Short Communication
Carbapenem-resistant-only Pseudomonas aeruginosa infection in patients formerly infected by carbapenem-susceptible strains

https://doi.org/10.1016/j.ijantimicag.2014.07.022Get rights and content

Highlights

  • Selective carbapenem resistance in P. aeruginosa occurs soon after carbapenem therapy.

  • OprD inactivation is the predominant cause of selective carbapenem resistance.

  • Carbapenem-resistant only P. aeruginosa clonal spread may exist in hospital settings.

Abstract

Pseudomonas aeruginosa isolates that were initially carbapenem-susceptible and later became selective carbapenem-resistant following antimicrobial therapy were identified from individual cases during the same hospitalisation. Cross-resistance to other β-lactams was not found and their susceptibilities remained identical in consecutive isolates. Real-time quantitative reverse transcription PCR was performed to investigate the role of OprD, an outer membrane protein regulating the entry of carbapenems, in the appearance of carbapenem-resistant-only P. aeruginosa (CROPA). Fifteen paired isolates of carbapenem-susceptible P. aeruginosa (CS-PA) and CROPA were identified. All of the cases had carbapenem exposure history within 1 month before the appearance of CROPA (mean 10 days). Reduced OprD expression was found in 93% (14/15) of the isolates, suggesting that oprD inactivation was the major contributor to selective carbapenem resistance. Of the 14 cases with CROPA due to oprD mutation, 71% (10/14) were persistent infection, as genotype analysis revealed that their paired strains were isogenic; 29% (4/14) represented re-infections as they were heterogenic, suggesting that oprD-deficient CROPA existed in the hospital and that carbapenem selective pressure promoted its spread to patients. We conclude that CROPA may occur soon after the use of carbapenems to treat CS-PA infections and that oprD mutation is the major mechanism of resistance in CROPA. Restriction of empirical use of carbapenems by antibiotic stewardship is important to halt the occurrence of CROPA.

Introduction

Pseudomonas aeruginosa is one of the major nosocomial pathogens. Infections by P. aeruginosa are often difficult to treat because of their limited susceptibility to commonly used antimicrobial agents [1]. There are a limited number of antimicrobial agents with reliable activity against P. aeruginosa, including antipseudomonal penicillins, cephalosporins, fluoroquinolones and carbapenems. Carbapenems are often the most consistently effective agents for the treatment of serious P. aeruginosa infections [2]. However, resistance to carbapenems has risen steadily among P. aeruginosa and is often associated with resistance to other antibiotics [3].

Carbapenem resistance in P. aeruginosa typically occurs through the loss of OprD [4], an outer membrane protein regulating the entry of carbapenems. Loss of OprD is the major determinant of non-metallo-β-lactamase-mediated resistance to carbapenems [5]. In addition to inactivation of OprD, overexpression of the intrinsic efflux system (MexAB–OprM) or production of carbapenem-hydrolysing β-lactamases (AmpC) may also be related to the occurrence of carbapenem resistance [6].

Carbapenem resistance often occurs following prolonged treatment of P. aeruginosa-infected patients, and cross-resistance to other β-lactams has been observed [3]. However, rapid onset of selective carbapenem resistance during treatment occurred in 15 patients in our hospital; in all cases the isolates were initially carbapenem-susceptible (CS). Susceptibilities to all other tested β-lactams remained identical in the carbapenem-resistant (CR) and CS pairs.

This study sought to investigate the mechanism accounting for the occurrence of selective carbapenem resistance in P. aeruginosa isolates, with a hypothesis that this resistance results from a loss of OprD expression. Clinical features and risk factors associated with the occurrence of CR-only P. aeruginosa (CROPA) in these cases were also analysed.

Section snippets

Bacterial strains and microbiological investigation

From 2007 to 2011, clinical isolates of P. aeruginosa from the microbiology laboratory of Chang Gung Memorial Hospital (Taoyuan, Taiwan) isolated from the same patient during the same hospitalisation that were initially CS and then became CR were collected for investigation. All isolates were identified by standard methods [7] and antimicrobial susceptibilities were determined by the standard disc diffusion method and Etest [8]. Susceptibility and resistance were defined according to the

Results

P. aeruginosa that were initially CS and then became selectively CR during the same hospitalisation were isolated from various specimens of 15 hospitalised patients, including blood (6 cases), ascites (4), wound discharge (2), sputum (1), pleural effusion (1) and pericardial effusion (1). Table 1 shows a comparison of demographic and clinical features of the cases with persistent infection and re-infection. Of the 15 cases, the mean age was 51.3 ± 23.9 years. Re-infection cases tended to be older

Discussion

This study showed that CROPA may appear during or soon after carbapenem treatment of patients infected with CS-PA. Most of the cases represented persistent infection, as isogenic paired strains were demonstrated.

It was demonstrated that mutational inactivation of oprD, resulting in loss of the porin OprD and affecting carbapenem penetration across the outer membrane, was the main mechanism of carbapenem resistance [13]. Sequence analysis of the oprD gene showed various routes of inactivation,

Acknowledgment

The authors thank the members of the Molecular Infectious Disease Research Center (Chang Gung Memorial Hospital, Taoyuan, Taiwan) for their suggestions and assistance in this study.

Funding: This study was supported by grants from the National Science Council, Taiwan [NSC 101–2314–B–182A–046].

Competing interests: None declared.

Ethical approval: Not required.

References (15)

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