Preface

Microorganisms are ubiquitous and play an important but "bifacial" role in the existence of plants, animals, and humans. Some microorganisms are necessary for maintaining vital functions but may play a negative role and lead to infectious diseases and even death. Thus, all existing species must protect themselves from excess colonization of microorganisms that can cause infection and death. The immune system defends the organism against different types of infections. Antimicrobial peptides (AMPs) are an important part of the innate immune system of different organisms living in nature. Unfortunately, the immune system cannot always defeat the infection. Infectious diseases were an enormous problem in the past. Before the discovery of antibiotics, extracts of herbs and plants and chemical compounds were used as therapies for infectious diseases. Everything changed with the discovery of penicillin, which marked the start of the "antibiotic era." Antibiotics were determined to be "miracle drugs" and intensively used in medicine for the successful treatment of bacterial infection. However, this intensive usage in human and veterinary medicine, sometimes as a preventive step, has led to drug resistance.

Microorganisms are good at adapting to different stress conditions and have different mechanisms of antibiotic resistance such as alteration of membrane permeability, enzymatic degradation of antibiotics, modification of bacterial proteins that are antimicrobial targets, and active efflux. Modern medicine is impossible without antibiotics, but conclusions of the "antibiotic era" aren't optimistic. Global antibiotic usage led to the development of superbugs, and the number of resistant bacteria and multidrug-resistant (MDR) bacteria is rapidly increasing. As such, we still have problems with infectious disease treatment despite an arsenal of antibiotics, many of which have become ineffective against modern (resistant) microorganisms.

Another problem with modern antibacterial treatment is that we currently use antibiotics discovered more than three decades ago. A new antibiotic is necessary, but unfortunately, the development of new antibiotics is costly, takes years, and is an unprofitable process for pharmaceutical companies.

This book presents a comprehensive overview of AMPs with potential for medical use. The AMPs described are isolated from different sources, such as ascidians, cyanobacteria, frogs, and more.

Chapter by Rajesh and Vanathi describes the structure and mechanism of action of bioactive peptides derived from ascidians and associated cyanobacteria that are structurally unique and have antibacterial, antifungal, and anticancer activity. Chapter by Rangel and De Simone discusses peptides from different sources that are active against the ESKAPE group of antibiotic-resistant bacteria, especially anti-Acinetobacter peptides, as *A. baumannii* has emerged as a highly troublesome nosocomial pathogen revealing drug-resistant (DR), MDR, extensively drugresistant (XDR), and pan-drug-resistant (PDR) phenotypes. Chapter by Ouertani et al. discusses bacteriocins, which are bacterial AMPs. The chapter focuses on the structure-function relationship and mechanism of action of AMPs.

This timely book presents the current situation of AMPs as an emerging group of therapeutic agents. It is a useful resource for clinicians, researchers, technicians, scientists, and students. Finally, we would like to thank all the contributing authors for their time and original research.

> **Shymaa Enany** Faculty of Pharmacy, Department of Microbiology and Immunology, Suez Canal University, Ismailia, Egypt
