic, antispasmodic, antibacterial, antifungal, antiseptic and antitumor activities. The present article presents a detailed review of the scientific literature which reveals the pharmacological properties of thymol and its multiple therapeutic actions against various cardiovascular, neurological, rheumatological, gastrointestinal, metabolic and malignant diseases at both biochemical and molecular levels. The noteworthy effects of thymol are largely attributed to its anti-inflammatory (via inhibiting recruitment of cytokines and chemokines), antioxidant (via scavenging of free radicals, enhancing the endogenous enzymatic and non-enzymatic antioxidants and chelation of metal ions), antihyperlipidemic (via increasing the levels of high density lipoprotein cholesterol and decreasing the levels of low density lipoprotein cholesterol and low density lipoprotein cholesterol in the circulation and membrane stabilization) (via maintaining ionic homeostasis) effects. This review presents an overview of the current in vitro and in vivo data supporting thymol's therapeutic activity and the challenges concerning its use for prevention and its therapeutic value as a dietary supplement or as a pharmacological agent or as an adjuvant along with current therapeutic agents for the treatment of various diseases. It is one of the potential candidates of natural origin that has shown promising therapeutic potential, pharmacological properties and molecular mechanisms as well as pharmacokinetic properties for the pharmaceutical development of thymol.

Potency and selectivity indices of acetone leaf extracts of nine selected South African trees against six opportunistic Enterobacteriaceae isolates from commercial chicken eggs

Background

The rise in antimicrobial resistance in a plethora of nosocomial and opportunistic bacterial pathogens often isolated from commercial eggs, poses a serious public health concern. The existence of these contaminants may also serve as a drawback in the current efforts of improving the well-being of immunocompromised patients. The aim of this study was to determine the efficacy of plant extracts that had good activity on Escherichia coli in previous word on pathogens isolated from eggs for possible use in combating pathogens from eggs.

Methods

Acetone leaf extracts of nine trees with good activities against Escherichia coli were tested for their in vitro antibacterial activity against six opportunistic bacterial isolates from commercial eggs (Stenotrophomonas maltophilia, Klebsiella pneumoniae, Salmonella serotype Typhimurium, Proteus mirabilis, Enterobacter cloacae and Escherichia coli) using a serial microdilution method with tetrazolium violet as indicator of growth. Cytotoxicity was determined using a tetrazolium-based colorimetric assay against Vero kidney cells, and selectivity index calculated.

Results

The MIC values range of the different extracts against Stenotrophomonas maltophilia was 0.08-0.31 mg/ml, Klebsiella pneumonia 0.08-0.63 mg/ml, Salmonella ser. Typhimurium 0.08-0.63 mg/ml, Proteus mirabilis 0.02-1.25 mg/ml, Enterobacter cloacae 0.08-0.31 mg/ml and Escherichia coli 0.08-0.16 mg/ml respectively. Escherichia coli was the most sensitive while Proteus mirabilis was most resistant pathogen to the different test extracts, with mean MIC values of 0.08 mg/ml and 0.46 mg/ml respectively. Cremaspora triflora extracts had good activity against all the pathogenic egg isolates, with the exception of Proteus mirabilis. Maesa lanceolata and Elaeodendron croceum had the best total antibacterial activity (TAA), while generally the selectivity index of the extract was low (SI < 1).

Conclusion

The exceptional activity of C. triflora extracts suggests that the plant has potential as a therapeutic agent against some members of the Enterobacteriaceae. Further pharmacological investigations may be interesting in the search for new antimicrobial leads.

