Antimicrobial and antibiofilm activity of secondary metabolites of lichens against methicillin-resistant Staphylococcus aureus strains from cystic fibrosis patients.

Aim: Three secondary metabolites of lichens - usnic acid, atranorin and fumarprotocetraric acid - were evaluated for their in vitro antibacterial and antibiofilm activities against three strains each of methicillin-susceptible and methicillin-resistant Staphylococcus aureus (MRSA) from cystic fibrosis patients. Materials & methods: Antibacterial activity was assessed by broth microdilution, while antibiofilm activity was evaluated by spectrophotometry or viable count. Results: Usnic acid was significantly more active than atranorin against planktonic cells, while fumarprotocetraric acid exhibited no activity. Atranorin was the most effective in counteracting adhesion to polystyrene, although usnic acid was more active against MRSA. Usnic acid and atranorin showed comparable activity against biofilm formation, although atranorin was more active against MRSA. Usnic acid was significantly more active than atranorin against preformed biofilms. Conclusion: Secondary metabolites of lichens may be considered to be 'lead compounds' for the development of novel molecules for the treatment of S. aureus infections in cystic fibrosis patients.