Appl Microbiol Biotechnol DOI 10.1007/s00253-016-7807-y APPLIED MICROBIAL AND CELL PHYSIOLOGY Antimicrobial activity, cytotoxicity and chemical analysis of lemongrass essential oil (Cymbopogon flexuosus)andpurecitral Emmanuel C. Adukwu1 & Melissa Bowles1 & Valerie Edwards-Jones 2 & Heather Bone1 Received: 12 July 2016 /Accepted: 9 August 2016 # The Author(s) 2016. This article is published with open access at Springerlink.com Abstract The aim of this study was to determine the antimi- Keywords Acinetobacter baumannii . Lemongrass oil . crobial effects of lemongrass essential oil (C. flexuosus)andto Multi-drug resistance (MDR) . Toxicity . IC50 determine cytotoxic effects of both test compounds on human dermal fibroblasts. Antimicrobial susceptibility screening was carried out using the disk diffusion method. Antimicrobial re- Introduction sistance was observed in four of five Acinetobacter baumannii strains with two strains confirmed as multi-drug-resistant The global threat of antimicrobial resistance (AMR) and infec- (MDR). All the strains tested were susceptible to both lemon- tions caused by AMR bacteria has raised the need for urgent grass and citral with zones of inhibition varying between 17 to therapeutic discoveries and improvement of existing infection 80 mm. The mean minimum inhibitory concentration (MIC) control and antimicrobial practices. In recent years, the Gram- and minimum bactericidal concentration (MBC) of citral negative bacterium Acinetobacter baumannii has been identified (mic—0.14 % and mbc—0.3 % v/v) was lower than that of as a resilient and resistant pathogen (Perez et al. 2007). Lemongrass (mic—0.65 % and mbc—1.1 % v/v)determined Outbreaks in hospital and community settings caused by using the microtitre plate method. Cell viability using human multi-drug-resistant, extensively drug-resistant and pan-drug- dermal fibroblasts (HDF; 106-05a) was determined following resistant A. baumannii have been reported worldwide. exposure to both compounds and a control (Grapeseed oil) AccordingtoVilaandPachon(2011), new approaches and an- using the XTT assay and the IC50 determined at 0.095 % (v/ timicrobial agents are needed for the control of A. baumannii v) for citral and 0.126 % (v/v) for lemongrass. Grapeseed oil infections as the few existing treatments have not been success- had no effect on cell viability. Live cell imaging was performed ful in managing infections caused by A. baumannii. using the LumaScope 500 imaging equipment and changes in The failure of existing antibiotics in managing infections HDF cell morphology such as necrotic features and shrinkage caused by AMR organisms has increased interest in alternative were observed. The ability of lemongrass essential oil (EO) and treatments. This is demonstrated by the breadth of literature citral to inhibit and kill MDR A. baumannii highlights its po- published in the area of natural antimicrobials especially the tential for use in the management of drug-resistant infections; antimicrobial effects of plant based essential oils (EOs). The however, in vitro cytotoxicity does suggest further tests are effects EOs are wide ranging and include antibacterial, antifun- needed before in vivo or ex vivo human exposure. gal, antibiofilm, antiparasitic, antioxidant, antiviral, anticancer and many other reported effects; however, the information on bioactivity and toxicity of essential oils is not as extensively studied. Despite this, the commercial use and applications of * Emmanuel C. Adukwu EOs continues to grow, e.g. EOs are used in household [email protected] cleaning products, cosmetics, perfumery, insecticides, disinfec- tant wipes, food and in management of infections in animals. 1 Faculty of Applied Sciences, University of the West of England, There are between 400 and 500 commercially produced Coldharbour Lane, Bristol BS16 1QY, UK essential oils (Tisserand and Young 2013) and one EO with 2 Manchester Metropolitan University, Manchester, UK a growing reputation is Lemongrass EO of the Cymbopogon Appl Microbiol Biotechnol species. The antimicrobial effect of whole lemongrass EO has EO and component been shown in previous studies with a wide range of in vitro activity including effects against AMR pathogens (Doran The lemongrass EO (C. flexuosus) used in this study was et al. 2009; Warnke et al. 2009;Adukwuetal.2012). The donated by Amphora Aromatics, Bristol, UK, whilst citral strong antimicrobial activity of lemongrass has been attributed (95 %; synonym—3,7-dimethyl-2,6-octadienal, geranial and to a high citral content (Marongiu et al. 2006;Adukwuetal. neral mixture; CAS Number 5392–40-5) was purchased from 2012; Kumar et al. 2013; Kpoviessi et al. 2014). Both lemon- Sigma-Aldrich, Dorset, UK. The EO and citral were stored in grass EO and citral are generally regarded as safe (GRAS) for a cool dark place and the containers kept tightly closed in a dry use as flavouring substances and is also an approved com- and well-ventilated place according to the safety data pound for use as a food