[Development of resistances to antibiotic drugs: causes, consequences and importance to the public health system].

Most bacteria contain genes involved in natural resistance to antimicrobial agents. Resistance has clinical importance when the organism is able to survive in the presence of in vivo concentrations of antimicrobial agents. Antimicrobial agents can select individual bacteria or bacterial populations that present natural or acquired resistance to them. Resistance is due to multiple genetic and biochemical causes. Two of the most important genetic processes in bacterial resistance are mutagenesis and the acquisition of new genes by horizontal transfer (usually by conjugation, and to a lesser extent transformation or transduction). Biochemical mechanisms of resistance include decreased permeability, antimicrobial modification, target change, protection or over-production, expression of efflux pumps and modifications of certain metabolic pathways. Resistance impacts the health care system in many ways: it requires that clinical microbiologists have reliable tools to detect and analyse the problem; it results in reduced options for empirical and targeted antimicrobial treatment; it forces the use of broad-spectrum antimicrobials; it increases infectious morbidity and mortality and health expenses; and it demands new antimicrobials to control this serious problem in the short term.