Safranal and its analogs inhibit Escherichia coli ATP synthase and cell growth

Safranal, a dominant component of saffron, is known to have antitumor, cytotoxic, and antibacterial properties. In this study, we examined safranal and its structural analogs-thymol, carvacrol, damascenone, cuminol, 2,6,6-trimethyl-2-cyclohexene-1,4-dione (TMCHD), 4-isopropylbenzyl bromide (IPBB), and 4-tert-butylphenol (TBP) induced inhibition of Escherichia coli membrane bound F1Fo ATP synthase. Safranal and its analogs inhibited wild-type enzyme to variable degrees. While safranal caused 100% inhibition of wild-type F1Fo ATP synthase, only about 50% inhibition occurred for αR283D mutant ATP synthase. Moreover, safranal, thymol, carvacrol, damascenone, cuminol, TMCHD, IPBB, and TBP all fully abrogated the growth of wild-type E. coli cells and had partial or no effect on the growth of null and mutant E. coli strains. Therefore, the antimicrobial properties of safranal, thymol, carvacrol, damascenone, cuminol, TMCHD, IPBB, and TBP can be linked to their binding and inhibition of ATP synthase. Total loss of growth in wild-type and partial or no growth loss in null or mutant E. coli strains demonstrates that ATP synthase is a molecular target for safranal and its structural analogs. Partial inhibition of the αArg-283 mutant enzyme establishes that αArg-283 residue is required in the polyphenol binding pocket of ATP synthase for the binding of safranal. Furthermore, partial growth loss for the null and mutant strains in the presence of inhibitors also suggests the role of other targets and residues in the process of inhibition.

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.

Phytochemicals for human disease: An update on plant-derived compounds antibacterial activity

In recent years, many studies have shown that phytochemicals exert their antibacterial activity through different mechanisms of action, such as damage to the bacterial membrane and suppression of virulence factors, including inhibition of the activity of enzymes and toxins, and bacterial biofilm formation. In this review, we summarise data from the available literature regarding the antibacterial effects of the main phytochemicals belonging to different chemical classes, alkaloids, sulfur-containing phytochemicals, terpenoids, and polyphenols. Some phytochemicals, besides having direct antimicrobial activity, showed an in vitro synergistic effect when tested in combination with conventional antibiotics, modifying antibiotic resistance. Review of the literature showed that phytochemicals represent a possible source of effective, cheap and safe antimicrobial agents, though much work must still be carried out, especially in in vivo conditions to ensure the selection of effective antimicrobial substances with low side and adverse effects.

Cinnamaldehyde, Carvacrol and Organic Acids Affect Gene Expression of Selected Oxidative Stress and Inflammation Markers in IPEC-J2 Cells Exposed to Salmonella typhimurium

Essential oils and organic acids are used as feed additives to improve health status and reduce colonization with pathogens. Although bactericidal in vitro, concentrations achieved in the animal gut are probably not lethal to pathogens. The aim of this study was to investigate the effects of cinnamaldehyde, carvacrol and cinnamic, lactic and propionic acids on the ability of Salmonella typhimurium ATCC 14028 (ST) to invade intestinal epithelial cells (IPEC-J2) and on the expression levels of immune related genes in the cells. The minimum inhibitory concentration (MIC) and non-inhibitory concentration (NIC) were determined and influence on the invasion capacity of ST was investigated. The structure of fimbriae and flagella was analysed by electron microscopy, and expression levels of HSP70, IkBa, IL-8 and IL-10 in the IPEC-J2 cells were carried out by q-PCR. Cinnamaldehyde, carvacrol and cinnamic and propionic acids inhibited ST invasion but not cell viability, bacterial viability and motility or the development of flagella. Propionic acid and cinnamaldehyde in combination with cinnamic acid caused structural impairment of fimbriae. Cinnamaldehyde up-regulated expression of HSP70 irrespective of the presence of organic acids or ST; exposure to carvacrol induced HSP70 only in the presence of propionic acid and ST. © 2016 The Authors. Phytotherapy Research published by John Wiley & Sons Ltd.

Essential Oils and Their Components as Modulators of Antibiotic Activity against Gram-Negative Bacteria

Gram-negative bacteria cause infections that are difficult to treat due to the emergence of multidrug resistance. This review summarizes the current status of the studies investigating the capacity of essential oils and their components to modulate antibiotic activity against Gram-negative bacteria. Synergistic interactions are particularly discussed with reference to possible mechanisms by which essential oil constituents interact with antibiotics. Special emphasis is given to essential oils and volatile compounds that inhibit efflux pumps, thus reversing drug resistance in Gram-negative bacteria. In addition, indifference and antagonism between essential oils/volatile compounds and conventional antibiotics have also been reported. Overall, this literature review reveals that essential oils and their purified components enhance the efficacy of antibiotics against Gram-negative bacteria, being promising candidates for the development of new effective formulations against Gram-negative bacteria.

