# Biofilms: Novel Strategies Based on Antimicrobial Peptides
## 3 Antimicrobial Peptides as Anti-Biofilm Agents  
Antimicrobial peptides [10; 11; 12; 13] are a promising class of compounds, widely distributed in nature, mainly as a part of innate immunity of organisms and are currently receiving considerable attention as a potential alternative to conventional antibiotics to prevent and treat bacterial infections [3]. They have a broad spectrum activity against many microbes ranging from viruses to parasites, and are also less susceptible to evolved antibacterial resistance because of their fast mechanism of action, rapid protease digestion with low bioavailability, and peptide aggregation [14]. Moreover, they exhibit a low probability to develop antimicrobial resistance likely thanks to their membrane-associated activity. Many AMPs are positively charged, amphiphilic molecules that kill bacteria through membrane disruption and/or pore formation, but also other modes of action are envisioned, such as penetration into the bacterial cytoplasm and inhibition of proteins, cell wall, or enzymes synthesis [15]. Many studies also showed that AMPs are effective against multi-drug resistant bacteria and fungi not only because they act disrupting membranes due to the electrostatic negatively charged bacterial surfaces, but also interfering with metabolic processes such as inhibition of cell wall, nucleic acid or protein biosynthesis rendering more difficult for microorganism to develop resistance. However, positive charges although being essential for antimicrobial action also represent an impediment to AMPs penetration into biofilms [16]. The main reason lies in the fact that polysaccharide (EPS) of the biofilm matrix is negatively charged and can trap positive AMPs preventing them from displaying their action against bacteria. Moreover, many AMPs are subject to hydrolytic and proteolytic breakdown. Several studies have described AMPs and how to modify their sequence in order to obtain novel antibiofilm agents with increased activity and proteolytic stability. Among them, LL-37 has shown an activity against _S. aureus_ bacteria as well as in preventing action against biofilm formation. However, despite the high potency of this peptide, the use of LL-37 is limited because of its poor stability; indeed, it is easily cleaved by endogenous enzymes highly present in the gut (trypsin and pepsin), pancreas (elastace) and serum (plasmin). Further, also enzymes secreted by microorganisms including _S. aureus_ (aureolysin, V8 protease) have shown the ability to cleave LL-37. Another disadvantage is associated with non-specific cytotoxicity (e.g., leukocyte, T-cell, and red blood cell viability). Shorter derivatives of LL-37 can overcome some of these limitations by improving stability, reducing toxicities, and lowering production costs [17; 18; 19; 20; 21; 22; 23]. [PERSON] et al. [24] identified the short cationic peptide 1037 with a weak antimicrobial activity on Gram-positive and Gram-negative bacteria but with a good anti-biofilm activity with death of cells. Moreover, they showed a decreased expression of a variety of genes involved in biofilm formation for _P. aeruginosa_. In a recent study, [PERSON] at al. demonstrated that a membrane-penetrating peptide gH625 and its analogue gH625-GCGKKKKK were effective both on planktonic cells and _C. tropicalis/Serratia marcenscens_ and _C. tropicalis/S. aureus_ polymicrobial biofilms [25]. They showed an eradication activity stronger for the modified analogue, also at concentration lower than their minimal inhibitory concentration (MIC) on planktonic cells. Many AMPs and peptido-mimetics present an action against microbial biofilms. They act in the first phase of biofilm formation and display a preventive action, or disperse existing ones, reduce biomass or kill microbial cells within biofilms as schematized in Figure 1. Some peptides perform different antimicrobial actions according to the concentration; for instance, at certain concentrations they are able to limit biofilm formation without affecting planktonic growth of the bacteria [26].