# Action of Antimicrobial Peptides against Bacterial Biofilms
## 2 Antimicrobial Peptides  
Antimicrobial peptides (AMPs) have a varying number (from five to over a hundred) of amino acids, most commonly L forms, with molecular weights between 1-5 KDa. AMPs have a broad spectrum of activity ranging from viruses to parasites [45]. AMPs are generally cationic in nature, and often referred as cationic host defense peptides because of their role in the immune response [46]. They are also known as cationic amphipatible peptides [47], cationic AMPs [48], and \(\alpha\)-helical AMPs [49]. Recently, a few anionic antimicrobial peptides have been reported which have a net charge ranging from \(-1\) to \(-7\), and a length from 5 to circa 7 amino acid residues [50]. AMPs have been recognized as promising alternatives to conventional antibiotics due to their multiple target sites and non-specific mechanism of action which reduces the chances of resistance development. AMPs exhibit strong anti-biofilm activity against multidrug resistant as well as clinically isolated bacterial biofilms [51]. AMPs can interfere in the early stages of biofilm formation to prevent the initial adhesion of bacteria to surfaces [51]. They can destroy mature biofilms by encouraging microbial detachment or killing [52]. Here we focus on the anti-biofilm action of AMPs against different Gram-positive and Gram-negative bacteria, with emphasis on their mechanism of action.  
Based on their secondary structure, AMPs are generally categorized into four groups (1) \(\alpha\)-helical AMPs; (2) \(\beta\)-sheet AMPs; (3) extended AMPs; and (4) cationic loop AMPs [53]. Alpha-helical peptides are the largest group of AMPs representing 30-50% of all AMPs of known secondary structure [54; 55; 56]. These peptides commonly consist of 12-40 amino acids and contain an abundance of helix stabilizing amino acids such as alanine, leucine, and lysine [56]. Beta-sheet AMPs usually consist of two to ten cysteine residues that from one to five inter-chain disulfide bonds that help the peptides to form the beta-sheet [57]. Beta-sheet antimicrobial peptides include the defensin family of peptides [58; 59]. Defensins consist of two to three antiparallel beta-sheets however, in some cases alpha-helical or unstructured segments can be found at their N- or C-termini [60]. Compared with \(\alpha\)-helical antimicrobial peptides, the defensins adopt a globular structure in aqueous solutions [60; 61]. Despite extensive variations in length, amino acid composition and net positive charge, \(\beta\)-strands are observed in all \(\alpha\)- and \(\beta\)-defensins [62; 63]. Extended AMPs are not folded into \(\alpha\)-helix or \(\beta\)-sheet structures. These AMPs often contain high numbers of arginine, tryptophan, proline or cystine residues [64]. Some of these AMPs can fold into defined amphipatible molecules in bacterial membranes, but often these are not membrane active [65]. The proline-rich insect-derived pyrrhocoricin, drosocin and apidaecin peptides penetrate membranes and exert their antimicrobial activities by interacting with intracellular proteins such as the heat-shock protein DnaK and GroEL to inhibit the DnaK ATPase and chaperone-assisted protein folding related activities, respectively [66; 67]. Cationic loop AMPs are proline-arginine rich peptides, and because of their high numbers of proline residues, they rarely form amphipathic characteristics and tend to adopt polyproline helical type-II structures [68].