Stress Conditions Induced by Carvacrol and Cinnamaldehyde on Acinetobacter baumannii

Acinetobacter baumannii has emerged as a major cause of nosocomial infections. The ability of A. baumannii to display various resistance mechanisms against antibiotics has transformed it into a successful nosocomial pathogen. The limited number of antibiotics in development and the disengagement of the pharmaceutical industry have prompted the development of innovative strategies. One of these strategies is the use of essential oils, especially aromatic compounds that are potent antibacterial molecules. Among them, the combination of carvacrol and cinnamaldehyde has already demonstrated antibacterial efficacy against A. baumannii. The aim of this study was to determine the biological effects of these two compounds in A. baumannii, describing their effect on the rRNA and gene regulation under environmental stress conditions. Results demonstrated rRNA degradation by the carvacrol/cinnamaldehyde mixture, and this effect was due to carvacrol. Degradation was conserved after encapsulation of the mixture in lipid nanocapsules. Results showed an upregulation of the genes coding for heat shock proteins, such as groES, groEL, dnaK, clpB, and the catalase katE, after exposure to carvacrol/cinnamaldehyde mixture. The catalase was upregulated after carvacrol exposure wich is related to an oxidative stress. The combination of thiourea (hydroxyl radical scavenger) and carvacrol demonstrated a potent bactericidal effect. These results underline the development of defense strategies of the bacteria by synthesis of reactive oxygen species in response to environmental stress conditions, such as carvacrol.

A Drug Combination Screen Identifies Drugs Active against Amoxicillin-Induced Round Bodies of In Vitro Borrelia burgdorferi Persisters from an FDA Drug Library

Although currently recommended antibiotics for Lyme disease such as doxycycline or amoxicillin cure the majority of the patients, about 10–20% of patients treated for Lyme disease may experience lingering symptoms including fatigue, pain, or joint and muscle aches. Under experimental stress conditions such as starvation or antibiotic exposure, Borrelia burgdorferi can develop round body forms, which are a type of persister bacteria that appear resistant in vitro to customary first-line antibiotics for Lyme disease. To identify more effective drugs with activity against the round body form of B. burgdorferi, we established a round body persister model induced by exposure to amoxicillin (50 μg/ml) and then screened the Food and Drug Administration drug library consisting of 1581 drug compounds and also 22 drug combinations using the SYBR Green I/propidium iodide viability assay. We identified 23 drug candidates that have higher activity against the round bodies of B. burgdorferi than either amoxicillin or doxycycline. Eleven individual drugs scored better than metronidazole and tinidazole which have been previously described to be active against round bodies. In this amoxicillin-induced round body model, some drug candidates such as daptomycin and clofazimine also displayed enhanced activity which was similar to a previous screen against stationary phase B. burgdorferi persisters not exposure to amoxicillin. Additional candidate drugs active against round bodies identified include artemisinin, ciprofloxacin, nifuroxime, fosfomycin, chlortetracycline, sulfacetamide, sulfamethoxypyridazine and sulfathiozole. Two triple drug combinations had the highest activity against amoxicillin-induced round bodies and stationary phase B. burgdorferi persisters: artemisinin/cefoperazone/doxycycline and sulfachlorpyridazine/daptomycin/doxycycline. These findings confirm and extend previous findings that certain drug combinations have superior activity against B. burgdorferi persisters in vitro, even when pre-treated with amoxicillin. These findings may have implications for improved treatment of Lyme disease.

Antibacterial and antifungal activities of thymol: A brief review of the literature

Thymol (2-isopropyl-5-methylphenol) is the main monoterpene phenol occurring in essential oils isolated from plants belonging to the Lamiaceae family (Thymus, Ocimum, Origanum, and Monarda genera), and other plants such as those belonging to the Verbenaceae, Scrophulariaceae, Ranunculaceae, and Apiaceae families. These essential oils are used in the food industry for their flavouring and preservative properties, in commercial mosquito repellent formulations for their natural repellent effect, in aromatherapy, and in traditional medicine for the treatment of headaches, coughs, and diarrhea. Many different activities of thymol such as antioxidant, anti-inflammatory, local anaesthetic, antinociceptive, cicatrizing, antiseptic, and especially antibacterial and antifungal properties have been shown. This review aims to critically evaluate the available literature regarding the antibacterial and antifungal effects of thymol.

