Involvement of an Active Efflux System in the Natural Resistance of Pseudomonas aeruginosa to Aminoglycosides

Abstract

A mutant, named 11B, hypersusceptible to aminoglycosides, tetracycline, and erythromycin was isolated after Tn501insertion mutagenesis of Pseudomonas aeruginosa PAO1. Cloning and sequencing experiments showed that 11B was deficient in an, at that time, unknown active efflux system that contains homologs of MexAB. This locus also contained a putative regulatory gene,mexZ, transcribed divergently from the efflux operon. Introduction of a recombinant plasmid that carries the genes of the efflux system restored the resistance of 11B to parental levels, whereas overexpression of these genes strongly increased the MICs of substrate antibiotics for the PAO1 host. Antibiotic accumulation studies confirmed that this new system is an energy-dependent active efflux system that pumps out aminoglycosides. Furthermore, this system appeared to function with an outer membrane protein, OprM. While the present paper was being written and reviewed, genes with a sequence identical to our pump genes, mexXY of P. aeruginosa, have been reported to increase resistance to erythromycin, fluoroquinolones, and organic cations inEscherichia coli hosts, although efflux of aminoglycosides was not examined (Mine et al., Antimicrob. Agents Chemother. 43:415–417, 1999). Our study thus shows that the MexXY system plays an important role in the intrinsic resistance of P. aeruginosato aminoglycosides. Although overexpression of MexXY increased the level of resistance to fluoroquinolones, disruption of themexXY operon in P. aeruginosa had no detectable effect on susceptibility to these agents.

Aminoglycosides versus bacteria – a description of the action, resistance mechanism, and nosocomial battleground

“…Bacterial species constitutively expressing such transporters are intrinsically resistant to low levels of various antibiotics. However, mutations in the regulatory genes of the pumps or induction of expression in the presence of substrate, can lead to the overexpression of the originally constitutive or pump genes [49,50]. In the last several years, aminoglycosides were shown to be substrates for a number of multidrug efflux pumps, including members of the five superfamilies of bacterial transporters.…”

Adaptive resistance to cationic compounds in Pseudomonas aeruginosa

“…2). These proteins form a functional tripartite efflux machinery that is capable of extruding a variety of antibiotics, including aminoglycosides, macrolides, ␤-lactams (penicillins except carbenicillin and sulbenicillin, cephalosporins except cefsulodin, ceftazidime and carbapenems), quinolones, lincomycin, chloramphenicol, tetracyclines and tigecycline [26][27][28][29].…”

Antimicrobial resistance among non-fermentative Gram-negative bacilli isolated from the respiratory tracts of Italian inpatients: a 3-year surveillance study by the Italian Epidemiological Survey

“…The mechanisms underlying resistance to ␤-lactam agents include the production of ␤-lactamases, outer membrane permeability mutations and efflux pumps [9][10][11][12][13][14][15][16][17][18][19][20][21]. The latter two mechanisms also play a role in resistance to aminoglycosides and quinolones [22][23][24]. Addition of mutations in DNA gyrase genes and the presence of aminoglycoside-modifying enzymes can lead to the emergence of multidrug-resistant strains of P. aeruginosa [15,17,[25][26][27] and very few drugs (e.g., polymyxins) are effective against infections caused by these organisms [28,29].…”