Role of the efflux pumps in antimicrobial resistance in E. coli

Patrick Plésiat Teaching hospital Jean Minjoz, Besançon, France.

Efflux is a universal mechanism for maintaining cell homeostasis. Of the 37 putative efflux systems encoded by the genome of Escherichia coli, 16 have been shown to export clinically relevant antibiotics. A number of these systems are composed of a single transporter protein. This protein which is located in the cytoplasmic membrane draws the energy for drug extrusion from the proton motive force or ATP hydrolysis. In the other systems, the transporter interacts with an outer membrane protein and a periplasmic adaptor to allow the extrusion of the antibiotic molecules directly into the external medium, thus bypassing the outer membrane permeability barrier. The bacterial drug transporters fall into five groups according to their amino acid sequences: the major facilitator superfamiliy (MFS); the small multidrug resistance (SMR) family; the resistance nodulation cell division (RND) family; the ATP-binding cassette (ABC) family; the multidrug and toxic compound extrusion (MATE) family. In E. coli, 7 MFS, 1 SMR, 6 RND, 1 ABC, and 1 MATE transporters have thus been reported to form efflux systems able to accomodate antibiotics and to confer some degree of resistance to various drugs when overproduced from plasmid-cloned genes. However, despite this plethora of systems only the constitutively expressed AcrAB-TolC tripartite pump contributes to the natural resistance of the bacterium to inhibitors. As many RND pumps found in other gram-negative species (e.g., Pseudomonas aeruginosa, Neisseria gonorrhoeae), AcrAB-TolC may transport a wide range of structurally-unrelated toxic compounds including oxacillin, chloramphenicol, tetracycline, erythromycin, nalidixic acid, norfloxacin, novobiocin, rifampicin, trimethoprim, dyes, organic solvants. The genes coding for the transporter AcrB and the adaptor AcrA form a single operon the expression of which is partially repressed by the product (AcrR) of a divergently oriented, self-regulated repressor gene acrR. In addition to the local control exerted by AcrR, acrAB transcription is modulated by complex regulatory circuitries involving loci marRAB, soxRS, and rob. Increase in the levels of AraC/XylS transcriptional activators MarA or SoxS following mutations e.g. in repressor genes marR and soxR respectively results in overproduction of AcrAB-TolC and increased efflux of the substrates of the pump. Because of the relatively high permeability of the porin uptake pathway in E. coli, mutants overexpressing AcrAB-TolC often exhibit a low and poorly significant resistance to agents such as tetracyclines and fluoroquinolones (MICs x four- to eightfold). However, the observation that fluoroquinolone resistant clinical strains of E. coli frequently show an upregulated AcrAB-TolC system in addition to alterations in targets DNA gyrase and Topoisomerase IV suggests that the pump may provide some advantages in the clinical setting. Interestingly, the AcrAB-TolC pump has been reported to modulate the post-antibiotic effect and the killing activity of antibiotics against E. coli. The use of efflux inhibitors could therefore be useful to prevent the emergence of fluoroquinolone resistant mutants harboring target alterations, and to improve the efficacy of these antibiotics.