Antimicrobial peptides: promising molecules for the treatment of biofilm-associated infections.

Introduction: Biofilms are complex bacterial communities living attached to biotic or abiotic surfaces and embedded in a self-produced extracellular matrix. It is now clear that biofilm formation may account for a large majority (up to 80%) of infections treated by physicians in the developed world and contributes significantly to morbidity and mortality especially in hospital settings. The treatment of biofilm infections is particularly challenging, as cells in this mode of growth are intrinsically refractory to antimicrobial drugs. The difficulty of successfully treating biofilm associated infections and the increasing resistance of microbes to traditional treatments call for the discovery of compounds with novel mechanisms of action. In this regard, over the last years, antimicrobial peptides (AMPs) have attracted considerable interest and are taken in increasingly consideration as new antibiofilm drugs. Materials and Methods: AMPs from humans or frog skin were tested for their ability to inhibit biofilm formation or to eradicate preformed biofilms. Antibiofilm activity was assessed by evaluating biofilm biomass (by crystal violet staining) biofilm architecture (by confocal microscopy) or by counting biofilm-associated viable cells. Synergy studies between AMPs and other antimicrobial compounds were also performed. Results: Different AMPs have shown ability to inhibit biofilm formation and/or eradicate preformed biofilm of clinical isolates or laboratory strains of Gram-positive and Gram-negative bacteria. Some peptides exerted their antibiofilm activity at concentrations equal or very close to their MIC values, indicating a classical bactericidal mechanism of antibiofilm action. In contrast, other peptides inhibited biofilm formation at sub-MIC values suggesting mechanisms of action specifically targeting the biofilm mode of growth. Encapsulation of a model peptide into chitosan nanoparticles demonstrated linear releasing kinetics of the loaded molecule and prolonged antibacterial activity in in vitro models. Discussion and Conclusions: In recent years, there has been increased interest in the use of AMPs as antibiofilm molecules. Nevertheless, optimization of AMP-antibiofilm activity is a research area still at an early stage and intensive discovery and design of new peptides is highly desirable. In this regards, a new database recently developed by a collaborative work from different research institutions in Pisa, and fully dedicated on AMPs tested on microbial biofilms, will represent a valid tool for peptide optimization and will be briefly presented.