Contribution of the MexX-MexY-OprM Efflux System to Intrinsic Resistance in Pseudomonas aeruginosa

Abstract

To test the possibility that MexX-MexY, a new set of efflux system components, is associated with OprM and contributes to intrinsic resistance in Pseudomonas aeruginosa, we constructed a series of isogenic mutants lacking mexXY and/or mexAB and/or oprM from a laboratory strain PAO1, and examined their susceptibilities to ofloxacin, tetracycline, erythromycin, gentamicin, and streptomycin. Loss of either MexXY or OprM from the MexAB-deficient mutant increased susceptibility to all agents tested, whereas loss of MexXY from the MexAB-OprM-deficient mutant caused no change in susceptibility. Introduction of an OprM expression plasmid decreased the susceptibility of the mexAB-oprM-deficient-/mexXY-maintaining mutant, yet caused no change in the susceptibility of a mexAB-oprM-and mexXY-deficient double mutant. Immunoblot analysis using anti-MexX polyclonal rabbit serum generated against synthetic oligopeptides detected expression of MexX in the PAO1 cells grown in medium containing tetracycline, erythromycin, or gentamicin, although expression of MexX was undetectable in the cells incubated in medium without any agent. These results suggest that MexXY induced by these agents is functionally associated with spontaneously expressed OprM and contributes to the intrinsic resistance to these agents.A variety of multicomponent efflux systems are coded on the chromosomes of gram-negative bacteria and contribute to intrinsic and acquired resistance against antimicrobial agents, disinfectants, organic solvents, and heavy metals (17)(18)(19)(20)24). Pseudomonas aeruginosa is a clinically significant pathogen exhibiting highly intrinsic resistance to various antimicrobial agents. One of the mechanisms contributing to its intrinsic resistance is a spontaneous expression of the efflux system MexA-MexB-OprM encoded on a mexA-mexB-oprM operon of the chromosome of P. aeruginosa (10). This system energydependently extrudes many antimicrobial agents from the cell interior in cooperation with the periplasmic, inner membrane, and outer membrane components organized through the two membranes. While the disruption of each component gene of the mexA-mexB-oprM operon increases the susceptibility to many antimicrobial agents, the disruption of oprM increases the susceptibility to a greater extent than the disruption of mexA or mexB (5,21,30,31). Due to the presence of a weak promoter in the mexB gene upstream of oprM gene, the polar effect from the disruptions of mexA and mexB does not entirely suppress the expression of the oprM gene (31). Thus, OprM can contribute to the intrinsic resistance by cooperation with unknown periplasmic and inner membrane components. Recently, mexG-mexH (GenBank accession no. AF073776 To test the possibility that MexXY is associated with OprM and contributes to intrinsic resistance, we constructed a series of isogenic mutants lacking mexXY and/or mexAB and/or oprM from laboratory strain PAO1 and compared their susceptibilities to antimicrobial agents. We also showed that the expression of MexXY is induced by expo...

Relevance of resistance levels to carbapenems and integron-borne bla IMP-1, bla IMP-7, bla IMP-10 and bla VIM-2 in clinical isolates of Pseudomonas aeruginosa

“…low permeability of the outer membrane due to loss of OprD, the activity of the efflux system due to overexpression of the MexAB-OprM and MexEF-OprN pumps, or the production of chromosomal AmpC blactamases, as well as the cooperation of such factors (Li et al, 2000;Masuda et al, 1999;Nikaido, 1989Nikaido, , 1996Pai et al, 2001;Quale et al, 2006). Further experiments are needed to clarify the details of the underlying mechanism(s).…”

A PhoPQ-Regulated ABC Transporter System Exports Tetracycline in Pseudomonas aeruginosa

“…A set of RND pumps in P. aeruginosa has been reported to be associated with Tet susceptibility, which include MexAB-OprM, MexCD-OprJ, MexXY-OprM, MexVW-OprM, MexPQ-OprE, MexMN-OprM, MexJK-OprM, MuxABC-OpmB, and MexGHIOprD (5,63,64). MexXY-OprM is the primary system for intrinsic resistance to Tet, and deletion of mexXY causes an 8-fold increase in susceptibility to Tet (63,64).…”

Why are bacteria refractory to antimicrobials?

“…Inducible multidrug efflux pumps are responsible for the intrinsic antibiotic resistance of many organisms and mutation of the regulatory elements that control the production of efflux pumps can lead to an increase in antibiotic resistance. For example, the MexAB-OprM in P. aeruginosa is normally positively regulated by the presence of antibiotics [13], yet mutations in its regulator, mexR, lead to the overexpression of Mex-AB-OprM, conferring increased resistance to b-lactam antibiotics [14]. The broad-spectrum nature of efflux pumps and their ability to be transferred between microorganisms make them a serious threat to the efficacy of antibiotics in the clinical setting.…”