processes Review Contribution of Essential Oils to the Fight against Microbial Biofilms—A Review 1, 1 2 3 4 Diana Camelia Nut, ă *, Carmen Limban , Cornel Chirit, ă , Mariana Carmen Chifiriuc , Teodora Costea , 5 5 5 Petre Ionit, ă , Ioana Nicolau and Irina Zarafu 1 Department of Pharmaceutical Chemistry, Faculty of Pharmacy, “Carol Davila” University of Medicine and Pharmacy, TraianVuia no.6, 020956 Bucharest, Romania; [email protected] 2 Department of Pharmacology and Clinical Pharmacy, Faculty of Pharmacy, “Carol Davila” University of Medicine and Pharmacy, TraianVuia no.6, 020956 Bucharest, Romania; [email protected] 3 Department of Microbiology, Faculty of Biology, Universtity of Bucharest, AleeaPortocalelor no.1-3, 060101 Bucharest, Romania; [email protected] 4 Department of Pharmacognosy, Phytochemistry, Phytotherapy, Faculty of Pharmacy, “Carol Davila” University of Medicine and Pharmacy, TraianVuia no.6, 020956 Bucharest, Romania; [email protected] 5 Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Regina Elisabeta no.4-12, 030018 Bucharest, Romania; [email protected] (P.I.); [email protected] (I.N.); [email protected] (I.Z.) * Correspondence: [email protected] Abstract: The increasing clinical use of artificial medical devices raises the issue of microbial con- tamination, which is a risk factor for the occurrence of biofilm-associated infections. A huge amount of scientific data highlights the promising potential of essential oils (EOs) to be used for the de- velopment of novel antibiofilm strategies. We aimed to review the relevant literature indexed in Citation: Nut˘a,D.C.; Limban, C.; , PubMed and Embase and to identify the recent directions in the field of EOs, as a new modality to Chirit, ˘a,C.; Chifiriuc, M.C.; Costea, T.; eradicate microbial biofilms. We paid special attention to studies that explain the mechanisms of the Ionit, ˘a,P.; Nicolau, I.; Zarafu, I. microbicidal and antibiofilm activity of EOs, as well as their synergism with other antimicrobials. Contribution of Essential Oils to the Fight against Microbial Biofilms—A The EOs are difficult to test for their antimicrobial activity due to lipophilicity and volatility, so we Review. Processes 2021, 9, 537. have presented recent methods that facilitate these tests. There are presented the applications of https://doi.org/10.3390/pr9030537 EOs in chronic wounds and biofilm-mediated infection treatment, in the food industry and as air disinfectants. This analysis concludes that EOs are a source of antimicrobial agents that should not Academic Editors: Elwira Sieniawska, be neglected and that will probably provide new anti-infective therapeutic agents. Greige-Gerges Helene and Adriana Trifan Keywords: essential oils; bacterial biofilm; antimicrobial; medical devices Received: 12 February 2021 Accepted: 15 March 2021 Published: 18 March 2021 1. Introduction In the past decade, essential oils (EOs) use for the prophylaxis and therapy of biofilm- Publisher’s Note: MDPI stays neutral associated infections (BAIs) have become very popular. The universally accepted definition with regard to jurisdictional claims in of a biofilm refers to a sessile multicellular community of microbial cells with a modified published maps and institutional affil- iations. transcriptome and phenotype (exhibiting increased resistance to both therapeutic doses of current antimicrobials and immune effectors) that adhere to a surface and, among them, being protected by an auto-secreted extracellular polymeric matrix [1,2]. It is considered that BAIs represent up to 85% of the total microbial infections, occurring after microbial colonization of either viable tissues or medical devices and having serious consequences [3], Copyright: © 2021 by the authors. because they are persistent and hard or impossible to treat, even in immunocompetent Licensee MDPI, Basel, Switzerland. individuals. This article is an open access article On the other hand, the food manufacturing industry is facing the formation of micro- distributed under the terms and conditions of the Creative Commons bial biofilms that can affect industrial processes, and researchers are in constant search of Attribution (CC BY) license (https:// new ways to eradicate this phenomenon. The protective mechanisms of microbial cells creativecommons.org/licenses/by/ within biofilms are multifactorial and differ from those that occurred in planktonic cells 4.0/). and include matrix impermeability, modified transcription rate, selection of persister cells, Processes 2021, 9, 537. https://doi.org/10.3390/pr9030537 https://www.mdpi.com/journal/processes Processes 2021, 9, x FOR PEER REVIEW 2 of 19 within biofilms are multifactorial and differ from those that occurred in planktonic