The existence of natural antimicrobial substances, contributing to the mechanisms of host defenses, has been recognized since the late nineteenth century. In 1963, the in vitro antibacterial activity of leukocyte extracts was attributed to basic proteins. Since the late 1980s, cationic peptides with antimicrobial properties have been subsequently identified in other host cells and tissues and in virtually every living species (Lehrer, 2004). The properties of these “Nature’s antibiotics” and their multiple functions in host defenses of multicellular organisms support the rationale of developing entirely novel peptide-based therapeutics harnessing the effector mechanisms of innate immunity (Hancock and Sahl, 2006). The term antimicrobial peptides covers different forms of natural macromolecules; ribosomally synthesized and non-post translationally modified innate immunity peptides, or their synthetic analogs, are predominantly considered here. Their antimicrobial and immunomodulatory activities will not be dissociated in general and they will be indistinctively described as (cationic) antimicrobial or host defense peptides.
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