Mechanism of Action of Thymol on Cell Membranes Investigated through Lipid Langmuir Monolayers at the Air-Water Interface and Molecular Simulation

A major challenge in the design of biocidal drugs is to identify compounds with potential action on microorganisms and to understand at the molecular level their mechanism of action. In this study, thymol, a monoterpenoid found in the oil of leaves of Lippia sidoides with possible action in biological surfaces, was incorporated in lipid monolayers at the air-water interface that represented cell membrane models. The interaction of thymol with dipalmitoylphosphatidylcholine (DPPC) at the air-water interface was investigated by means of surface pressure-area isotherms, Brewster angle microscopy (BAM), polarization-modulation reflection-absorption spectroscopy (PM-IRRAS), and molecular dynamics simulation. Thymol expands DPPC monolayers, decreases their surface elasticity, and changes the morphology of the lipid monolayer, which evidence the incorporation of this compound in the lipid Langmuir film. Such incorporation could be corroborated by PM-IRRAS since some specific bands for DPPC were changed upon thymol incorporation. Furthermore, potential of mean force obtained by molecular dynamics simulations indicates that the most stable position of the drug along the lipid film is near the hydrophobic regions of DPPC. These results may be useful to understand the interaction between thymol and cell membranes during biochemical phenomena, which may be associated with its pharmaceutical properties at the molecular level.

SaxA-Mediated Isothiocyanate Metabolism in Phytopathogenic Pectobacteria

Pectobacteria are devastating plant pathogens that infect a large variety of crops, including members of the family Brassicaceae. To infect cabbage crops, these plant pathogens need to overcome the plant's antibacterial defense mechanisms, where isothiocyanates are liberated by hydrolysis of glucosinolates. Here, we found that a Pectobacterium isolate from the gut of cabbage root fly larvae was particularly resistant to isothiocyanate and even seemed to benefit from the abundant Brassica root metabolite 2-phenylethyl isothiocyanate as a nitrogen source in an ecosystem where nitrogen is scarce. The Pectobacterium isolate harbored a naturally occurring mobile plasmid that contained a sax operon. We hypothesized that SaxA was the enzyme responsible for the breakdown of 2-phenylethyl isothiocyanate. Subsequently, we heterologously produced and purified the SaxA protein and characterized the recombinant enzyme. It hydrolyzed 2-phenylethyl isothiocyanate to yield the products carbonyl sulfide and phenylethylamine. It was also active toward another aromatic isothiocyanate but hardly toward aliphatic isothiocyanates. It belongs to the class B metal-dependent beta-lactamase fold protein family but was not, however, able to hydrolyze beta-lactam antibiotics. We discovered that several copies of the saxA gene are widespread in full and draft Pectobacterium genomes and therefore hypothesize that SaxA might be a new pathogenicity factor of the genus Pectobacterium, possibly compromising food preservation strategies using isothiocyanates.

Plant Phenols as Antibiotic Boosters: In Vitro Interaction of Olive Leaf Phenols with Ampicillin

The antimicrobial properties of olive leaf extract (OLE) have been well recognized in the Mediterranean traditional medicine. Few studies have investigated the antimicrobial properties of OLE. In this preliminary study, commercial OLE and its major phenolic secondary metabolites were evaluated in vitro for their antimicrobial activities against Escherichia coli and Staphylococcus aureus, both individually and in combination with ampicillin. Besides luteolin 7-O-glucoside, OLE and its major phenolic secondary metabolites were effective against both bacteria, with more activity on S. aureus. In combination with ampicillin, OLE, caffeic acid, verbascoside and oleuropein showed additive effects. Synergistic interaction was observed between ampicillin and hydroxytyrosol. The phenolic composition of OLE and the stability of olive phenols in assay medium were also investigated. While OLE and its phenolic secondary metabolites may not be potent enough as stand-alone antimicrobials, their abilities to boost the activity of co-administered antibiotics constitute an imperative future research area.

Polyphenolic Secondary Metabolites Synergize the Activity of Commercial Antibiotics against Clinical Isolates of β-Lactamase-producing Klebsiella pneumoniae

Emergence of worldwide antimicrobial resistance prompted us to study the resistance modifying potential of plant-derived dietary polyphenols, mainly caffeic acid, ellagic acid, epigallocatechin-3-gallate (EGCG) and quercetin. These compounds were studied in logical combination with clinically significant antibiotics (ciprofloxacin/gentamicin/tetracycline) against Klebsiella pneumoniae, after conducting phenotypic screening of a large number of clinical isolates and selecting the relevant strains possessing extended-spectrum β-lactamase (ESBL) and K. pneumoniae carbapenemase (KPC)-type carbapenemase enzymes only. The study demonstrated that EGCG and caffeic acid could synergize the activity of tested antibiotics within a major population of β-lactamase-producing K. pneumoniae. In spectrofluorimetric assay, ~17-fold greater ciprofloxacin accumulation was observed within K. pneumoniae cells pre-treated with EGCG in comparison with the untreated control, indicating its ability to synergize ciprofloxacin to restrain active drug-efflux. Further, electron micrograph of ESBL-producing K. pneumoniae clearly demonstrated the prospective efficacy of EGCG towards biofilm degradation.

