Quinolones were introduced into clinical practice in the late 1960s. Although quinolone resistance was described early, no transferable mechanism of quinolone resistance (TMQR) was confirmed until 1998. To date, five different TMQRs have been described in the literature, including target protection (Qnr), quinolone modification (AAC(6')-Ib-cr), plasmid-encoded efflux systems (e.g. QepA or OqxAB, amongst others), effect on bacterial growth rates and natural transformation. Although TMQRs usually only result in a slight increase in the minimum inhibitory concentrations of quinolones, they possess an additive effect and may facilitate the acquisition of full quinolone resistance. The emergence of new related genes may continue in the next years.
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