Abstract
This chapter discusses the evolution of antibiotic resistance. Antibiotic resistance refers to the property of bacteria which prevents the inhibition of their growth by antimicrobial agents used in the clinical setting. Resistance can thus be achieved either if the antibiotic concentration reaching the target is too low or if the interaction between the antibiotic and the target is not efficient enough to produce the inhibition of bacterial growth. This includes intrinsic and acquired resistance. The evolution of antibiotic resistance is a consequence of the selection of resistant organisms. Bacteria with increased mutation rates are usually known as mutators. Recombination is a powerful mechanism for the evolution of antibiotic resistance genes. Enzymes involved in antibiotic resistance frequently show a high molecular plasticity, easily generating variants with an enlarged spectrum of action for new drugs within a chemical family, particularly under intense exposure. Genetic variability in a population does not increase inevitably along time, since it is the result of factors acting in opposite directions: some processes introduce new variation in the populations while others remove it. Fitness is a relative measure of reproductive success and is usually evaluated as the difference between the population growth rate of a reference and the target strain. Genetic drift is the result of the sampling process that occurs in every population in which the total number of individuals is limited. One of these factors indicates that compensation of fitness cost occurs, so that the advantage of drug-susceptible strains in an antibiotic-free environment is reduced. The chapter states that experimental evolution with model microorganisms has been a successful approach to test evolutionary hypotheses. These experiments allow studying evolution in real time, producing accurate measures of key parameters like fitness, generation times, population sizes, or mutation rates. The evolution of antibiotic resistance is unpredictable.
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