Table 1.

Main resistance mechanisms in quinolones

Resistance mechanism	Characteristics
Chromosomal mutations (quinolone resistance-determining region) High resistance level	They act over genes encoding the DNA gyrase targets (Gyr A and Gyr B) and topoisomerase IV (Par C and Par E). They appear for random transcription errors
Reduction of intracellular concentration Low resistance level	Decrease in drug penetration by reduction of porin expression. It can be selected after treatment with other antibiotics Increased expression of active ejection pumps. They are usually linked to the dose of quinolones Some confer unique resistance to quinolones (NorA-B-C) and others also to structurally unrelated antibiotics
Plasmids (plasmid-mediated quinolone resistance)	Coding of active ejection pumps (QacA-B, PmrA)
Low resistance level	Encoding proteins in the target of action, decreasing the availability of the target to the antibiotic Coding of enzymes modifying the quinolone structure. They are derived from the aminoglycoside-modifying enzymes