This chapter reviews the problem of resistance to antimicrobial agents by describing pharmacologic concepts and mechanism of action of selected antimicrobial agents. In particular this chapter focuses on the basis and mode of action of antimicrobial agents, mechanism of acquisition of resistance to antimicrobial agents and ways to prevent emergency and spreading of antimicrobial resistance.

Microorganisms coexist with other living organisms and exhibit the greatest genetic and metabolic activity. Microbes have evolved various mechanisms to survive pressure exerted by competitive environmental challenges. Infection is the invasion of the host by harmful microorganisms (microbial pathogens), which then multiply in close association with the host's tissues. Infections may differ in severity and may range from in apparent to fulminating. There has been a continual battle between humans and the multitude of microbial pathogens. Antimicrobial agents are among few drugs that cure by eliminating the infective microorganisms. Development of antimicrobial agents for clinical use has been successful in targeting essential components of general areas of microbial metabolism namely: cell wall
synthesis, protein synthesis, ribonucleic acid (RNA) synthesis, deoxyribonucleic acid (DNA) synthesis and intermediary metabolism.

The successful use of antimicrobial agents to inhibit and eliminate the infectious organisms has been facing challenges and difficulties because microbial pathogens are developing various forms of resistance to the drugs and as use of antimicrobial drugs increases, so do the level and complexity of the resistance. Emergence of resistance to multiple antimicrobial agents in pathogens has become an emergency public health problem as there are fewer, or sometimes no, effective antimicrobial agents available for infections caused by these pathogens. Microbial pathogens may manifest resistance to antimicrobial agents through various mechanisms such as by acquiring mutations and selection or acquiring resistance genes from other microcrobial pathogens. Patients infected with resistant pathogens often suffer treatment
failure which usually have detrimental outcome especially to those critically ill patients.

Currently most widely used antimicrobial agents are subject to resistance and even some newer agents are facing the same challenge. The resistance has generally been met through the discovery of novel antimicrobial agents and by use of derivatives prepared by semisynthetic methods, which are not affected by existing resistance mechanisms. Understanding the mode of drug action and the how these resistance mechanisms work will highlight the challenges facing the
chemotherapy of infectious diseases and the way to tackle these problems.

This chapter reviews the problem of resistance to antimicrobial agents by describing pharmacologic concepts and mechanism of action of selected antimicrobial agents. In particular this chapter focuses on the basis and mode of action of antimicrobial agents, mechanism of acquisition of resistance to antimicrobial agents and ways to prevent emergency and spreading of antimicrobial resistance.