additive and for human consumption information. (Food and drug Administration 2015a; 2015b). Generally, cytotoxic activity of EOs and components on Susceptibility testing human cell lines have been studied with a larger proportion ofthesestudiesfocusingontheeffectsofteatreeoil The disk diffusion assay was used to determine antimicrobial (Söderberg et al. 1996; Lis-Balchin et al. 2000;Hammer susceptibility of the A. baumannii to a selection of antibiotics et al. 2006;Loughlinetal.2008; Nielsen 2008). Kpoviessi (Table 1) using the British Society for Antimicrobial et al. (2014) investigated the cytotoxic activity of lemongrass Chemotherapy guidelines, version 13 (2014) and to whole EO from four Cymbopogon species; C.citratus, C. giganteus, lemongrass EO and citral. Using the agar overlay assay, bac- C. nardus and C. schoenantus against a human non-cancer terial lawns were prepared with the inoculum size adjusted to diploid fibroblast cell line (W138) showing moderate toxicity approximately 1.5 × 108 CFU/ml. Ten microliters of lemon- of C. citratus against this W138 cell line. The cytotoxic effect grass EO and citral were deposited onto 6-mm filter paper of C. flexuosus, the EO in focus in our study and known to discs before placing them on the surface of Iso-sensitest agar possess antimicrobial activity, was not determined in the (Oxoid; Basingstoke, UK). The agar plates were then incubat- Kpoviessi study, and the effect of the oil on dermal fibroblast ed at 37 °C for 24 h and the diameter of the zone of inhibition is yet to be demonstrated to our knowledge. Citral on the other (ZOI) measured in millimetres using a Vernier calliper. Each hand has been reported to cause several adverse reactions such experiment was performed in triplicate. The controls were as sensitisation and allergic contact dermatitis (Tisserand and bacterial cultures without treatment. Balacs 1995; Heydorn et al. 2003). The focus of this study was to investigate the antimicrobial Minimum inhibitory concentration and minimum activity of C. flexuosus and citral against A. baumannii and to bactericidal concentration determine the cytotoxic activity of both C. flexuosus and citral on human fibroblasts which is important due to the growing The method used in this study for determination of inhibitory usage of EOs in household applications and cosmetics as well and bactericidal activity of both lemongrass EO and citral was as proposed usage in health care applications. similar to that use in Adukwu et al. (2012) with similar con- centration ranges 0.03, 0.06, 0.12, 0.5, 1, 2 and 4 % (v/v), and the only difference was the microplate reader. In this study, we Materials and methods used the Tecan Infinite 200 PRO, Reading, UK for optical density measurements. For minimum bactericidal concentra- Bacterial strains tion (MBC) determination, 10 μl was taken from each well (treated and untreated) after incubation and spot inoculated on Five A. baumannii strains from the University of the West of BHI agar and incubated for 24 h at 37 °C. The concentration at England, Bristol, UK, microbiology culture collection were which no growth was observed on subculture was determined used in this study. These were ATCC® BAA-1709™ (human as the MBC. isolate), ATCC® BAA-1710™ (human isolate), NCTC 12156 (ATCC 19606; type strain), ATCC 17978 (lung infection model; human isolate) and SM 37212, a clinical isolate ob- Cytotoxic activity tained from the Pathology department at Southmead Hospital, Bristol, UK. The strains were maintained on brain heart infu- Cell cultures sion (BHI) agar (CM1136; Oxoid Ltd, Basingstoke, UK) and sub-cultured on a weekly basis. For inoculum preparation, Primary cell cultures of human dermal fibroblasts (HDF; 106- single colonies were picked from a BHI agar plate into BHI 05a) were obtained from ECACC (European Collection of broth (CM1135; Oxoid Ltd., Basingstoke, UK) and incubated Cell Cultures, Porton Down, Sailsbury, UK). HDFs were cul- overnight at 37 °C. tured in DMEM (Sigma, Poole, UK, D6546) supplemented Appl Microbiol Biotechnol Table 1 Inhibition zones (mm) of A. baumannii strains after 24 h exposure to selected antibiotics following BSAC guidelines (version 13, June 2014) Antibiotic Disc content NCTC 12156 ATCC 17978 SM 37212 ATCC® ATCC® (μg) (ATCC 19606) BAA-1709™ BAA-1710™ Ciprofloxacin 1 18.30 (R) 17.6 (R) 0.00 (R) 30.10 (S) 0.00 (R) Gentamicin 10 20.70 (S) 19.6 (R) 7.42 (R) 31.70 (S) 7.10 (R) Meropenem 10 26.50 (S) 27.7 (R) 21.00 (R) 36.80 (S) 25.70 (S) Piperacillin/Tazobactam 75/10 24.50 (S) 25.00 (S) 18.90 (R) 86.00 (S) 20.00 (I) Criteria for defining MDR, XDR and PDR in Acinetobacter spp. (Magiorakos et al. 2012); MDR: non-susceptible to ≥1agentin≥3 antimicrobial categories; XDR: non-susceptible to ≥1 agent in all but ≤2 categories; PDR: non-susceptible to all antimicrobial agents listed (R)resistant,(S) sensitive, (I) intermediate with 10 % (v/v)FBS(Sigma)and1%(v/v) GIBCO® 490-nm readings. The data set was then blank corrected GlutaMAX™ (ThermoFisher Scientific, Paisley, UK).