In vitro interaction of usnic acid in combination with antimicrobial agents against methicillin-resistant Staphylococcus aureus clinical isolates determined by FICI and ΔE model methods

• Published in: Phytomedicine (Stuttgart) Vol. 19; no. 3-4; pp. 341 - 347
• Main Authors: Segatore, Bernardetta, Bellio, Pierangelo, Setacci, Domenico, Brisdelli, Fabrizia, Piovano, Marisa, Garbarino, Juan A., Nicoletti, Marcello, Amicosante, Gianfranco, Perilli, Mariagrazia, Celenza, Giuseppe
• Format: Journal Article
• Language: English
• Published: Germany Elsevier GmbH 15.02.2012; Urban & Fischer Verlag
• Subjects: Antagonism; Anti-Bacterial Agents - pharmacology; Anti-infective agents; Antibiotics; Antimicrobial activity; Antimicrobial agents; Benzofurans - pharmacology; Care and treatment; Checkerboard assay; Chemotherapy; Clindamycin; Clindamycin - pharmacology; Clinical isolates; Culture Media - chemistry; Data processing; Diagnosis; Dibenzofurans; Dosage and administration; Drug development; Drug resistance; Drug Resistance, Multiple, Bacterial; Drug Synergism; Erythromycin; Erythromycin - pharmacology; Gentamicin; Gentamicins - pharmacology; Health aspects; Levofloxacin; Methicillin resistance; Methicillin-Resistant Staphylococcus aureus - drug effects; Methicillin-Resistant Staphylococcus aureus - growth & development; Microbial Sensitivity Tests - methods; Models, Biological; MRSA; Ofloxacin - pharmacology; Oxacillin; Oxacillin - pharmacology; Staphylococcus aureus; Staphylococcus aureus infections; Usnic acid; Italy; Antimicrobial activity; Usnic acid; Checkerboard assay; MRSA
• ISSN: 0944-7113; 1618-095X
• DOI: 10.1016/j.phymed.2011.10.012

The in vitro antimicrobial activities of usnic acid were evaluated in combination with five therapeutically available antibiotics, using checkerboard microdilution assay against methicillin-resistant clinical isolates strains of Staphylococcus aureus. MIC90, MIC50, as well as MBC90 and MBC50, 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 ΔE 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.