*2.2.1 α-Defensins (HNPs and HD5)*

The subfamily of human neutrophil peptides (HNPs), also known as αdefensins, are secreted and released from polymorphonuclear neutrophil (PMN) granules upon activation and are conventionally involved in microbial killing [164]. Two important CAMPs HNP-1 and HNP-2, which differ in only one initial amino acid, can inhibit the growth of the standard strain of *A. baumannii* ATCC 19606 at a concentration of 50 μg/ml. Interestingly, the colistin-resistant mutant of *A. baumannii* ATCC 19606 is much more sensitive (MIC = 3.25 μg/ml) to HNP-1 than the standard strain [86]. Human defensin 5 (HD5) has a relatively low anti-*A. baumannii* effect (MIC = 320 μg/ml). However, an analog called HD5d5 obtained by sequence modification presented a stronger bactericidal effect (MIC = 40 μg / ml) against *A. baumannii*, exerting the effect through damage to the membrane, accumulation in the cytoplasm, and reduction of catalase and superoxide dismutase activities [165, 166].
