*2.2.2 β-Defensins*

Human β-Defensin (HBD) 2, 3 of this subfamily have anti-*Acinetobacter* effects. HBD-2 is primarily produced by the epithelial lining of the respiratory and urinary tracts, and engaging is more effective on MDR clinical isolates than non-MDR isolates [167]. Longer than most of the natural AMPs, HBD-3 combined helix and β structure [147]. Even though the anti-*Acinetobacter* bactericidal effect is inhibited by exposure to human serum, it can kill all MDR and non-MDR *A. baumannii* clinical isolates at 4 μg/ml during 1.5 h in the serum-free environment. Thus, this peptide has the potential to be further studied for wounds infected by *A. baumannii* since it demonstrated wound-healing effects [97, 168].

### *2.2.3 α-Helical and antiparallel β-sheet defensins*

CL-defensin, belonging to the family of insect defensins, is predicted to have a characteristic N-terminal loop, an α-helix, and an antiparallel β-sheet, which was supported by circular dichroism spectroscopy [95]. In addition, this peptide induces pore formation in other Gram-positive bacteria and causes a small amount of membrane permeabilization in *A. baumannii* [95].

#### **2.3 Frog antimicrobial peptides**

#### *2.3.1 Magainin and pexiganan (its analog)*

The Magainin-1 and 2 are cationic, α-helical, and amphipathic AMPs ionophores isolated from the skin of the African clawed frog (*Xenopus laevis*) [168, 169]. The primary mechanism of antimicrobial activity is probably pore formation in outer and inner membranes, although the exact mechanism of action is not yet precise [98, 170]. Despite both have anti-*Acinetobacter* training, Magainin-2 is much stronger and able to inhibit the growth of sensitive and MDR strains of *A. baumannii* at 4.9–64 μg/ml, while reported as 256 μg/ml for Magainin-1 [86, 98]. Magainin-2 has some advantages, such as anticancer effect, stability at physiological salt

concentrations, lack of hemolytic activity, and toxicity for mammalian cells [98]. Furthermore, Magainin-2 can inhibit and eliminate the biofilm of *A. baumannii* [98]. Pexiganan AMP or MSI-78 is a synthetic analog of Magainin-2 with a potent and broad spectrum of action [171, 172]; it kills bacteria by forming toroidal pores in their cell membranes [172, 173]. Several studies have been performed on anti-*Acinetobacter* activity due to its being highly active against *Acinetobacter*. Pexiganan can inhibit the growth of MDR and sensitive clinical isolates of A*. baumannii* at a concentration of 1–8 μg/ml [100, 101, 174]. Jáskiewicz et al. studied the antimicrobial activity of eight peptides on *A. baumannii* ATCC 19606 reference strains. Among these, CAMEL and pexiganan showed potent antimicrobial and anti-biofilm activity [102].

#### *2.3.2 Brevinin-2 related peptide (B2RP)*

B2RP is an α-helical AMP isolated from the skin secretions of the mink frog *Rana septentrionalis* [175] and carpenter frog Rana virgatipes [176]. This peptide forms an α-helical structure adjacent to the target cell, resulting in the perturbation of the phospholipid bilayer that may lead to growth inhibition of bacterial death, and the application of this peptide for systemic use is limited due to the moderate toxicity for human red blood cells [177]. B2RP inhibited the growth of a susceptible strain of *A. baumannii* at 29 μg/ml concentration but inhibited the MDR isolates more efficiently at 7–13.9 μg/ml [103]. The analogs of these peptides (D4K, K16A, L18K) resulted in twofolds higher anti-*A. baumannii* activity and much lower hemolytic activity [103]. A study reported that the analog of B2RP with D4K substitution inhibited sensitive and colistin-resistant [103] and XDR isolates of *A. baumannii* [105].

#### *2.3.3 B2RP-ERa*

B2RP-ERa is a cationic AMP from the Brevinin family isolated from the skin of the Asian frog *Hylarana erythraea* [106, 178]. Shorter and with lower molecular weight, B2RP-ERa is structurally similar to B2RP. B2RP-ERa is an antiinflammatory peptide with no toxic effect on peripheral blood mononuclear cells [179] with low hemolytic activity [178], which could inhibit the growth of sensitive and drug-resistant *Acinetobacter* strains at 8–32 and 8–64 μg/ml, respectively [104, 106].

#### *2.3.4 Alyteserins*

Alyteserins are a class of cationic AMPs, which firstly reported their presence in norepinephrine-stimulated skin secretions of the midwife toad [180]. However, initial studies show that Alyteserin-1c has more significant inhibitory activity against Gram-negative bacteria, while Alyteserin-2a is more active against Grampositive bacteria [180], the anti-*A. baumannii* effects of these Alyteserins have already been proven [107, 108]. Alyteserin-1c is a cationic α-helical AMP with low hemolytic activity on human red blood cells firstly isolated from *Alytes obstetricans* [107, 180, 181]. The MIC and MBC against clinical isolates of MDR *A. baumannii* have been reported as 11.3–22.6 μg/ml [107]. Substitution of E4K on this AMP reduced the hemolytic activity, and enhanced the antimicrobial and cationic activity [107]. The analog [E4K] inhibits the growth of colistin-sensitive, colistinresistant, and XDR *A. baumannii* isolates at concentrations of 4–16 μg/ml, 4–16 μg/ ml [104], and 8–64 μg/ml, respectively [105]. Alyteserin-2a is also a tiny α-helical AMP that displays relatively weak antimicrobial and hemolytic activities. Despite its anti-*A. baumannii* potential was not high mainly, some structural changes resulted in lower toxicity against human erythrocytes and higher bactericidal effect (4–8 folds) against MDR isolates with MIC of 6.8–13.6 μg/ml [108].
