**2. Mechanism of action**

AMPs are considered as promising antimicrobial agents due to which their mechanism of action has been explored widely. Characterization of AMPs is very crucial to enhance their utilization as therapeutic agents [6]. Antimicrobial peptides exert both bacteriostatic and bactericidal effects and they develop less resistance to microbes than conventional antibiotics [8].

These peptides are positively charged amphiphilic molecules possessing both hydrophilic and hydrophobic residues. Cationic peptides being positively charged interact with negatively charged cell membranes through electrostatic attraction then undergo membrane adsorption and conformational change. These peptides complete their activity after binding to the cell membrane through different mechanisms such as the barrel stave model, the carpet model, the toroidal pore model, etc. [4]. The mechanisms of action of AMPs differ from antibiotics. There are several hypothetical mechanisms of action of these peptides, including the plasma membrane disruption, intracellular antimicrobial mechanism, the inhibition of the synthesis of macromolecules such as protein, nucleic acids, and enzyme activity, and antimicrobial effect via participating in immune regulatory effects [9, 10].

AMPs are divided into four major types based on their secondary structure including linear α-helical peptides, β-sheet peptides, linear extended peptides, and both α-helix and β-sheet peptides [11]. According to extensive research on members of all four groups of AMPs, the permeabilization of microbial cytoplasmic membranes appears to be the main mechanism for most AMPs to kill cells [2]. The helical peptides damage membranes through the carpet or barrel-stave pore model. Their main function is to introduce amphipathic helices into bacterial cell membranes, which disrupts the structure of the membranes [12]. The β-sheet peptides can act in a variety of ways, including prevention of cell wall formation and binding to particular lipid components in membranes [2]. They translocate across lipid bilayers which are associated with the development of temporary pores. The αβ family contains both α and β structures. Elongated AMPs are linear and rich in one or more amino acids, like glycine, tryptophan, arginine, and histidine. The members of this category have a flexible structure in the aqueous environment that allows them to convert into an amphipathic structure when they come into contact with a membrane. They do not act directly on pathogen membranes, but rather permeate them and interact with cytoplasmic proteins.

The MOA of AMPs can be broadly classified into two categories –first is direct killing and second is immunological regulation as shown in **Figure 1**. The direct killing MOA is further classified into two classes – membrane targeting and nonmembrane targeting. Membrane permeabilizing peptides are capable to create transient pores on the membrane, mostly recognized by cationic peptides, such as defensin, LL37, melittin. Non-membrane targeting peptides can pass through the cell membrane and interfere with crucial cellular processes that ultimately lead to the death of cells without permeabilizing the membrane such as pleurocidin, pyrrhocidin, and mersacidin [9].

*Antimicrobial Peptides: Mechanism of Action DOI: http://dx.doi.org/10.5772/intechopen.99190*

**Figure 1.** *Mechanism of action of antimicrobial peptides.*
