[Fluoroquinolones and Gram-negative bacteria: antimicrobial activity and mechanisms of resistance]

Fluoroquinolones acts by interacting with type II topoisomerases (DNA gyrase and topoisomerases IV). Related to this mechanism of action, bacteria have developed resistance mechanisms consisting in some target mutations (GyrA/GyrB for DNA gyrase and ParC/ParE for topoisomerase IV) or in a reduced access to the target itself, by either decreased permeability or augmented expression of efflux pumps, such as AcrAB and MexAB. Along with these classical mechanisms of chromosomal resistance, the presence of fluoroquinolones resistant proteins (Qnr) has been recently evidenced, codified by transmissible genes by means of plasmids, especially in Enterobacter spp., Escherichia coli and Klebsiella pneumoniae, whereas Proteus mirabilis and non fermenter Gram-negative, like Acinetobacter spp. and Pseudomonas aeruginosa, are not involved in such a kind of resistance. Qnr proteins determine a slight increase in MIC values, which often remains below the susceptibility breakpoint. More relevant is their impact on MPC values. Additionally, new specific resistance mechanisms have been described. AAC(6')-Ib-cr represents the first enzyme able to inactivate, by acetylation, antimicrobials of two different classes, aminoglycosides and fluoroquinolones. However, ciprofloxacin and norfloxacin, but not levofloxacin, are susceptible to this enzyme action. Finally, the presence of another resistance mechanism has been reported, an efflux-pump plasmid-mediated, codified by the QepA gene, which acts by a selective mechanism. Only hydrophilic fluoroquinolones, i.e. norfloxacin and ciprofloxacin, but not all the other ones, i.e. levofloxacin, moxifloxacin, etc, are affected by this mechanism. In the light of these new information, it is clear that, in terms of bacterial resistance, it is not any more possible to assimilate one fluoroquinolones to another, since different molecules can be diversely active, due to the specific resistance mechanism.