Neem (Azadirachta indica A. Juss) Oil to Tackle Enteropathogenic Escherichia coli

Neem (Azadirachta indica A. Juss) oil (NO) was assayed against forty-eight isolates of Escherichia coli by standardised disc diffusion test and microdilution test. By molecular biology characterization, fourteen isolates resulted in diarrheagenic E. coli with sixteen primer pairs that specifically amplify unique sequences of virulence genes and of 16S rRNA. The NO showed biological activity against all isolates. The bacterial growth inhibition zone by disc diffusion method (100 µL NO) ranged between 9.50 ± 0.70 and 30.00 ± 1.00 mm. The antibacterial activity was furthermore determined at lower NO concentrations (1 : 10–1 : 10,000). The percent of growth reduction ranged between 23.71 ± 1.00 and 99.70 ± 1.53. The highest bacterial growth reduction was 1 : 10 NO concentration with 50 µL of bacterial suspension (ca. 1 × 10⁶ CFU/mL). There is significant difference between the antibacterial activities against pathogenic and nonpathogenic E. coli, as well as NO and ciprofloxacin activities. Viable cells after the different NO concentration treatments were checked by molecular biology assay using PMA dye. On the basis of the obtained results, NO counteracts E. coli and also influences the virulence of E. coli viable cells after NO treatment. The NO metabolomic composition was obtained using fingerprint HPTLC.

Antibiotic-resistant bacteria: prevalence in food and inactivation by food-compatible compounds and plant extracts

Foodborne antibiotic-resistant pathogenic bacteria such as Campylobacter jejuni, Bacillus cereus, Clostridium perfringens, Escherichia coli, Salmonella enterica, Staphylococcus aureus, Vibrio cholerae, and Vibrio parahemolyticus can adversely affect animal and human health, but a better understanding of the factors involved in their pathogenesis is needed. To help meet this need, this overview surveys and interprets much of our current knowledge of antibiotic (multidrug)-resistant bacteria in the food chain and the implications for microbial food safety and animal and human health. Topics covered include the origin and prevalence of resistant bacteria in the food chain (dairy, meat, poultry, seafood, and herbal products, produce, and eggs), their inactivation by different classes of compounds and plant extracts and by the use of chlorine and physicochemical methods (heat, UV light, pulsed electric fields, and high pressure), the synergistic antimicrobial effects of combinations of natural antimicrobials with medicinal antibiotics, and mechanisms of antimicrobial activities and resistant effects. Possible areas for future research are suggested. Plant-derived and other safe natural antimicrobial compounds have the potential to control the prevalence of both susceptible and resistant pathogens in various environments. The collated information and suggested research will hopefully contribute to a better understanding of approaches that could be used to minimize the presence of resistant pathogens in animal feed and human food, thus reducing adverse effects, improving microbial food safety, and helping to prevent or treat animal and human infections.

Synergy among thymol, eugenol, berberine, cinnamaldehyde and streptomycin against planktonic and biofilm-associated food-borne pathogens

Essential oils have been found to exert antibacterial, antifungal, spasmolytic, and antiplasmodial activity and therapeutic effect in cancer treatment. In this study, the antibacterial activities of four main essential oils' components (thymol (Thy), eugenol (Eug), berberine (Ber), and cinnamaldehyde (Cin)) were evaluated against two food-borne pathogens, Listeria monocytogenes and Salmonella Typhimurium, either alone or in combination with streptomycin. Checkerboard assay demonstrated that Thy and Cin elicited a synergistic effect with streptomycin against L. monocytogenes, while a synergy existed between Cin or Eug and streptomycin against Salm. Typhimurium. Further experiments showed that this synergy was sufficient to eradicate biofilms formed by these two bacteria. Thus, our data highlighted that the combinations of specific components from essential oils and streptomycin were useful for the treatment of food-borne pathogens, which might help prevent the spread of antibiotic resistance through improving antibiotic effectiveness.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study has shown the synergistic effect of four components of essential oil (thymol, eugenol, berberine and cinnamaldehyde) combined with streptomycin on planktonic and biofilm-associated food-borne pathogens Listeria monocytogenes and Salmonella Typhimurium. These findings indicate that combination of specific components of essential oils with streptomycin may provide alternative methods to overcome the problem of food-borne bacteria both in suspension and in biofilm.

