Antibiotic resistance is often considered to be a trait acquired by previously susceptible bacteria, on the basis of which can be attributed to the horizontal acquisition of new genes or the occurrence of spontaneous mutation. In addition to acquired resistance, bacteria have a trait of intrinsic resistance to different classes of antibiotics. An intrinsic resistance gene is involved in intrinsic resistance, and its presence in bacterial strains is independent of previous antibiotic exposure and is not caused by horizontal gene transfer. Recently, interest in intrinsic resistance genes has increased, because these gene products not only may provide attractive therapeutic targets for development of novel drugs that rejuvenate the activity of existing antibiotics, and but also might predict future emergence of resistant pathogens if they become mobilized. In the present review, we summarize the conventional examples of intrinsic resistance, including the impermeability of cellular envelopes, the activity of multidrug efflux pumps or lack of drug targets. We also demonstrate that transferases and enzymes involved in basic bacterial metabolic processes confer intrinsic resistance in Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. We present as well information on the cryptic intrinsic resistance genes that do not confer resistance to their native hosts but are capable of conferring resistance when their expression levels are increased and the activation of the cryptic genes. Finally, we discuss that intrinsic genes could be the origin of acquired resistance, especially in the genus Acinetobacter.
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