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Lipids and Essential Oils as Antimicrobial Agents

Lipids and Essential Oils as Antimicrobial Agents byHalldor Thormar (ed.). West Sussex, UK: Wiley; 2011. Hardcover; 315 pages. ISBN: 9780470741788. $145.00.

Oils are important for life. They are composed of complex chemicals in liquid form. Fixed oils are composed of lipids that are vital for the cell and functions of living organisms. The main types of lipids are fats, oils, phospholipids, and steroids. Animal fats and vegetable oils are known as triglycerides, which are composed of fatty acids and glycerol. Phospholipids, the major components of cell membranes, consist of 2 fatty acids. When triglycerides and phospholipids undergo a chemical reaction known as hydrolysis, they produce fatty acids with antimicrobial properties. These fatty acids are abundantly available in nature and are essential for the body.

Essential oils consist of volatile terpenoid and non-terpenoid constituents not essential for life functions, but they have other useful properties. Terpenoids are composed of hydrocarbons and a number of smaller units known as isoprene molecules. Essential oils can be found in aromatic plants and are responsible for their fragrance. Unlike fixed oils that are obtained by expression or solvent extraction from plant materials such as seeds, essential oils are obtained by water or steam distillation or expression in the case of citrus oils. In Lipids and Essential Oils as Antimicrobial Agents, the authors examine both categories of oils with a focus on their antimicrobial properties.

Lipids and Essential Oils as Antimicrobial Agents consists of 11 chapters ably written by experts in their respective fields. Each chapter provides comprehensive and valuable information on various aspects of antimicrobial lipids and essential oils in an easily digestible manner.

The first 8 chapters deal with various aspects of lipids as antimicrobial agents beginning with the chemical aspects of lipids and their actions on biological membranes. The first chapter is titled “Membranes as Targets of Antimicrobial Lipids.” The author offers a brief but comprehensive explanation of the physical and chemical properties of lipids and their behavior and function in living cells. At certain concentrations, surfactants—a class of molecules that acts on the surface of cells—can act as detergents, breaking apart lipid membranes. The author concludes that antimicrobial lipids act in a similar way to kill microorganisms.

In the following chapter, the author discusses the history of antimicrobial lipids research from the 1880s to 1960, focusing mainly on the antimicrobial activity and life-saving functions of soaps. Striking examples of antimicrobial soaps include the use of oleate soaps on anthrax and cholera germs and chaulmoogra oil (Hydnocarpus wightiana, Achariaceae) for treating leprosy. Skin lipids with oleic acid and other long-chain fatty acids have been shown to possess self-disinfecting activity on human skin and its appendages like hair, nails, and earwax. Chapter 3 discusses the more recently discovered antibacterial, antifungal, and antiviral activities of lipids. Monoglycerides such as monolaurin and monocaprin and fatty acids like lauric and linolenic acids demonstrate antibacterial activities against gram-positive bacteria genera such as Staphylococci (Staphylococcaceae), Streptococci (Streptococcaceae), and Bacilli (Bacillaceae), as well as Propionibacterium (Propionibacteriaceae), a bacterium that causes acne. Capric, lauric, palmitoleic, and arachidonic acids, monocaprin, monolaurin, and monopalmitolein were found to be active against gram-negative bacteria such as Neisseria gonorrhoeae (Neisseriaceae, which causes gonorrhea), Salmonella (Enterobacteriaceae), and Escherichia coli (Enterobacteriaceae), mainly in acidic conditions. While monocarpin and monolaurin did not show significant activity against E. coli at 30oC and physiological pH, by increasing the temperature to 50oC and lowering the pH,  remarkable antibacterial activity was observed, demonstrating that acidic conditions remove the barriers in outer membranes of the cell wall and allow lipids to access and saturate the cell, causing disintegration of the membranes.