Antimicrobial activity of essential oils and carvacrol, and synergy of carvacrol and erythromycin, against clinical, erythromycin-resistant Group A Streptococci

In the present study, we have evaluated the in vitro antibacterial activity of essential oils from Origanum vulgare, Thymus vulgaris, Lavandula angustifolia, Mentha piperita, and Melaleuca alternifolia against 32 erythromycin-resistant [Mininum Inhibitory Concentration (MIC) ≥1 μg/mL; inducible, constitutive, and efflux-mediated resistance phenotype; erm(TR), erm(B), and mef(A) genes] and cell-invasive Group A streptococci (GAS) isolated from children with pharyngotonsillitis in Italy. Over the past decades erythromycin resistance in GAS has emerged in several countries; strains combining erythromycin resistance and cell invasiveness may escape β-lactams because of intracellular location and macrolides because of resistance, resulting in difficulty of eradication and recurrent pharyngitis. Thyme and origanum essential oils demonstrated the highest antimicrobial activity with MICs ranging from 256 to 512 μg/mL. The phenolic monoterpene carvacrol [2-Methyl-5-(1-methylethyl) phenol] is a major component of the essential oils of Origanum and Thymus plants. MICs of carvacrol ranged from 64 to 256 μg/mL. In the live/dead assay several dead cells were detected as early as 1 h after incubation with carvacrol at the MIC. In single-step resistance selection studies no resistant mutants were obtained. A synergistic action of carvacrol and erythromycin was detected by the checkerboard assay and calculation of the Fractional Inhibitory Concentration (FIC) Index. A 2- to 2048-fold reduction of the erythromycin MIC was documented in checkerboard assays. Synergy (FIC Index ≤0.5) was found in 21/32 strains and was highly significant (p < 0.01) in strains where resistance is expressed only in presence of erythromycin. Synergy was confirmed in 17/23 strains using 24-h time-kill curves in presence of carvacrol and erythromycin. Our findings demonstrated that carvacrol acts either alone or in combination with erythromycin against erythromycin-resistant GAS and could potentially serve as a novel therapeutic tool.

Antifungal Activity of Some Constituents of Origanum vulgare L. Essential Oil Against Postharvest Disease of Peach Fruit

Plant essential oils (EOs) can potentially replace synthetic fungicides in the management of postharvest fruit and vegetable diseases. The aim of this study was to evaluate in vitro and in vivo effectiveness of thymol, carvacrol, linalool, and trans-caryophyllene, single constituents of the EO of Origanum vulgare L. ssp. hirtum against Monilinia laxa, M. fructigena, and M. fructicola, which are important phytopathogens and causal agents of brown rot of pome and stone fruits in pre- and postharvest. Moreover, the possible phytotoxic activity of these constituents was assessed and their minimum inhibitory concentration (MIC) was determined. In vitro experiment indicated that thymol and carvacrol possess the highest antifungal activity. Results of in vivo trials confirmed the strong efficacy of thymol and carvacrol against brown rot of peach fruits. The thymol MIC resulted to be 0.16 μg/μL against M. laxa and M. fructigena and 0.12 μg/μL against M. fructicola, whereas for carvacrol they were 0.02 μg/μL against the first two Monilinia species and 0.03 μg/μL against the third. Results of this study indicated that thymol and carvacrol could be used after suitable formulation for controlling postharvest fruit diseases caused by the three studied Monilinia species.

