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Essential oils: their antibacterial properties and potential applications in foods--a review.

Biological effects of essential oils - A review

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

Gut Microbiota in Health and Disease

Aims: The minimum inhibitory concentration (MIC) of oregano essential oil (OEO) and two of its principle components, i.e. thymol and carvacrol, against Pseudomonas aeruginosa and Staphylococcus aureus was assessed by using an innovative technique. The mechanism of action of the above substances was also investigated. Methods and Results: The applied technique uses 100-well microtitre plate and collects turbidimetric growth data. To produce the inhibition profiles, a wide range of concentrations were tested for each of the three compounds, as well as for carvacrol‐thymol mixtures. Following a specific mathematical analysis of the observed inhibition profiles from all compounds, it was suggested that mixtures of carvacrol and thymol gave an additive effect and that the overall inhibition by OEO can be attributed mainly to the additive antimicrobial action of these two compounds. Addition of low amounts of each additive: (a) increased permeability of cells to the nuclear stain EB, (b) dissipated pH gradients as indicated by the CFDA-SE fluorescent probe irrespective of glucose availability and (c) caused leakage of inorganic ions. Conclusions: Mixing carvacrol and thymol at proper amounts may exert the total inhibition that is evident by oregano essential oil. Such inhibition is due to damage in membrane integrity, which further affects pH homeostasis and equilibrium of inorganic ions. Significance and Impact of the Study: The knowledge of extent and mode of inhibition of specific compounds, which are present in plant extracts, may contribute to the successful application of such natural preservatives in foods, since certain combinations of carvacrol‐ thymol provide as high inhibition as oregano essential oil with a smaller flavour impact.

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A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol.

Photodynamic therapy (PDT) employs a non-toxic dye, termed a photosensitizer (PS), and low intensity visible light which, in the presence of oxygen, combine to produce cytotoxic species. PDT has the advantage of dual selectivity, in that the PS can be targeted to its destination cell or tissue and, in addition, the illumination can be spatially directed to the lesion. PDT has previously been used to kill pathogenic microorganisms in vitro, but its use to treat infections in animal models or patients has not, as yet, been much developed. It is known that Gram-(−) bacteria are resistant to PDT with many commonly used PS that will readily lead to phototoxicity in Gram-(+) species, and that PS bearing a cationic charge or the use of agents that increase the permeability of the outer membrane will increase the efficacy of killing Gram-(−) organisms. All the available evidence suggests that multi-antibiotic resistant strains are as easily killed by PDT as naive strains, and that bacteria will not readily develop resistance to PDT. Treatment of localized infections with PDT requires selectivity of the PS for microbes over host cells, delivery of the PS into the infected area and the ability to effectively illuminate the lesion. Recently, there have been reports of PDT used to treat infections in selected animal models and some clinical trials: mainly for viral lesions, but also for acne, gastric infection by Helicobacter pylori and brain abcesses. Possible future clinical applications include infections in wounds and burns, rapidly spreading and intractable soft-tissue infections and abscesses, infections in body cavities such as the mouth, ear, nasal sinus, bladder and stomach, and surface infections of the cornea and skin.

Photodynamic therapy: a new antimicrobial approach to infectious disease?

Carvacrol, (+)-carvone, thymol, and trans-cinnamaldehyde were tested for their inhibitory activity against Escherichia coli O157:H7 and Salmonella typhimurium. In addition, their toxicity to Photobacterium leiognathi was determined, utilizing a bioluminescence assay. Their effects on the cell surface were investigated by measuring the uptake of 1-N-phenylnaphthylamine (NPN), by measuring their sensitization of bacterial suspensions toward detergents and lysozyme, and by analyzing material released from cells upon treatment by these agents. Carvacrol, thymol, and trans-cinnamaldehyde inhibited E. coli and S. typhimurium at 1-3 mM, whereas (+)-carvone was less inhibitory. trans-Cinnamaldehyde was the most inhibitory component toward P. leiognathi. Carvacrol and thymol disintegrated the outer membrane and released outer membrane-associated material from the cells to the external medium; such release by (+)-carvone or trans-cinnamaldehyde was negligible. Of the tested components, carvacrol and thymol decreased the intracellular ATP pool of E. coli and also inreased extracellular ATP, indicating disruptive action on the cytoplasmic membrane.

