from Keith Poole writing in Microbial Efflux Pumps: Current Research:
Antibiotic efflux systems are common in Pseudomonas aeruginosa, with chromosomally-encoded multidrug efflux systems of the Resistance Nodulation Division (RND) family, specifically MexAB-OprM, MexCD-OprJ, MexEF-OprN and MexXY-OprM, of particular importance in clinical settings. Despite the broad substrate specificity of many of these, their clinical importance is limited to fluoroquinolone resistance (MexAB-OprM, MexCD-OprJ and MexEF-OprN), β-lactam resistance (MexAB-OprM, MexXY-OprM) and aminoglycoside resistance (MexXY-OprM). Expression of these systems is governed by the products of regulatory genes (mexAB-oprM: mexR, nalC, nalD; mexCD-oprJ: nfxB; mexEF-oprN: mexT; mexXY: mexZ) whose mutation is typically responsible for acquired multidrug resistance in lab and clinical isolates. With few exceptions these efflux systems are not inducible by substrate antimicrobials, consistent with antimicrobial efflux not being their intended function. Indeed, recent data highlight their induction by environmental stresses (oxidative stress, nitrosative stress, envelope stress) suggestive of a role in stress response systems in this organism. Significantly, such stresses may provide a selective pressure for antibiotic-resistant efflux mutants in vivo independent of antibiotic exposure. Given the importance of these efflux systems in intrinsic and acquired multidrug resistance in P. aeruginosa, strategies aimed at interfering with efflux-mediated resistance are being investigated.
Further reading: Microbial Efflux Pumps: Current Research