Neem (Azadirachtaindica A. Juss) Oil: A Natural Preservative to Control Meat Spoilage

Plant-derived extracts (PDEs) are a source of biologically-active substances having antimicrobial properties. The aim of this study was to evaluate the potential of neem oil (NO) as a preservative of fresh retail meat. The antibacterial activity of NO against Carnobacterium maltaromaticum, Brochothrix thermosphacta, Escherichia coli, Pseudomonas fluorescens, Lactobacillus curvatus and L. sakei was assessed in a broth model system. The bacterial growth inhibition zone (mm) ranged from 18.83 ± 1.18 to 30.00 ± 1.00, as was found by a disc diffusion test with 100 µL NO. The bacterial percent growth reduction ranged from 30.81 ± 2.08 to 99.70 ± 1.53 in the broth microdilution method at different NO concentrations (1:10 to 1:100,000). Viable bacterial cells were detected in experimentally-contaminated meat up to the second day after NO treatment (100 µL NO per 10 g meat), except for C. maltaromaticum, which was detected up to the sixth day by PCR and nested PCR with propidium monoazide (PMA™) dye. In comparison to the previously published results, C. maltaromaticum, E. coli, L. curvatus and L. sakei appeared more susceptible to NO compared to neem cake extract (NCE) by using a broth model system.

Hydrocolloid-based nutraceutical delivery systems: Effect of counter-ions on the encapsulation and release

Nutraceuticals provide health benefits, especially for the prevention and treatment of chronic diseases such as diabetes, obesity, cardiovascular disease and cancer. Their incorporation in food supplements, functional foods and medicinal foods is a major technological challenge due to lower water solubility, instability during processing and storage conditions. Carriers that can effectively overcome these predicaments and protect them during product development, consumption and delivery are in high demand. Toward this end, our research approach is to entrap nutraceuticals in the ordered networks of hydrocolloids. We have examined the effect cations in regulating the encapsulated amounts and release characteristics. Iota-carrageenan and eugenol have been chosen as models of hydrocolloid and nutraceutical, respectively, in the presence of Na and Ca ions. The results suggest that carrageenan maintains its network organization even after encapsulating the eugenol molecules. Increased eugenol amounts are found in the Na carrageenan complex compared to the Ca complex, and the release rate is faster from the former but it is more controlled from the latter. These differences highlight the vital role of cations on the encapsulation efficiency and release profiles of hydrocolloid-based nutraceutical carriers. The outcome offers an elegant opportunity for developing novel and value-added food systems employing low-in-cost, nontoxic and heavily consumed food grade hydrocolloids.

Antioxidative and antimicrobial activities of the extracts from the seed coat of Bambara groundnut (Voandzeia subterranea)

Antioxidative and antimicrobial activities of extracts from Bambarra groundnut seed coat, prepared using water, ethanol and acetone as the media at various temperatures (30, 60, 90 °C) were investigated.

The antibacterial properties of isothiocyanates

Isothiocyanates (ITCs) are natural plant products generated by the enzymic hydrolysis of glucosinolates found in Brassicaceae vegetables. These natural sulfur compounds and their dithiocarbamate conjugates have been previously evaluated for their anti-cancerous properties. Their antimicrobial properties have been previously studied as well, mainly for food preservation and plant pathogen control. Recently, several revelations concerning the mode of action of ITCs in prokaryotes have emerged. This review addresses these new studies and proposes a model to summarize the current knowledge and hypotheses for the antibacterial effect of ITCs and whether they may provide the basis for the design of novel antibiotics.

Antimicrobial resistance (AMR) and plant-derived antimicrobials (PDAms) as an alternative drug line to control infections

Infectious diseases caused by antimicrobial-resistant microbes (ARMs) and the treatment are the serious problems in the field of medical science today world over. The development of alternative drug line to treat such infectious diseases is urgently required. Researches on ARMs revealed the presence of membrane proteins responsible for effusing the antibiotics from the bacterial cells. Such proteins have successfully been treated by plant-derived antimicrobials (PDAms) synergistically along with the commercially available antibiotics. Such synergistic action usually inhibits the efflux pump. The enhanced activity of plant-derived antimicrobials is being researched and is considered as the future treatment strategy to cure the incurable infections. The present paper reviews the advancement made in the researches on antimicrobial resistance along with the discovery and the development of more active PDAms.

