Ethylhexylglycerin Impairs Membrane Integrity and Enhances the Lethal Effect of Phenoxyethanol

RESEARCH ARTICLE Ethylhexylglycerin Impairs Membrane Integrity and Enhances the Lethal Effect of Phenoxyethanol Solveig Langsrud1*, Katrin Steinhauer2, Sonja LuÈ thje2, Klaus Weber2, Peter Goroncy- Bermes2, Askild L. Holck1 1 NofimaÐNorwegian Institute of Food, Fisheries and Aquaculture Research, Osloveien 1, N-1430 Aas, Norway, 2 SchuÈlke & Mayr GmbH, Research & Development, Robert-Koch Str. 2, 22851 Norderstedt, Germany a11111 * [email protected] Abstract Preservatives are added to cosmetics to protect the consumers from infections and prevent OPEN ACCESS product spoilage. The concentration of preservatives should be kept as low as possible and Citation: Langsrud S, Steinhauer K, LuÈthje S, this can be achieved by adding potentiating agents. The aim of the study was to investigate Weber K, Goroncy-Bermes P, Holck AL (2016) the mechanisms behind potentiation of the bactericidal effect of a commonly used preser- Ethylhexylglycerin Impairs Membrane Integrity and Enhances the Lethal Effect of Phenoxyethanol. vative, 2-phenoxyethanol (PE), by the potentiating agent ethylhexylglycerin (EHG). Sub- PLoS ONE 11(10): e0165228. doi:10.1371/journal. lethal concentrations of EHG (0.075%) and PE (0.675%) in combination led to rapid killing pone.0165228 of E. coli (> 5 log reduction of cfu after 30 min), leakage of cellular constituents, disruption Editor: Paul McNeil, Augusta University Medical of the energy metabolism, morphological deformities of cells and condensation of DNA. College of Georgia, UNITED STATES Used alone, EHG disrupted the membrane integrity even at low concentrations. In conclu- Received: April 13, 2016 sion, sub-lethal concentrations of EHG potentiate the effect of PE through damage of the Accepted: October 7, 2016 cell membrane integrity. Thus, adding EHG to PE in a 1:9 ratio has a similar effect on membrane damage and bacterial viability as doubling the concentration of PE. This study pro- Published: October 26, 2016 vides insight about the mechanism of action of a strong potentiating agent, EHG, which is Copyright: © 2016 Langsrud et al. This is an open commonly used in cosmetics together with PE. access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are Introduction within the paper and in its supporting information files. Survival and growth of microorganisms in cosmetics may lead to product degradation and Funding: This study was funded by SchuÈlke & infection of the end-user [1, 2]. Thus, preservatives, such as 2-phenoxyethanol (PE), organic Mayr GmbH, Norderstedt, Germany and acids isothiazolinones and parabens are used to maintain the function and ensure that the Foundation for Research Levy on Agricultural products are safe to use [3]. However, adverse effects have been described for some preserva- Products (FRLAP, Grant no. 224921/F40). FRLAP tives used in various formulations including cosmetics. For example, Lundov et al. [4] reported had no role in study design, data collection and an increase in prevalence of contact allergy to methylisothiazolinone within the last years. It is analysis, decision to publish, or preparation of the manuscript. SchuÈlke & Mayr, through the co- therefore desirable to keep the concentrations of preservatives as low as possible to reduce authors participated in planning, interpretation of exposure of consumers while at the same time maintaining antimicrobial safety of cosmetics. results and the decision to publish. This can be achieved for example by using combinations of preservatives acting synergistically PLOS ONE | DOI:10.1371/journal.pone.0165228 October 26, 2016 1 / 16 Membrane Effects of Ethylhexylglycerin Competing Interests: The authors KS, SLuÈ, KW, or by adding potentiating agents. The latter are agents which themselves have little lethal or and PG-B are employees of SchuÈlke & Mayr GmbH inhibitory effects at the concentrations applied, but enhance the efficiency of other preserva- and participated in planning, interpretation of tives. For example, the chelator EDTA can potentiate the effect of a range of preservatives, such results and the decision to publication. This does not alter our adherence to PLOS ONE policies on as quaternary ammonium compounds, parabens and phenolics and is therefore often used in sharing data and materials. cosmetic formulations [3]. 