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Complimentary and personal copy for Marketa Houdkova, Ladislav Kokoska www.thieme.com Volatile Antimicrobial Agents and In Vitro Methods for Evaluating Their Activity in the Vapour Phase: A Review DOI 10.1055/a-1158-4529 Planta Med This electronic reprint is provided for non- commercial and personal use only: this reprint may be forwarded to individual colleagues or may be used on the authorʼs homepage. This reprint is not provided for distribution in repositories, including social and scientific networks and platforms. Publishing House and Copyright: © 2020 by Georg Thieme Verlag KG Rüdigerstraße 14 70469 Stuttgart ISSN 0032‑0943 Any further use only by permission of the Publishing House Reviews Volatile Antimicrobial Agents and In Vitro Methods for Evaluating Their Activity in the Vapour Phase: A Review Authors Marketa Houdkova, Ladislav Kokoska Affiliation sult of literature analysis, the antimicrobial efficacy of vapours Department of Crop Sciences and Agroforestry, Czech from 122 different plant species and 19 pure compounds ex- University of Life Sciences Prague, Prague, Czech Republic amined in 61 studies using different in vitro tests against a broad spectrum of microorganisms was identified and sum- Key words marized. In addition, 11 different techniques found in the lit- antibacterial, broth dilution, diffusion, essential oils, plant erature are described in detail. An original classification of volatiles, volatilization methods based on the solid and liquid matrix volatilization principle is proposed because carrier medium/matrix selec- received November 8, 2019 tion is crucial for the volatilization of any agents tested. This revised March 19, 2020 review should be useful for medicinal, pharmaceutical, food, accepted April 14, 2020 and agricultural experts working in areas related to the man- agement of infectious diseases (especially respiratory and skin Bibliography infections), food preservation (active packaging), and protec- DOI https://doi.org/10.1055/a-1158-4529 tion of agriculture products (controlled atmosphere). It may published online | Planta Med © Georg Thieme Verlag KG also stimulate the interest of pharmaceutical, cosmetic, food, Stuttgart · New York | ISSN 0032‑0943 and agriculture industries in the research and development of Correspondence new antimicrobial agents of natural origin. Since several orig- Prof. Ladislav Kokoška inal apparatuses previously developed for antimicrobial sus- Department of Crop Sciences and Agroforestry, Faculty of ceptibility testing in the vapour phase are described in this re- Tropical AgriSciences, Czech University of Life Sciences Prague view, labware manufacturers may also be interested in this Kamycka 129, 165 21 Praha 6 – Suchdol, Czech Republic topic. The review also provides specific guidelines and recom- Phone: + 420 224 382 180, Fax: + 420 234 381 829 mendations for researchers studying the antimicrobial activ- [email protected] ity of volatile agents. The article will therefore appeal to com- munities of industrial stakeholders, pharmacists, physicians, food experts, agriculturists, and researchers in related areas ABSTRACT such as pharmacology, medicinal chemistry, microbiology, This review summarizes data on the in vitro antimicrobial ef- natural product chemistry, food preservation and plant pro- fectiveness of volatile agents of plant origin and in vitro meth- tection. ods for evaluating their activity in the vapour phase. As a re- Electronic reprint for personal use Introduction products in addition to a few specific compounds not represented in these major classes such as alkanes, alkenes, alcohols, esters, al- Volatile agents (VAs) are abundant chemicals that are emitted by dehydes, and ketones of various biogenetic origin [4]. The physical organisms in all terrestrial and marine ecosystems as an important properties of these VAs enable them to freely cross cellular mem- factor, allowing communication and interaction among plants, branes and be released into the surrounding environment [5]. microorganisms, animals, and the environment [1, 2]. The pri- In recent years, natural substances including VAs have been in- mary functions of VAs are defence against herbivores and patho- tensively studied for the purpose of reducing the utilization of gens and the attraction of pollinators and seed dispersers, signal- synthetic antimicrobial agents in pharmacy, foods, and agricul- ling involved in plant communication, and they also can act as ture. They are considered relatively safe, and are easily decom- wound sealers in some plants [3]. This relatively large group of posed, environmentally friendly, and non-phytotoxic [6]. More- natural products consisting of lipophilic compounds of low molec- over, VAs have the benefit