Phytomedicine 19 (2012) 341–347 Contents lists available at SciVerse ScienceDirect Phytomedicine j ourna l ho mepage: www.elsevier.de/phymed In vitro interaction of usnic acid in combination with antimicrobial agents against methicillin-resistant Staphylococcus aureus clinical isolates determined by FICI and E model methods Bernardetta Segatore a,1 , Pierangelo Bellio a,1 , Domenico Setacci a , Fabrizia Brisdelli a , Marisa Piovano b , Juan A. Garbarino b , Marcello Nicoletti c , Gianfranco Amicosante a , Mariagrazia Perilli a , Giuseppe Celenza a,∗ a Department of Biomedical Sciences and Technologies, University of l’Aquila, L’Aquila, Italy b University F. Santa Maria, Valparaiso, Chile c Department of Environmental Ecology, University Sapienza, Rome, Italy a r t i c l e i n f o Keywords: Usnic acid MRSA Checkerboard assay Antimicrobial activity a b s t r a c t The in vitro antimicrobial activities of usnic acid were evaluated in combination with five therapeuti- cally available antibiotics, using checkerboard microdilution assay against methicillin-resistant clinical isolates strains of Staphylococcus aureus. MIC 90 , MIC 50 , as well as MBC 90 and MBC 50 , were evaluated. A synergistic action was observed in combination with gentamicin, while antagonism was observed with levofloxacin. The combination with erythromycin showed indifference, while variability was observed for clindamycin and oxacillin. Data from checkerboard assay were analysed and interpreted using the fractional inhibitory concentration index (FICI) and the response surface approach using the E model. Discrepancies were found between both methods for some combinations. These could mainly be explained by the failure of FIC approach, being too much subjective and sensitive to experimental errors. These findings, beside confirm the well known antimicrobial activity of usnic acid, suggest, however, that this substance might be a good candidate for the individuation of novel templates for the development of new antimicrobial agents or combinations of drugs for chemotherapy. © 2011 Elsevier GmbH. All rights reserved. Introduction Antimicrobial resistance has emerged among pathogenic bac- teria since the beginning of the antibiotic era, as a consequence of the selective pressure generated by the therapeutic use, abuse and sometimes misuse, of antibiotics for humans and animals. Nowadays, bacteria expressing phenotype Multidrug Resistance (MDR) are among the most important cause of infections in noso- comial and community settings (Johnson 2011; Ippolito et al. 2010; Navarro et al. 2008) and new drugs are urgently needed. Despite most of Staphylococcus aureus were susceptible to peni- cillin, in 1944 Kirby described the production of penicillinase in staphylococci (Kirby 1944). One year later the introduction of methicillin in clinical therapy, the first methicillin resistant S. aureus (MRSA) was described (Jevons et al. 1963). As a result of its intrinsic ability to overcome antibiotic chemotherapy, S. aureus ∗ Corresponding author at: Department of Biomedical Sciences and Technologies, University of l’Aquila, Via Vetoio, 1, 67100 l’Aquila, Italy. Tel.: +39 0862433444. E-mail address: celenza@univaq.it (G. Celenza). 1 B. Segatore and P. Bellio contributed equally to this work. continuously expands its ecological niche. It is resistant to many adverse environmental conditions, so that MRSA strains are mainly associated with hospital acquired infections (HA-MRSA). In 2002, fully vancomycin-resistant strains of S. aureus (VRSA) due to acqui- sition of vanA gene from vancomycin-resistant enterococci were reported in USA (CDCP 2002). The rate of mortality of septicemia caused by VISA rised from 30% for MRSA, to almost 80%. Thus, the emergence of resistant S. aureus bacteria has serious consequences both in terms of therapeutic failures and impact on Health Care System. To meet the growing challenge of S. aureus, the identification of novel targets for small molecules is one of the most important approach to face the problem (García-Lara et al. 2005; Gibbons 2008; Silver 2011; Wright and Sutherland 2007). To overcome antibiotic-mediated resistance, a valuable alter- native would be the use of combination of drugs. Thus, substances that can increase susceptibility to currently licensed agents would be a very attractive and valuable option. Several studies have demonstrated that a number of natural products, which failed as antimicrobials, are able to dramati- cally increase the effectiveness of chemotherapeutic agents against Gram-negative bacteria (Gibbons and Udo 2000; Hemaiswarya 0944-7113/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.phymed.2011.10.012