*2.1.5 Bovine cathelicidins (Indolicidin and Bactenecin)*

Indolicidin is a short tryptophan-rich cationic AMP encoded by a member of the cathelicidin gene family, isolated from cytoplasmic granules of the bovine neutrophils [158, 159]. Indolicidin acts by displacing divalent cations from their binding sites on the surface of the cell membrane and causes bacterial death through channel formation in the cytoplasmic membrane [88]. Indolicidin not only forms pores in the membrane but can also inhibit DNA processing enzymes [160, 161]. This peptide is among the potent anti-*A. baumannii* AMPs with MIC of 4–32 and 16 μg/ ml against sensitive and colistin-resistant clinical isolates, respectively [86]. In a study by Giacometti et al. were investigated the *in vitro* activity of indolicidin and other AMPs alone and in combination with antimicrobial agents, the MIC of indolicidin against 12 MDR clinical isolates was reported as 2–64 μg/ml [87]. Isolated from bovine, ovine, and caprine neutrophil granules, Bactenecin is a short cyclic, arginine-rich cationic AMP [89] with a type I β-turn structure and forms a loop due to the disulfide bond between cysteines 3 and 11 [90]. These AMPs act by permeabilizing the cell membrane and inhibiting protein and RNA synthesis in bacteria [70]. Vila-Farres et al. (2012) reported the anti-*A. baumannii* effect of this peptide can inhibit sensitive and colistin-resistant strains of *A. baumannii* at 16 and 64 μg/ml, respectively [86].

#### **2.2 Defensins**

Defensins are an evolutionarily ancient class of AMPs present in animals, plants, and fungi involved in the immune system of living organisms and

contain six (invertebrates) to eight conserved cysteine residues in their structure. They are categorized into three subfamilies of α, β, and θ-defensins [162]. Most defensins bind to the cell membrane and make pores, leading to bacterial death [163].
