Hindawi Publishing Corporation Journal of Biomedicine and Biotechnology Volume 2012, Article ID 373682, 12 pages doi:10.1155/2012/373682 Research Article Investigation of Antimicrobial Activity and Statistical Optimization of Bacillus subtilis SPB1 Biosurfactant Production in Solid-State Fermentation Dhouha Ghribi, 1, 2 Lobna Abdelkefi-Mesrati, 2, 3 Ines Mnif, 1, 2 Radhouan Kammoun, 2 Imen Ayadi, 4 Imen Saadaoui, 3 Sameh Maktouf, 1 and Semia Chaabouni-Ellouze 1 1 Unit´ e Enzyme et Bioconversion, Ecole Nationale d’Ing´ enieurs de Sfax, BP W, 3038 Sfax, Tunisia 2 Institut Sup´ erieur de Biotechnologie de Sfax, BP 261, 3000 Sfax, Tunisia 3 Equipe des Biopesticides, Centre de Biotechnologie de Sfax, BP 1177, 3018 Sfax, Tunisia 4 Unit´ e de Bioinformatique, Centre de Biotechnologie de Sfax, BP 1177, 3018 Sfax, Tunisia Correspondence should be addressed to Dhouha Ghribi, dhouhag@yahoo.fr Received 13 November 2011; Accepted 20 December 2011 Academic Editor: Guihua H. Bai Copyright © 2012 Dhouha Ghribi et al. This 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. During the last years, several applications of biosurfactants with medical purposes have been reported. Biosurfactants are considered relevant molecules for applications in combating many diseases. However, their use is currently extremely limited due to their high cost in relation to that of chemical surfactants. Use of inexpensive substrates can drastically decrease its production cost. Here, twelve solid substrates were screened for the production of Bacillus subtilis SPB1 biosurfactant and the maximum yield was found with millet. A Plackett-Burman design was then used to evaluate the effects of five variables (temperature, moisture, initial pH, inoculum age, and inoculum size). Statistical analyses showed that temperature, inoculum age, and moisture content had significantly positive effect on SPB1 biosurfactant production. Their values were further optimized using a central composite design and a response surface methodology. The optimal conditions of temperature, inoculum age, and moisture content obtained under the conditions of study were 37 ◦ C, 14 h, and 88%, respectively. The evaluation of the antimicrobial activity of this compound was carried out against 11 bacteria and 8 fungi. The results demonstrated that this biosurfactant exhibited an important antimicrobial activity against microorganisms with multidrug-resistant profiles. Its activity was very effective against Staphylococcus aureus, Staphylococcus xylosus, Enterococcus faecalis, Klebsiella pneumonia, and so forth. 1. Introduction Solid-state fermentation which involves growth of microor- ganisms on moist solid substrates in the absence of free flow- ing water, recently, has received a considerable attention of researchers due to its several advantages over the submerged fermentation [1]. Solid substrates and wastes from different origins could be treated by solid-state fermentation and useful products could be produced. Solid-state fermentation had a long history of production of traditional foods by using different organisms and is also found to have an increasing application in the production of enzymes, antibi- otics, surfactants, biocides, and so forth [1]. It had been reported that in solid-state fermentation, especially in fungal one, the productions of metabolites, such as enzymes, antibi- otics, organic acids, and aroma compounds, are higher than that in submerged fermentation [2]. Bacillus subtilis strains produced a broad spectrum of bioactive compounds with great potential for biotechnologi- cal and biopharmaceutical applications including surfactin, fengycin, iturin, mycosubtilins, and bacillomycins, which are amphiphilic membrane-active biosurfactants with potent antimicrobial activities [3]. These biosurfactants possessed, also, antiviral [4], antitumor [5], hemolytic [6], blood anti- coagulant, and fibrinolytic [7] activities. Moreover, biosur- factants of Bacillus subtilis have numerous environmental and biotechnological applications [8] and have shown particular utility in oil recovery [9], remediation of soil