Characterization of the action of selected essential oil components on gram-negative bacteria

The effect of lactic acid on the outer membrane permeability of Escherichia coli O157:H7, Pseudomonas aeruginosa, and Salmonella enterica serovar Typhimurium was studied utilizing a fluorescent-probe uptake assay and sensitization to bacteriolysis. For control purposes, similar assays were performed with EDTA (a permeabilizer acting by chelation) and with hydrochloric acid, the latter at pH values corresponding to those yielded by lactic acid, and also in the presence of KCN. Already 5 mM (pH 4.0) lactic acid caused prominent permeabilization in each species, the effect in the fluorescence assay being stronger than that of EDTA or HCl. Similar results were obtained in the presence of KCN, except for P. aeruginosa, for which an increase in the effect of HCl was observed in the presence of KCN. The permeabilization by lactic and hydrochloric acid was partly abolished by MgCl2. Lactic acid sensitized E. coli and serovar Typhimurium to the lytic action of sodium dodecyl sulfate (SDS) more efficiently than did HCl, whereas both acids sensitized P. aeruginosa to SDS and to Triton X-100. P. aeruginosa was effectively sensitized to lysozyme by lactic acid and by HCl. Considerable proportions of lipopolysaccharide were liberated from serovar Typhimurium by these acids; analysis of liberated material by electrophoresis and by fatty acid analysis showed that lactic acid was more active than EDTA or HCl in liberating lipopolysaccharide from the outer membrane. Thus, lactic acid, in addition to its antimicrobial property due to the lowering of the pH, also functions as a permeabilizer of the gram-negative bacterial outer membrane and may act as a potentiator of the effects of other antimicrobial substances.

https://www.thermoscientific.com/content/dam/tfs/LPG/LCD/LCD%20Documents/Peer%20Reviewed%20Papers%20OR%20Third-Party%20Papers/Microplate%20Instrumentation/Microplate%20Readers/D00570~.pdf

Lactic acid permeabilizes gram-negative bacteria by disrupting the outer membrane

Antimicrobial activity and mechanisms of phenolic extracts of 12 Nordic berries were studied against selected human pathogenic microbes. The most sensitive bacteria on berry phenolics were Helicobacter pylori and Bacillus cereus. Campylobacter jejuni and Candida albicans were inhibited only with phenolic extracts of cloudberry, raspberry, and strawberry, which all were rich in ellagitannins. Cloudberry extract gave strong microbicidic effects on the basis of plate count with all studied strains. However, fluorescence staining of liquid cultures of virulent Salmonella showed viable cells not detectable by plate count adhering to cloudberry extract, whereas Staphylococcus aureus cells adhered to berry extracts were dead on the basis of their fluorescence and plate count. Phenolic extracts of cloudberry and raspberry disintegrated the outer membrane of examined Salmonella strains as indicated by 1-N-phenylnaphthylamine (NPN) uptake increase and analysis of liberation of [14C]galactose- lipopolysaccharide. Gallic acid effectively permeabilized the tested Salmonella strains, and significant increase in the NPN uptake was recorded. The stability of berry phenolics and their antimicrobial activity in berries stored frozen for a year were examined using Escherichia coli and nonvirulent Salmonella enterica sv. Typhimurium. The amount of phenolic compounds decreased in all berries, but their antimicrobial activity was not influenced accordingly. Cloudberry, in particular, showed constantly strong antimicrobial activity during the storage.

Berry Phenolics: Antimicrobial Properties and Mechanisms of Action Against Severe Human Pathogens

Berry fruits are rich sources of bioactive compounds, such as phenolics and organic acids, which have antimicrobial activities against human pathogens. Among different berries and berry phenolics, cranberry, cloudberry, raspberry, strawberry and bilberry especially possess clear antimicrobial effects against, e.g. Salmonella and Staphylococcus. Complex phenolic polymers, like ellagitannins, are strong antibacterial agents present in cloudberry and raspberry. Several mechanisms of action in the growth inhibition of bacteria are involved, such as destabilisation of cytoplasmic membrane, permeabilisation of plasma membrane, inhibition of extracellular microbial enzymes, direct actions on microbial metabolism and deprivation of the substrates required for microbial growth. Antimicrobial activity of berries may also be related to antiadherence of bacteria to epithelial cells, which is a prerequisite for colonisation and infection of many pathogens. Antimicrobial berry compounds may have important applications in the future as natural antimicrobial agents for food industry as well as for medicine. Some of the novel approaches are discussed.

Hanna-Leena Alakomi

Papers

Characterization of the action of selected essential oil components on gram-negative bacteria

Lactic acid permeabilizes gram-negative bacteria by disrupting the outer membrane

Berry Phenolics: Antimicrobial Properties and Mechanisms of Action Against Severe Human Pathogens

Bioactive berry compounds—novel tools against human pathogens

Toxicity of selected plant volatiles in microbial and mammalian short-term assays