Usnic acid, a lichen secondary metabolite inhibits Group A Streptococcus biofilms

Group A Streptococci (GAS) are involved in a number of life threatening diseases and biofilm formation by these pathogens are considered as an important virulence determinant as it mediates antibiotic resistance among them. In the present study, we have explored the ability of (+)-usnic acid, a lichen secondary metabolite, as an antibiofilm agent against four serotypes of Streptococcus pyogenes causing pharyngitis. Usnic acid inhibited the biofilms of M serotypes M56, st38, M89 efficiently and the biofilm of M74 to a lesser extent. Confocal imaging of the treated samples showed that usnic acid reduced the biomass of the biofilms when compared to that of the control. Fourier Transfer Infrared (FT-IR) spectroscopy indicated that usnic acid reduced the cellular components (proteins and fatty acids) of the biofilms. Interestingly, the FT-IR spectrum further revealed that usnic acid probably acted upon the fatty acids of the biofilms as evident from the disappearance of a peak at 2,455–2,100 cm−1 when compared to the control only in serotypes M56, st38 and M89 but not in M74. The present study shows, for the first time, that usnic acid can act as an effective antibiofilm agent against GAS.