2-Phenoxyethanol is a widely used and well studied compound used for antimicrobial protection of cosmetics [5, 6]. Data from 2010 showed that about 25% of 36 800 cosmetic products sold in USA and 33% of 204 different products in Sweden contained this preservative [5, 7]. According to the cosmetic directive in EU, concentrations of PE up to 1% is allowed for preservative purposes in cosmetics. To obtain sufficient antimicrobial effect it is often combined with other antibacterial agents [8–10]. The mode of action of PE has been investigated extensively, initially by W. Hugo and H. E. Street in the 1950s followed up by P. Gilbert et al. in the 1970s [11–17] and was never completely elucidated, but the research revealed that several mechanisms were involved in inhibitory and bactericidal activity. The lethal action of PE on E. coli was associated with gross membrane damage resulting in leakage of cytoplasmic constituents. It was pointed out that membrane leakage alone could not explain the lethal activity. Experi- ments at sub-lethal concentrations showed that PE prevented cell growth by inhibiting DNA and RNA biosynthesis [12] as well as the energy metabolism through inhibition of malate dehydrogenase [15] and disruption of the proton gradient [14]. Together the studies showed that PE acted on several targets in the cell depending on the concentrations and most likely cell death was a result of a combination of these mechanisms leading to non-reversible injuries to the cell. Several investigations have been performed to find synergistic effects between PE and other preservatives. Fitzgerald et al. demonstrated a synergistic effect of PE and chlorhexidine resulting in increased cell death and leakage of potassium and pentose of cells exposed to a combination of the agents compared to each agent separately [18]. Increased leakage could partly explain the synergistic effect. Combining PE with either diazolidinyl urea or methylchloroi- sothiazolinone/methylisothiazolinone resulted in preservation of cosmetic cream at concentrations below the preservatives’ MIC-values and 10 to 20 times below maximum permitted concentrations. The mechanisms behind the interaction effects were however not studied [19]. PE is often combined with the antifungal preservative chlorphenesin [9]. It has been reported that although PE alone is not skin irritant, even at high concentrations [20], a combination with chlorphenesin results in a synergistic sensory skin irritation [9]. An alternative to building protection against microbial growth by combining different preservatives is to use potentiating agents. There are several advantages of combining preservatives with potentiating agents, for example wider spectrum of activity, enhanced activity at lower concentrations and reduced likelihood of allergic reactions or irritation of the skin for the end user. Ethylhexylglycerin (EHG, 3-[2-(Ethylhexyl)oxyl]-1,2-propandiol), a 1-alkyl glyceryl ether, is a multifunctional additive for cosmetics and is used as a potentiating agent in combination with PE to obtain better protection against microbial growth. A synergistic effect between PE and EHG on the viability of a range of different microorganisms has been reported, both in laboratory tests and in cosmetic products [21]. It has been suggested that the potentiation effect is due to the surfactant properties of EHG, affecting the surface tension properties of bacteria improving the contact between PE and the membrane [21]. Thus, the effect of combination of PE and EHG is well established regarding antimicrobial protection of e.g. cosmetics, but the mechanism of action behind the synergistic effect is not known. A few reports on contact allergy to EHG have been published ([22] and references therein). The aim of the study was to investigate how ethylhexylglycerin (EHG) potentiates the bactericidal effect of the commonly used preservative 2-phenoxyethanol (PE). In conclusion, EHG PLOS ONE | DOI:10.1371/journal.pone.0165228 October 26, 2016 2 / 16 Membrane Effects of Ethylhexylglycerin damages the cell membrane integrity and together with PE a synergistic action leads to rapid killing of E. coli associated with leakage of cellular constituents, disruption of the energy metabolism, morphological deformities of cells and condensation of DNA. Materials and Methods The experiments were repeated three times on different days and with freshly prepared cultures and solutions if not stated otherwise. Cultivation and preparation of cells E. coli ATCC 11229 was inoculated (one colony from Tryptone Soy Agar) in Tryptone Soya Broth (TSB) and incubated over night at 30°C with shaking. Stationary phase cultures were cen- trifuged (13 000g, 3 min) and the pellets resuspended in sodium phosphate buffer (0.1 mol l-1, pH 7.5) if not stated otherwise. Antibacterial agents The antibacterial agents tested were ethylhexylglycerin (EHG, Sensiva SC50), phenoxyethanol (PE) and a mixture of PE (90%) and EHG (10%) (EUX, commercial name Euxyl PE 9010). All agents were

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