Research Article In Vitro Antimicrobial and Modulatory Activity of the Natural Products Silymarin and Silibinin Dayanne Rakelly de Oliveira, 1 Saulo Relison Tintino, 2 Maria Flaviana Bezerra Morais Braga, 2 Aline Augusti Boligon, 3 Margareth Linde Athayde, 3 Henrique Douglas Melo Coutinho, 2 Irwin Rose Alencar de Menezes, 4 and Roselei Fachinetto 1,5 1 Programa de P´ os-Graduac ¸˜ ao em Ciˆ encias Biol´ ogicas, Bioqu´ ımica Toxicol´ ogica, Universidade Federal de Santa Maria, 97105-700 Santa Maria, RS, Brazil 2 Laborat´ orio de Microbiologia e Biologia Molecular, Universidade Regional do Cariri (URCA), 63100-000 Crato, CE, Brazil 3 Departamento de Farm´ acia Industrial, Universidade Federal de Santa Maria, 97105-700 Santa Maria, RS, Brazil 4 Laborat´ orio de Microbiologia e Biologia Molecular, Departamento de Qu´ ımica Biol´ ogica, Universidade Regional do Cariri, 63100-000 Crato,CE, Brazil 5 Programa de P´ os-Graduac ¸˜ aoo em Farmacologia, Universidade Federal de Santa Maria, 97105-700 Santa Maria, RS, Brazil Correspondence should be addressed to Henrique Douglas Melo Coutinho; hdmcoutinho@gmail.com Received 26 September 2014; Revised 12 February 2015; Accepted 15 February 2015 Academic Editor: Glen Jickling Copyright © 2015 Dayanne Rakelly de Oliveira et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Silymarin is a standardized extract from the dried seeds of the milk thistle (Silybum marianum L. Gaertn.) clinically used as an antihepatotoxic agent. Te aim of this study was to investigate the antibacterial and antifungal activity of silymarin and its major constituent (silibinin) against diferent microbial strains and their modulatory efect on drugs utilized in clinical practice. Silymarin demonstrated antimicrobial activity of little signifcance against the bacterial strains tested, with MIC (minimum inhibitory concentration) values of 512 g/mL. Meanwhile, silibinin showed signifcant activity against Escherichia coli with a MIC of 64 g/mL. Te results for the antifungal activity of silymarin and silibinin demonstrated a MIC of 1024 g/mL for all strains. Silymarin and silibinin appear to have promising potential, showing synergistic properties when combined with antibacterial drugs, which should prompt further studies along this line. 1. Introduction Microbial infections have become one of the principal problems of public health in the world, afecting all coun- tries, developing or developed. It can be related to the process of natural selection in bacterial development or the natural consequence of the adaptation of bacteria to exposure to antibiotics in the course of the indiscriminate use of antibiotics in humans and animals. Various cases related to resistance have been reported, including methicillin-resistant Staphylococcus aureus (MRSA) [1], penicillin-nonsusceptible Streptococcus pneumoniae (PNSSP), vancomycin-resistant enterococci (VRE), extended spectrum beta-lactamase- (ESBL-) producing Enterobacteriaceae, and Candida sp resistant to imidazoles. Staphylococcus aureus is a common cause of cutaneous and sof tissue infections, as well as invasive illness, such as bacteremia, septic arthritis, osteomyelitis, and necrotizing pneumonia [2, 3]. Escherichia coli is a Gram-negative bacillus that causes infections, especially neonatal, such as meningitis and septicemia, and even diarrheal diseases, in the whole world, particularly afecting children up to 5 years old. E. coli is typical of the intestinal fora and commensal of the vaginal fora [4, 5]. Also, Pseudomonas aeruginosa has ofen Hindawi Publishing Corporation BioMed Research International Volume 2015, Article ID 292797, 7 pages http://dx.doi.org/10.1155/2015/292797