Mobile genes coding for efflux-mediated antimicrobial resistance in Gram-positive and Gram-negative bacteria

Efflux mechanisms that account for resistance to a variety of antimicrobial agents are commonly found in a wide range of bacteria. Two major groups of efflux systems are known, specific exporters and transporters conferring multidrug resistance (MDR). The MDR systems are able to remove antimicrobials of different classes from the bacterial cell and occasionally play a role in the intrinsic resistance of some bacteria to certain antimicrobials. Their genes are commonly located on the bacterial chromosome. In contrast, the genes coding for specific efflux systems are often associated with mobile genetic elements which can easily be interchanged between bacteria. Specific efflux systems have mainly been identified with resistances to macrolides, lincosamides and/or streptogramins, tetracyclines, as well as chloramphenicol/florfenicol in Gram-positive and Gram-negative bacteria. In this review, we focus on the molecular biology of antimicrobial resistance mediated by specific efflux systems and highlight the association of the respective resistance genes with mobile genetic elements and their distribution across species and genus borders.