An emerging crisis of antibiotic resistance: In search of alternative antimicrobial sources

Antimicrobials produced by some of the microorganisms, plants, and animals impede bacterial growth or kill infectious microorganisms. Members of the Actinobacteria alone produce ~80% of the total antimicrobial compounds. Recently the overuse and misuse of natural, synthetic, and chemically modified antibiotics have posed a serious threat to human health as well as the environment. Therefore, screening of newer and natural antimicrobial agents from microbial, plant, and animal sources has been continually broadening. Terrestrial plants have vast ability to synthesize aromatic substances such as alkaloids, phenolic compounds, tannings, triterpenoids and terpinenes to defend against predation by microorganisms, insects, and animals. In this chapter, we will review recent works on naturally isolated antimicrobial compounds, such as antimicrobial peptides, enzymes, phytochemicals, plant and herbal extracts as potential antimicrobial agents for different infectious diseases. We will also review the mode of action of these antimicrobial agents and their target organisms in relation to bacterial, viral, fungal, and protozoan mediated human diseases.