# Action of Antimicrobial Peptides against Bacterial Biofilms
## 2 Antimicrobial Peptides
### Mechanism of Action against Biofilms  
In this review, we systemically analyzed all published data since 1970 to summarize all the possible anti-biofilm mechanisms of antimicrobial peptides. A total of 162 published reports were initially selected for this review following search criteria using 'antimicrobial peptide' OR 'peptide' AND'mechanism of action' AND 'biofilm' OR 'antibiofilm' in the databases PubMed, Scopus, Web of Science, MEDLINE, and Cochrane Library. The studies investigated the antimicrobial activity of AMPs against a variety of microorganisms. A total of 17 original reports qualified for our review which have articulated the mechanism of anti-biofilm action of AMPs. These reports are included in this review. Several overlapping anti-biofilm mechanisms of AMPs are reported in the literature. Following careful consideration, we found five major anti-biofilm mechanisms: (1) disruption or degradation of the membrane potential of biofilm embedded cells; (2) interruption of bacterial cell signaling systems; (3) degradation of the polysaccharide and biofilm matrix; (4) inhibition of the alarmone system to avoid the bacterial stringent response; (5) downregulation of genes responsible for biofilm formation and transportation of binding proteins. Certain synthetic AMPs can rapidly degrade pre-established biofilms of _P. aeruginosa_[52]. Although the mechanism of biofilm degradation is poorly understood, the rapid destruction of biofilm embedded cells [52] may indicate that they act by disrupting the membranes of the bacteria. Table 1 and Figure 1 summarize the mechanisms of biofilm inhibition and degradation of various AMPs. Mechanistic studies have tended to focus on the membrane-disrupting properties of AMPs [76; 77]. (i) disruption or degradation of the membrane potential of biofilm embedded cells  
Three bacteriocins (nisin A, lactic Q, and nukacin ISK-1) can destroy the membrane potential of biofilm embedded cells of _S. aureus_ (an MRSA strain) and can cause the release of ATP from the cells [78]. An engineered peptide RN3(5-17P22-36) [79] derived from the cationic proteins of eosinophil granules [80; 81] can kill bacteria via membrane disruption. However, this membrane depolarization of cells in biofilms was 2-3-fold less compared with planktonic bacteria at the same concentration [79]. A frog skin-derived AMP esculentin (Esc (1-21) can permeabilized the cytoplasmic membrane of _P. aeruginosa_ PAO1 in biofilms and cause release of \(\beta\)-galactosidasease [82]. However, this effect was slower and did not result in comparable \(\beta\)-galactosidase release compared to its action on planktonic cells [82]. The AMP (CSA)-13 can quickly penetrate into biofilms and permeabilized the cell membranes of biofilm cells of _P. aeruginosa_[83]. (ii) interruption of the bacterial cell signaling system  
Human cathelicidic LL-37 and indolicidin can prevent biofilm formation of _P. aeruginosa_ possibly by down-regulating the transcription of two major quorum-sensing systems, Las and Rhl [84]. Another mechanism by which AMPs have been shown to inhibit the formation of biofilms is by increasing twitching motility in _P. aeruginosa_ by stimulating the expression of genes needed for type IV pilli biosynthesis and function [84; 85]. The main function of type IV pilli is to increase the movement of bacteria on surfaces, which may facilitate removal of cells [86]. (iii) degradation of the polysaccharide and biofilm matrix  
AMPs can also act on the extracellular polymeric matrix of bacterial biofilms. For example, peptide PI can degrade the EPS produced by _Streptococcus mutans_ leading to reductions in biofilms formed on polystyrene or and saliva-coated hydroxyapatite [87]. An anti-biofilm peptide derived from maggots of the blowfly _Calliphora vicina_ can degrade the biofilm matrix produced by drug resistant Escherichia coli, Staphylococcus aureus and Acinetobacter baumannii_ but the mechanism of degradation was not investigated [88].