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

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          Abstract

          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.

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          Most cited references 114

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          Carbapenemases: the versatile beta-lactamases.

          Carbapenemases are beta-lactamases with versatile hydrolytic capacities. They have the ability to hydrolyze penicillins, cephalosporins, monobactams, and carbapenems. Bacteria producing these beta-lactamases may cause serious infections in which the carbapenemase activity renders many beta-lactams ineffective. Carbapenemases are members of the molecular class A, B, and D beta-lactamases. Class A and D enzymes have a serine-based hydrolytic mechanism, while class B enzymes are metallo-beta-lactamases that contain zinc in the active site. The class A carbapenemase group includes members of the SME, IMI, NMC, GES, and KPC families. Of these, the KPC carbapenemases are the most prevalent, found mostly on plasmids in Klebsiella pneumoniae. The class D carbapenemases consist of OXA-type beta-lactamases frequently detected in Acinetobacter baumannii. The metallo-beta-lactamases belong to the IMP, VIM, SPM, GIM, and SIM families and have been detected primarily in Pseudomonas aeruginosa; however, there are increasing numbers of reports worldwide of this group of beta-lactamases in the Enterobacteriaceae. This review updates the characteristics, epidemiology, and detection of the carbapenemases found in pathogenic bacteria.
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            The real threat of Klebsiella pneumoniae carbapenemase-producing bacteria.

            From early this decade, Enterobacteriaceae that produce Klebsiella pneumoniae carbapenemases (KPC) were reported in the USA and subsequently worldwide. These KPC-producing bacteria are predominantly involved in nosocomial and systemic infections; although they are mostly Enterobacteriaceae, they can also be, rarely, Pseudomonas aeruginosa isolates. KPC beta lactamases (KPC-1 to KPC-7) confer decreased susceptibility or resistance to virtually all beta lactams. Carbapenems (imipenem, meropenem, and ertapenem) may thus become inefficient for treating enterobacterial infections with KPC-producing bacteria, which are, in addition, resistant to many other non-beta-lactam molecules, leaving few available therapeutic options. Detection of KPC-producing bacteria may be difficult based on routine antibiotic susceptibility testing. It is therefore crucial to implement efficient infection control measures to limit the spread of these pathogens.
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              The phenolic hydroxyl group of carvacrol is essential for action against the food-borne pathogen Bacillus cereus.

              The natural antimicrobial compound carvacrol shows a high preference for hydrophobic phases. The partition coefficients of carvacrol in both octanol-water and liposome-buffer phases were determined (3.64 and 3.26, respectively). Addition of carvacrol to a liposomal suspension resulted in an expansion of the liposomal membrane. Maximum expansion was observed after the addition of 0.50 micromol of carvacrol/mg of L-alpha-phosphatidylethanolamine. Cymene, a biological precursor of carvacrol which lacks a hydroxyl group, was found to have a higher preference for liposomal membranes, thereby causing more expansion. The effect of cymene on the membrane potential was less pronounced than the effect of carvacrol. The pH gradient and ATP pools were not affected by cymene. Measurement of the antimicrobial activities of compounds similar to carvacrol (e.g., thymol, cymene, menthol, and carvacrol methyl ester) showed that the hydroxyl group of this compound and the presence of a system of delocalized electrons are important for the antimicrobial activity of carvacrol. Based on this study, we hypothesize that carvacrol destabilizes the cytoplasmic membrane and, in addition, acts as a proton exchanger, thereby reducing the pH gradient across the cytoplasmic membrane. The resulting collapse of the proton motive force and depletion of the ATP pool eventually lead to cell death.
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                Author and article information

                Contributors
                Role: Academic Editor
                Role: Academic Editor
                Role: Academic Editor
                Journal
                Medicines (Basel)
                Medicines (Basel)
                medicines
                Medicines
                MDPI
                2305-6320
                28 July 2016
                September 2016
                : 3
                : 3
                Affiliations
                [1 ]Department of Pharmacognosy, Faculty of Pharmacy, University of Medicine and Pharmacy Grigore T. Popa-Iasi, Universitatii 16, Iasi 700115, Romania; petruta.aelenei@ 123456fitermanpharma.ro (P.A.); adriana.trifan@ 123456umfiasi.ro (A.T.); bujoralexandra123@ 123456gmail.com (A.B.); anaclara70@ 123456yahoo.com (A.C.A.)
                [2 ]Regulatory Affairs Department, Fiterman Pharma LLC, Pacurari Road 127, Iasi 700544, Romania
                [3 ]Stejarul Biological Research Centre/National Institute of Research and Development for Biological Sciences, Alexandru cel Bun 6, Piatra Neamt 610004, Romania; elgille9@ 123456yahoo.com
                Author notes
                [* ]Correspondence: ancamiron@ 123456yahoo.com or anca.miron@ 123456umfiasi.ro ; Tel.: +40-232-301-600; Fax: +40-232-211-820
                Article
                medicines-03-00019
                10.3390/medicines3030019
                5456245
                © 2016 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

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