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A large group of low molecular weight natural compounds that exhibit antimicrobial activity has been isolated from animals and plants during the past two decades. Among them, cationic peptides are the most widespread. Interestingly, the variety and diversity of these peptides seem to be much wider than suspected. In fact, novel classes of peptides with varying chemical propertiescontinue to be isolated from different vertebrate and invertebrate species, as well as from bacteria. To the early characterized peptides, mostly cationic in nature, anionic peptides, aromatic dipeptides, processed forms of oxygen-binding proteins and processed forms of natural structural and functional proteins can now be added, just to name a few. In spite of the astonishing diversity in structure and chemical nature displayed by these molecules, all of them present antimicrobial activity, a condition that has led researchers to consider them as "natural antibiotics" and as such a new and innovative alternative to chemical antibiotics with a promising future as biotechnological tools. A resulting new generation of anti microbial peptides (AMPs) with higher specific activity and wider microbe-range of action could be constructed, and hopefully endogenously expressed in genetically-modified organisms.