of being bioactive in their vapour ular weight and high vapour pressure at ambient temperatures is phase. Thanks to this, volatiles have great potential for the devel- divided into several chemical classes, including terpenoids, phe- opment of novel agricultural, food, and pharmaceutical plant-de- nylpropanoids, fatty acid derivatives, and amino acid-derived rived products and technologies [7]. Houdkova M, Kokoska L. Volatile Antimicrobial Agents … Planta Med Reviews Among the most important representatives of VAs are essen- [31–33]. However, industrial antimicrobial applications based on tial oils (EOs) produced via plant metabolism. EOs are aromatic liq- the most typical physicochemical feature of these agents, which uids of complex composition including aliphatic hydrocarbons, is volatility, have not been fully developed yet. VAs could be appli- terpenoids, and phenylpropanoids [8]. Plant species producing cable for the development of new pharmaceutical preparations (e. EOs belong to various genera divided into around 60 families such g., inhalation therapy), disinfection and sterilization agents in as the Alliaceae, Apiaceae, Asteraceae, Chenopodiaceae, Cypera- healthcare facilities, the protection of stored agricultural products ceae, Lamiaceae, Lauraceae, Myrtaceae, Poaceae, Piperaceae, Ru- (e.g., controlled atmosphere storage), and for the preservation taceae, Verbenaceae, and Zingiberaceae [9, 10]. Commonly, they and shelf-life extension of food products (e.g., active packaging) are obtained via the distillation and cold pressing of whole plants [34]. Only a few fumigants recommended for organic greenhouse or their individual parts (bark, buds, flowers, fruits, leaves, roots, application, e.g., Eco-Oil (Organic Crop Protectants), Prev-AM seeds, stems, and wood) [11]. However, enfleurage, organic sol- (Oro Agri International), Requiem (Bayer AG), and inhaler nasal vent, and supercritical fluid extractions are another possible way sticks for ease of breathing (e.g., Vicks), based on the vapours of of isolating products containing volatile compounds such as plant volatiles have been commercialized. These preparations are oleoresins. The extraction process plays a vital role in the yield composed of ingredients such as camphor, limonene, menthol, and quality of EOs and related products [12]. Since ancient times, and the EOs of Abies sibirica, Citrus sp., and Melaleuca alternifolia EOs have been widely used in medicine, perfumery, cosmetics, [35,36]. and the food industry. As they often possess a broad spectrum of As far as research on the in vitro antimicrobial activity of VAs in biological effects [13], EOs have potential in various applications the vapour phase is concerned, their growth-inhibitory effect has for human, animal, and plant health, as well as food quality. Be- been studied against a broad spectrum of pathogenic microor- sides antimicrobial activity [14–16], EOs and their constituents ganisms associated with human infections (e.g., Candida albicans, have been observed to exhibit anaesthetic [17], antiparasitic Haemophilus influenzae, Pseudomonas aeruginosa, Staphylococcus [18], anti-inflammatory [19], antioxidant [20], antiulcer [21], anti- aureus, Streptococcus pneumoniae), foodborne diseases (e.g., Bacil- viral [22], cytotoxic [23], immunomodulatory [24], insecticidal lus cereus, Escherichia coli, Listeria monocytogenes, Salmonella enter- [25], and molluscicidal [26] properties. Despite their historical re- itidis), and plant diseases (e.g., Alternaria alternata, Aspergillus sp., cord as medicinal agents, European and American health author- Fusarium oxysporum, Penicillium sp.) [37]. The results of the anti- ities such as the European Pharmacopoeia and the United States microbial effects of EOs and volatile compounds previously tested Pharmacopoeia National Formulary list only a small number of by various authors in a gaseous phase are summarized in ▶ Tables EO-bearing plants and their preparations as medicinal [3]. 1 and 2, respectively. According to previously published data, a Their main advantages are that they do not need to be applied high level of efficiency was observed for EOs isolated from plant systemically to the body or directly to agricultural or food prod- species such as Armoracia rusticana, Brassica nigra, Cinnamomum ucts and that they naturally tend to be regularly distributed in zeylanicum, Cymbopogon citratus, Lavandula sp., Lippia berlandieri, the air conditions of the target area. In addition, EOs are a typical Origanum vulgare, Pulicaria mauritanica,andSyzygium aromaticum, example of complex mixtures producing an antimicrobial syner- as well for pure compounds such as carvacrol, 8-hydroxyquino- gistic effect, which is currently considered an effective tool in line, linalool, linalyl acetate,