cells and include matrix impermeability, modified transcription rate, selection of persister cells, Processes 2021, 9, 537 accumulation of antibiotic inactivating enzymes, increased horizontal transfer2 rate of 19 of re- sistance genes, etc. [4]. Therefore, biofilm cells can become up to100–1000 times more re- sistant to antimicrobial substances than planktonic cells [5]. This high phenotypic re- accumulationsistance, also ofcalled antibiotic tolerance, inactivating interferes enzymes, not only increased with the horizontal BAI treatment transfer ratebut ofalso resis- with the tanceefficacy genes, of surface etc. [4]. disinfection Therefore, biofilm processes cells can [6,7 become]. The discovery up to100–1000 of natural times more products resistant with an- totimicrobial antimicrobial activity substances represents than planktonica direction cells of current [5]. This research high phenotypic trying to resistance, limit microbial also dis- calledeases. tolerance, interferes not only with the BAI treatment but also with the efficacy of surface disinfection processes [6,7]. The discovery of natural products with antimicrobial Therefore, the purpose of this paper was to review the recent literature on EOs anti- activity represents a direction of current research trying to limit microbial diseases. biofilm activities. Therefore, the purpose of this paper was to review the recent literature on EOs antibiofilm activities. 2. Methods 2. MethodsFor this purpose, the PubMed (National Library of Medicine, Washington, DC) and EmbaseFor this(Elsevier) purpose, databases the PubMed were (National searched Library for all ofrelevant Medicine, articles Washington, written in DC) English, and us- Embaseing the following (Elsevier) databases keywords: were “essential searched oils”, for all an relevantd “biofilm”, articles and written then in“dentistry”, English, using “chronic thewound following infections”, keywords: “medical “essential devices”, oils”, “food and “biofilm”, industry”. and We then also “dentistry”, reviewed additional “chronic rel- woundevant articles infections”, identified “medical from devices”,the referenced “food citations. industry”. We We limited also reviewed our investigation additional to Eng- relevant articles identified from the referenced citations. We limited our investigation to lish-language journals. English-language journals. EOs (also called volatile or ethereal oils) are natural aromatic oily liquids with com- EOs (also called volatile or ethereal oils) are natural aromatic oily liquids with com- plex compositionscompositions obtained obtained from from different different plant plant organs organs by various by variou methods,s methods, including including expression, fermentation,fermentation, enfleurage enfleurage and and extraction. extraction. The The most most used used technique technique is steam is steam distillation [ 8[8].]. From From the the 20–60 20–60 low low molecular molecular weight weight components, components, which which can be can found be found in in the compositioncomposition of of EOs EOs in in different different amounts, amounts, at least at least one, one, such such as terpenes as terpenes and terpenoids and terpenoids or otherother aromaticaromatic compounds, compounds, exhibit exhibit antimicrobial antimicrobial activity activity [9] (Figure [9] (Figure1). 1). OH OCOCH3 OH OH OH OH Geraniol Linalool Linalyl acetate Nerol Carvacrol Thymol O O OH O OH Eucalyptol alpha-Terpineol Terpinen-4-ol Citral Piperitone OH CH3 CH2OH HO cis-nerolidol alpha-Pinene trans-Nerolidol Farnesol H O HO OCH3 OH Menthol Eugenol Cinnamaldehyde CHO CHO Decanal trans-2-Decenal Figure 1.1.The The main main components components of of essential essential oils oils (EOs). (EOs). Processes 2021, 9, 537 3 of 19 EOs of vegetal origin have been used for millennia in ethnomedicine as a natural antimicrobial and antiviral agents. Their antimicrobial activity is due to the alteration of microbial cell envelope leading to cellular lysis with cell contents leakage and proton motive force inhibition. Their benefits result from the fact that microbial resistance is less probably to be installed, as compared to chemical substances, the facile preparation, high biocompatibility and biodegradability [6]. Besides their antimicrobial activity, EOs also exhibit anti-inflammatory effects. In addition, their antibiofilm activity came to researchers’ attention in the last ten years [4]. 3. Discussion 3.1. EOs Mechanism of Action as Antibiofilm Agents The complex composition of EOs suggests that multiple mechanisms, probably acting synergically, are involved in their biological effects [3]. From the studied articles, we identified several types of mechanisms of