In Vitro Synergistic Activities of Essential Oils and Surfactants in Combination with Cosmetic Preservatives Against Pseudomonas aeruginosa and Staphylococcus aureus Vania Patrone • Raffaella Campana • Emanuela Vittoria • Wally Baffone Received: 24 September 2009 / Accepted: 22 October 2009 / Published online: 17 November 2009 Ó Springer Science+Business Media, LLC 2009 Abstract The aim of this study is to evaluate possible synergistic antimicrobial interactions between common cosmetic preservatives and selected essential oils or sur- factants. The antimicrobial efficacy of six essential oils, three surfactants and five preservatives against Pseudo- monas aeruginosa ATCC 9027 and Staphylococcus aureus ATCC 43387 was assessed by a broth micro-dilution assay. MICs for individual and combined antimicrobials were determined and then transformed to fractional inhibitory concentration (FIC) indexes. All essential oils exhibited antibacterial activity; among surfactants, bacteria resulted most susceptible to the cationic agent. Synergy was observed when essential oils of eucalyptus and mint were combined with methylparaben against P. aeruginosa, while essential oils of mint, oregano and sage combined with propylparaben and imidazolidinyl urea acted against S. aureus. Many binary mixtures of preservatives and sur- factants produced synergistic activity with the most effec- tive interactions involving the cationic and amphoteric compounds under study. FIC indexes demonstrated syner- gistic effects when preservatives were combined with either essential oils or surfactants against both bacterial strains. These results highlight the potential usefulness of essential oils and surfactants to enhance the activities of conventional biocides. This kind of study should contribute to the selection and optimization of preservative systems for cosmetic preparations. Introduction Antimicrobial preservation systems are included in cos- metics and toiletries to prevent and control the growth of microorganisms from contamination during manufacture, storage or consumer use. Microbial contaminants of con- cern include pathogens and opportunistic pathogens, but also non-pathogenic microorganisms which can alter the chemical or physical properties of a product resulting in phase separation, discoloration, formation of gas or off- odours, change in the pH of the formulation. Preservative systems usually contain various combinations and amounts of chemical biocides in order to provide a broad spectrum of activity. Commonly used preservatives include the parabens, imidazolidinyl urea, quaterium-15 (Q-15) form- aldyhyde solution, isothiazolinones. Most of preservatives offer a high antimicrobial efficacy, but many have the potential to cause adverse reactions on sensitive skin [13]. Moreover, the continuous emergence of bacterial strains resistant to conventional preservatives has become a major problem in recent years [14] and has prompted a search for alternative preservative systems. Much attention has been focused on exploiting the antimicrobial potential of mul- tifunctional ingredients which are not added to formula- tions primarily for their antimicrobial activity but which may contribute to product preservation [7, 17]. Essential oils, mostly used in cosmetics as fragrances, have been demonstrated to possess antimicrobial properties [5, 8, 16] and have been proposed as natural preservation agents [11]. Surfactants are widely used in personal care products as emulsifiers, solubilizers, wetting and cleaning agents, foam producers and conditioning aids. Among them, cationic surfactants such as quaternary ammonium compounds are known to exert excellent antimicrobial activity against both Gram-positive and Gram-negative bacteria, and are the V. Patrone Á R. Campana Á E. Vittoria Á W. Baffone (&) Department of Biomolecular Science, Division of Toxicological, Hygienic and Environmental Science, University of Urbino ‘‘Carlo Bo’’, Via S. Chiara 27, 61029 Urbino, Italy e-mail: wally.baffone@uniurb.it 123 Curr Microbiol (2010) 60:237–241 DOI 10.1007/s00284-009-9531-7