Please cite this article in press as: Aguiar JJS, et al. Antibacterial and modifying-antibiotic activities of the essential oils of Ocimum gratissimum L. and Plectranthus amboinicus L. Eur J Integr Med (2014), http://dx.doi.org/10.1016/j.eujim.2014.10.005 ARTICLE IN PRESS +Model EUJIM-367; No. of Pages 6 Available online at www.sciencedirect.com ScienceDirect European Journal of Integrative Medicine xxx (2014) xxx.e1–xxx.e6 Original article Antibacterial and modifying-antibiotic activities of the essential oils of Ocimum gratissimum L. and Plectranthus amboinicus L. José J.S. Aguiar, Cicera P.B. Sousa, Mariana K.A. Araruna, Maria K.N. Silva, Aline C. Portelo, Jeferson C. Lopes, Victória R.A. Carvalho, Fernando G. Figueredo, Vanessa C.N. Bitu, Henrique D.M. Coutinho ∗ , Thiago Adolfo Sobreira Miranda, Edinardo F.F. Matias Faculdade Leão Sampaio Unidade Saúde, Juazeiro do Norte, Ceará, Brazil Received 12 August 2014; received in revised form 9 October 2014; accepted 13 October 2014 Abstract Introduction: Developing resistance to antimicrobial agents is increasingly observed for many microorganisms is increasingly becoming a problem worldwide. The aim of the present work was to evaluate the antibacterial and antibiotic-modifying activity of essential oils of Ocimum gratissimum and Plectranthus amboinicus (Lamiaceae), alone and combined. Methods: Standard and multiresistant bacterial strains of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa were utilized, and antibiotics of the aminoglycoside class were tested, using the microdilution technique. Results: The antibacterial effect of the O. gratissimum L. and P. amboinicus L. oils alone and combined have a minimum inhibitory concen- tration (MIC) ≥ 1024 g/mL, except against E. coli ATCC10536, which showed a MIC = 128 g/mL and against S. aureus ATCC25923 with MIC = 512 g/mL. The drug-modifying effect of the essential oil of O. gratissimum L. resulted in an antagonism, reducing the effect of antibiotics, against all bacterial strains assayed. However, the essential oil of P. amboinicus L. showed a synergistic effect, potentiating the antibiotic activity of these drugs against the bacterial strains assayed. When the mixture of the O. gratissimum and P. amboinicus oils was combined with the antibiotic, a synergistic effect was observed. Conclusions: The data obtained are promising, but further studies are needed to isolate the active compounds and to conduct pharmacological tests in vivo, making it possible to develop new therapeutic alternatives for the treatment of diseases caused by multiresistant microorganisms. © 2014 Elsevier GmbH. All rights reserved. Keywords: Ocimum gratissimum; Plectranthus amboinicus; Antibiotic-modifying activity; Antibacterial; Essential oils Introduction Bacteria are prokaryotic microorganisms that reproduce by binary fission and characterized by the presence of filamentous double-strand circular DNA. They are differentiated by their cell wall, which is used to classify whether they are Gram-positive or Gram-negative [1]. These microscopic organisms are usually found on the surface of the skin, mucosae and intestinal tract of ∗ Corresponding author at: Laboratório de Microbiologia e Biologia Molecu- lar, Av. Cel Antonio Luiz, 1161, CEP: 63105-000, Crato, CE, Brazil. Tel.: +55 88831021212; fax: +55 8831021291. E-mail address: hdmcoutinho@gmail.com (H.D.M. Coutinho). humans and animals. The genus Staphylococcus is commonly distributed in nature and occurs in the normal microbiota [2]. The bacteria are usually found as single cells, doublets, or chains and are classified as important human pathogens, causing diseases with a fatal prognosis [3]. The bacterial species Escherichia coli is classified as a microorganism capable of causing severe infections and is asso- ciated with a variety of diseases, included sepsis, urinary tract infection (UTI), meningitis and gastroenteritis. On the other hand, Pseudomonas aeruginosa is a Gram-negative bacillus with polar flagella, which confer motility, where it causes vari- ous cutaneous infections, in cases where there are severe burns and also for example, in endocarditis in immunocompromised patients [1]. http://dx.doi.org/10.1016/j.eujim.2014.10.005 1876-3820/© 2014 Elsevier GmbH. All rights reserved.