Antiviral, antimicrobial, and antiprotozoal activities of fatty acids, fatty alcohols, and monoglycerides are also reviewed. Antimicrobial lipids in human breastmilk are discussed in Chapter 4. Milk lipids play and important role not only in nutrition but also in the protection of suckling neonates from infections. Breastmilk, which contains medium-chain saturated and long-chain unsaturated fatty acids and their respective monoglycerides, has the ability to destabilize microbial membranes. Their mechanism of action is explained in detail. The same issue is also discussed from another perspective in Chapter 6, in the section, “The Role of Human Milk Lipids in Innate Immunity,” where breast feeding is recommended to enhance the immune systems of newborn babies.

In chapter 5, the author deals with antimicrobial lipids of the skin and explains their molecular mechanism of action. Complex mixtures of lipids on skin are seen as contributing to localized natural immunity. Skin lipids have been shown to be effective against gram-positive bacteria, gram-negative bacteria, and yeasts. They appear to act through microbial plasma membranes and can enhance innate immunity. Effects of antimicrobial lipids in pulmonary mucosa, respiratory tract, in the lungs and on skin, and their role in natural immunity are also discussed in detail.

Chapter 7 comprises a compilation of practical aspects of lipids as active ingredients in pharmaceuticals, cosmetics, and health foods. It was interesting to read that lipids are sometimes used as animal feed supplements instead of antibiotics, and that capric acid acts as an antimethanogenic in ruminant animals. Agricultural applications of fatty acids are mentioned as well, with the author noting that linolenic and linoleic acids  exhibited activity against pathogenic fungi tested, while oleic acid had limited antifungal activity on certain fungi. The effects of lipids on a variety of diseases are reviewed in this chapter as well. The final chapter on lipids covers practical uses of antimicrobial lipids as disinfectants, antiseptics, and sanitizers. The dawn of soaps as germicides and the renewed interest in recent years in using antimicrobial fatty acids and monoglycerides to reduce bacterial contamination of foodstuffs and as sanitizers or disinfectants are discussed. The use of the lipid monocaprin either singly or in liquid soaps is recommended as a strong microbicidal.

Essential oils are covered in the final 3 chapters. Chapter 9 gives an overview of the chemistry and biological activity of essential oils. The chemistry of monoterpenes, sesquiterpenes, diterpenes, norterpenes, and phenylpropanoids as well as sulfur- and nitrogen-containing essential oil constituents is described. It would have been beneficial for the authors also to include non-terpenoid volatile components like alkanes, alkenes, and their oxygenated derivatives: benzenoids, fatty acids, and their esters as essential oil constituents. In this chapter, the author also reviews antibacterial, antifungal, antiviral, antiprotozoal, and anticancer properties associated with essential oils. Their use in pharmaceuticals and foods also appears in this chapter.

Chapter 10 is devoted specifically to the antiviral activities of essential oils and their components. Essential oils are complex mixtures of volatile chemicals, some of which may possess antimicrobial and/or antiviral activities. Microorganisms cannot easily develop resistance to essential oils like they do to single chemicals due to the compexity of their multicomponent chemistry. Therefore, they are preferable to single components for antimicrobial and/or antiviral effects. Lemon balm (Melissa officinalis, Lamiaceae) essential oil has shown antiviral activity against the herpes simplex viruse. Essential oils of chamomile (Matricaria recutita, Asteraceae), star anise (Illicium verum, Schisandraceae), dwarf pine (Pinus mugo, Pinaceae), manuka (Leptospermum scoparium, Myrtaceae), and tea tree (Melaleuca alternifolia, Myrtaceae) were found to be promising potential antiviral agents as well. A controlled clinical trial of a tea tree oil gel was found effective in the treatment of human herpes labialis. A comprehensive review of the mechanisms and practical aspects of antibacterial and antifungal activities of essential oils is given in the final chapter.

I recommend this book for scientists, researchers, and health professionals interested in pharmaceuticals, cosmetics, natural products, oils, or nutrition. Students also can benefit from the comprehensive reviews in order to understand and learn the similar actions of these 2 diverse groups of natural chemicals.—K. Husnu Can Baser, PhDProfessor of PharmacognosyAnadolu UniversityBadeBio Biotechnology Ltd.Eskisehir, Turkey