Inhibition of biofilm formation in Gram-positive bacteria by a natural antimicrobial agent

Artificial surfaces of implanted medical devices can be often colonized by bacteria and fungi. Once adhered, microrganisms grow forming biofilms and the local or systemic related infections are highly resistant to antibiotics. Among all the strategies to prevent these infections, the most promising are those based on device’s coating with antimicrobials that eluting in the surrounding microenvironment inhibit microbial growth and colonization. Looking for new antimicrobial substances able to adsorb on biomaterials, we focused our attention on usnic acid, a secondary lichen metabolite, already know to be active against planktonic forms a number of gram positive bacteria, including Staphylococcus aureus and Enterococcus faecalis. Thus, our research was addressed to evaluate usnic acid activity in the inhibition of biofilm formation by loading polyurethanes which are among the most employed polymers in medical device production. Previously, basic functional groups (amino groups) have been introduced on the surface of a polyurethane to establish electrostatic interactions with the acidic groups displayed by usnic acid. Then, round shaped discs of these so functionalized polymers were put into a flowcell, suitable for growing biofilms under a wide range of hydrodynamic conditions, and subsequently observed by confocal microscopy. Results showed that only polymers loaded with usnic acid inhibited S. aureus biofilm formation while the gram negative Pseudomonas aeruginosa, used as control, formed biofilm on both the untreated and usnic acid-loaded polymers. However, usnic acid was able to affect the morphology of P. aeruginosa biofilm indicating the possible interference of this drug with cell-cell communication pathways. These promising results seem to open new perspectives in the control of biofilm formation on medical devices.