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Antimicrobial agents have had a major impact on human health. Together with vaccines, they have contributed to reduced mortality, extended lifespan, and enhanced quality of life. Among drugs used in human medicine, however, they are distinctive in that their use promotes the occurrence of drug resistance in the pathogens they are designed to treat as well as in other “bystander” organisms. Indeed, the history of antimicrobial development has been driven in large part by the medical need engendered by the emergence of resistance to each generation of agents. Thus, the careful and appropriate use of antimicrobial drugs is particularly important not only for optimizing efficacy and minimizing adverse effects but also for minimizing the risk of resistance and preserving the value of existing agents. Although this chapter focuses on antibacterial agents, the optimal use of all antimicrobials depends on an understanding of each drug’s mechanism of action, spectrum of activity, mechanisms of resistance, pharmacology, and adverse effect profile. This information is applied in the context of the patient’s clinical presentation, underlying conditions, and epidemiology to define the site and likely nature of the infection or other condition and thus to choose the best therapy. Gathering of microbiologic information is important for refining therapeutic choices on the basis of documented pathogen and susceptibility data whenever possible; this information also makes it possible to choose more targeted therapy, thereby reducing the risk of selection of resistant bacteria. Durations of therapy are chosen according to the nature of the infection and the patient’s response to treatment and are informed by clinical studies when they are available, with the understanding that shorter courses are less likely than longer ones to promote the emergence of resistance. This chapter and the one that follows provide specific information that is necessary for making informed choices among antibacterial agents. The mechanisms of action of antibacterial agents are discussed in detail in the text of this chapter, and mechanisms of resistance are discussed in detail in Chap. 140. Both types of mechanisms are summarized for the most commonly used groups of agents in Table 139-1. A schematic of antibacterial targets is provided in Fig. 139-1.

TABLE 139-1Mechanisms of Action of and Resistance to Antibacterial Agents

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