Antimicrob Agents Chemother 45(2): 480-484

Carbapenem Resistance Mechanisms in <em>Pseudomonas aeruginosa</em> Clinical Isolates

Division of Infectious Disease, Department of Internal Medicine,^{and Department of Microbiology,}^{College of Medicine, University of Dankook, Chonan, Department of Clinical Pathology, Sungkyunkwan University College of Medicine,}^{and Department of Internal Medicine, Seoul National University College of Medicine,}^{Seoul, Korea, and Department of Microbiology, Kyoto Pharmaceutical University, Kyoto, Japan}⁵

^{Corresponding author. Mailing address: Division of Infectious Disease, Department of Internal Medicine, College of Medicine, University of Dankook, San 29, Anseo-dong, Chonan, Chungnam, 330-715, Korea. Phone: 82-41-550-3918. Fax: 82-41-556-3256. E-mail:}rk.oc.letinu@jhiap.

Received 2000 Mar 31; Revisions requested 2000 Aug 8; Accepted 2000 Nov 10.

Abstract

In order to define the contributions of the mechanisms for carbapenem resistance in clinical strains of Pseudomonas aeruginosa, we investigated the presence of OprD, the expressions of the MexAB-OprM and MexEF-OprN systems, and the production of the β-lactamases for 44 clinical strains. All of the carbapenem-resistant isolates showed the loss of or decreased levels of OprD. Three strains overexpressed the MexAB-OprM efflux system by carrying mutations in mexR. These three strains had the amino acid substitution in MexR protein, Arg (CGG) → Gln (CAG), at the position of amino acid 70. None of the isolates, however, expressed the MexEF-OprN efflux system. For the characterization of β-lactamases, at least 13 isolates were the depressed mutants, and 12 strains produced secondary β-lactamases. Based on the above resistance mechanisms, the MICs of carbapenem for the isolates were analyzed. The MICs of carbapenem were mostly determined by the expression of OprD. The MICs of meropenem were two- to four-fold increased for the isolates which overexpressed MexAB-OprM in the background of OprD loss. However, the elevated MICs of meropenem for some individual isolates could not be explained. These findings suggested that other resistance mechanisms would play a role in meropenem resistance in clinical isolates of P. aeruginosa.

Abstract

Pseudomonas aeruginosa is a clinically important pathogen with intrinsic resistance to various antimicrobial agents. This intrinsic multidrug resistance results from the synergy between broadly specific drug efflux pumps and a low degree of outer membrane permeability. For the carbapenem antimicrobials, the resistance is mostly mediated by OprD loss, which primarily confers a resistance to imipenem but also confers a low grade resistance to meropenem (12, 16). But the multidrug efflux systems which mediate the resistance to quinolone, chloramphenicol, and many other antimicrobial agents, also contribute to the carbapenem resistance. The strains which overexpress the MexAB-OprM system or express the MexEF-OprN system exhibit the carbapenem resistance by pumping the drug out or repressing the transcription of oprD, respectively (13, 20, 25). On the other hand, the NfxB mutants which expressed MexCE-OprJ became more susceptible to imipenem, bipenem, and some β-lactams (22). In addition to the OprD loss or drug efflux pumps, chromosomal AmpC β-lactamase plays an important role in carbapenem resistance in P. aeruginosa (16, 21). Although the contributions of the OprD loss, the efflux systems, and β-lactamase in the carbapenem resistance have been well characterized in the laboratory strains, little data is available for how such factors play together in the clinical isolates of P. aeruginosa (2, 4, 31).

Based on the carbapenem susceptibility patterns, the clinical isolates of carbapenem-resistant P. aeruginosa could be divided into three groups as the imipenem-resistant and meropenem-sensitive group, the imipenem-sensitive and meropenem-resistant group, and the imipenem-resistant and meropenem-resistant group (2, 4, 17). This suggests that carbapenem resistance occurs by several mechanisms in concert for the clinical isolates. Therefore, in order to evaluate the contributions of the carbapenem resistance mechanisms in clinical strains of imipenem-resistant P. aeruginosa, we investigated the presence of OprD, expressions of the MexAB-OprM and the MexEF-OprN systems, and the production of the β-lactamases.

ACKNOWLEDGMENTS

This work was supported by the Korean Research Foundation made in the program of 1997.

We are very grateful to G. A. Jacoby, who provided the strains carrying the plasmids encoding PSE-1, OXA-10, and OXA-2 β-lactamases; to D. M. Livermore, who provided OprD(−) mutant 2297 β-con D2; and to K. Poole for giving us the strain PAO1 and the nalB mutant OCR1. We thank E. Osano for giving us the strain harboring IMP-1 β-lactamase. We also thank H. Choy and J. Tran for their critical review of the manuscript.

ACKNOWLEDGMENTS

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