21st European Congress of Clinical Microbiology and Infectious Diseases

Secondary metabolites from lichens: drug interaction of usnic acid, lobaric acid and protolichesterinic acid with antibiotics against MRSA clinical isolates

Abstract number: P1062

Celenza G., Segatore B., Setacci D., Bellio P., Brisdelli F., Petitto V., Perilli M., Nicoletti M., Amicosante G.

Objectives: The objectives of this work were:

•to characterise the potential antibacterial activity of usnic acid, protolichesterinic acid and lobaric acid secondary metabolites from lichens against methicillin-resistant Staphylococcus aureus clinical isolates;

•to investigate the interaction of the bioactive compounds from lichens in combination with the antibiotics which susceptibility against MRSA is reduced.

Methods: MICs of secondary metabolites from lichens were characterised by microdiluton test in Mueller-Hinton broth, where bacteria were grown at 37°C 18 hours. MBCs were calculated by plating 100 uL of bacterial colture on MH agar medium from the determined MIC value of each compounds up to eigthfold the MIC value. The compounds were tested in combination with erythromycin, clindamycin, gentamicin, levofloxacin and oxacillin. All tested clinical isolates showed resistance to those antibacterial agents. Drug interactions were assessed by broth microdilution assays. The growth of each well was quantified by microplate reader at 600 nm. FICI was calculated. Synergy was defined as FICI  0.5, antagonism as FICI  4, no-interaction for values of FICI between 0.5 and 4.

Results: MIC performed on 10 clinical isolates of MRSA showed a value ranging from 2–4 mg/mL for usnic acid, 4–8 mg/mL for protolichesterinc acid, 16–32 mg/mL for lobaric acid. MBC for each compounds was ranging from the MIC value to twofold the MIC value calculated for each isolate. The checkerboard assay showed synergy between each natural compound in combination with LVX and GEN (FIC  0.5), while for ERY, CLI and OXA no-interactions were observed (FICI between 0.5 and 4). No antagonism has been observed. Beside the antibacterial activity the cytotoxic effects of the secondary metabolites were evaluated in human tumor celle lines (MCF-7, human breast adenocarcinoma; HCT-116, human colon carcinoma; HeLa, human cervix carcinoma. Usnic acid and protolichesterinic acid inhibited significantly the growth of HeLa and HCT-116 cells.

Conclusion: Although the antimicrobial activity of usnic acid against Gram-positive bacteria is known, the interaction between secondary metabolites from lichens in combination with antibiotics has been investigated for the first time in this study. This would pave the way for the introduction of new classes of molecules in clinical practice from natural sources. The potential biological activity of those compounds might be extended to eukaryotic cells.