Oligomerization of esculin improves its antibacterial activity and modulates antibiotic resistance

In this particular study, the antibacterial activity of esculin and oligomer fractions was assessed. MIC values of esculin and its oligomer fractions as well as of some antibiotics against Gram-positive and Gram-negative strains and against Escherichia coli multiresistant variants were determined by the standard broth microdilution method. Both esculin and oligoesculin fractions exhibited antibacterial effect against reference strains; Staphylococcus aureus, Enterococcus faecalis, Salmonella enteritidis and Salmonella typhimurium. It appears that E3 oligomer fraction had the greatest antibacterial activity against these reference strains. Besides, as E2 and E3 revealed the best antibacterial effect against multiresistant variants of E. coli, we decided to test the effect of each, combined to the antibiotic against which the variants were resistant. In the interaction study, E2 and E3 oligoesculin fractions were found to be effective in reducing the resistance of E. coli 6574 to ofloxacin and the resistance of E. coli 6228 to amoxicillin. Only E3 oligoesculin fraction showed a synergetic interaction with amoxicillin and tetracyclin against E. coli 6708, but no interaction was found either with E2 or E3 fractions against E. coli 6234. Our study allowed us to conclude that oligomerization of esculin increases its antibacterial potential, according to the degree of polymerization.

Chemistry and multibeneficial bioactivities of carvacrol (4-isopropyl-2-methylphenol), a component of essential oils produced by aromatic plants and spices

Aromatic plants produce organic compounds that may be involved in the defense of plants against phytopathogenic insects, bacteria, fungi, and viruses. One of these compounds, called carvacrol, which is found in high concentrations in essential oils such as oregano, has been reported to exhibit numerous bioactivities in cells and animals. This integrated overview surveys and interprets our present knowledge of the chemistry and analysis of carvacrol and its beneficial bioactivities. These activities include its antioxidative properties in food (e.g., lard, sunflower oil) and in vivo and the inhibition of foodborne and human antibiotic-susceptible and antibiotic-resistant pathogenic bacteria, viruses, pathogenic fungi and parasites, and insects in vitro and in human foods (e.g., apple juice, eggs, leafy greens, meat and poultry products, milk, oysters) and food animal feeds and wastes. Also covered are inhibitions of microbial and fungal toxin production and the anti-inflammatory, analgesic, antiarthritic, antiallergic, anticarcinogenic, antidiabetic, cardioprotective, gastroprotective, hepatoprotective, and neuroprotective properties of carvacrol as well as metabolic, synergistic, and mechanistic aspects. Areas for future research are also suggested. The collated information and suggested research might contribute to a better understanding of agronomical, biosynthetic, chemical, physiological, and cellular mechanisms of the described health-promoting effects of carvacrol, and facilitate and guide further studies needed to optimize the use of carvacrol as a multifunctional food in pure and encapsulated forms, in edible antimicrobial films, and in combination with plant-derived and medical antibiotics to help prevent or treat animal and human diseases.

Essential oils, a new horizon in combating bacterial antibiotic resistance

For many years, the battle between humans and the multitudes of infection and disease causing pathogens continues. Emerging at the battlefield as some of the most significant challenges to human health are bacterial resistance and its rapid rise. These have become a major concern in global public health invigorating the need for new antimicrobial compounds. A rational approach to deal with antibiotic resistance problems requires detailed knowledge of the different biological and non-biological factors that affect the rate and extent of resistance development. Combination therapy combining conventional antibiotics and essential oils is currently blooming and represents a potential area for future investigations. This new generation of phytopharmaceuticals may shed light on the development of new pharmacological regimes in combating antibiotic resistance. This review consolidated and described the observed synergistic outcome between essential oils and antibiotics, and highlighted the possibilities of essential oils as the potential resistance modifying agent.

Thymoquinone vapor significantly affects the results of Staphylococcus aureus sensitivity tests using the standard broth microdilution method

The broth microdilution (BMD) method is widely used for the determination of minimum inhibitory concentrations of antimicrobial agents, including volatile oils and their components. In this series of various experiments, we have demonstrated the influence of thymoquinone (TQ) vapor on the results of the BMD test performed with Staphylococcus aureus as a model organism. The spread of vapor from the TQ containing wells (32-512 μg/mL) caused the complete inhibition of staphylococcal growth in adjoining wells initially containing bacterium-inoculated pure Mueller-Hinton broth only and thus produced false positive results of the test. The ability of TQ to pass into the adjoined wells was subsequently confirmed by gas chromatography-mass spectrometry, whereas TQ at concentrations up to 84 μg/mL was detected in these wells after five hours. Based on these results, we suppose that vapors of TQ as well as of other naturally occurring volatile compounds and their mixtures (for example essential oils and plant extracts) can significantly influence results of the standard BMD assay. These observations, therefore, call for development of new appropriate BMD method suitable for assessment of antimicrobial activity of volatile substances.