The use of natural products as herbal drug preparations dates back thousands of years ago; however, their application to modern drug discovery started after the discovery of penicillin by Sir Alexander Fleming in 1928. Since then, a number of antibiotics have been isolated and characterized. It was a medical miracle, but at the same time, the problem of treating infectious diseases has been emerged due to development of resistance to antibiotics in bacterial populations. Taking this into consideration, a global attention has taken place towards finding new chemicals, with lead structures, which either directly or after small modifications can be used for development of new drugs.

Natural products are very significant in modern drug development, especially for antimicrobial and antitumor agents. They provide new chemical entities (NCEs) with completely different structures from known drugs. Medicinal chemistry of bioactive natural products cover a wide range of fields, including isolation and characterization of bioactive compounds from natural sources, structure modification for optimization of their activity and other physical properties, and total and semi-synthesis for a thorough scrutiny of structure activity relationship. Numerous methods have been employed to acquire compounds for drug discovery, including isolation from plants and other natural sources, synthetic chemistry, combinatorial chemistry, computational chemistry, molecular modeling and high throughput screening (HTS).

The book represents a comprehensive introduction to several medicinal plants and their applications in medical and biological fields. Antimicrobial activities such as antibacterial, antiviral, antifungal and anticancerous have been discussed in detail as evaluated by scientists throughout the world. In the present work, antibacterial activity of various selected medicinal plants has been evaluated against clinically significant bacterial species. The plants were selected on the basis of their availability, moreover, plant parts which are economically viable and easily available vere screened for the current study. This book covers all the important indings of the current research work and also explored these results very clearly. All topics are systematically discussed which will be helpful to carry out more innovative researches in the world of antimicrobials. Our primary intention in writing such a book is to attract research students and to spur their interests in bioactive natural products research by providing them with illustrative examples. In addition, we hope that experts in natural products may find this book's information useful to them,in terms of updated research results on several classes of renowned naturalproducts. This research monograph is highly useful for the medical andpharmaceutical industries.