Evaluation of CHROMagar™ KPC for the detection of carbapenemase-producing Enterobacteriaceae in rectal surveillance cultures

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Abstract

In this study, the performance of the chromogenic medium CHROMagar™ KPC was evaluated and was compared with in-house-daily prepared McConkey agar plates supplemented with imipenem (1 mg/L) for the detection of carbapenemase-producing Enterobacteriaceae. In this surveillance study, rectal swabs were cultured on both media and polymerase chain reaction (PCR) for blaKPC and blaVIM was used to confirm the genotype of growing colonies of Enterobacteriaceae. CHROMagar KPC was also tested with 17 genotypically characterised carbapenemase-producing and non-producing Gram-negative bacteria. It was shown that CHROMagar allows rapid detection of carbapenemase-producing Enterobacteriaceae, although blaKPC- and blaVIM-harbouring isolates could not be differentiated by colour or colony morphology. The positive and negative predictive values of the tested methods for the detection of carbapenemase-producing Enterobacteriaceae were, respectively, 100% and 98.8% for CHROMagar KPC and 94.7% and 88.6% for imipenem-supplemented McConkey agar. CHROMagar KPC medium is a useful screening medium for carbapenemase-producing Enterobacteriaceae in stools in settings with a high proportion of patients colonised with a variety of carbapenemase-producers.

Introduction

Infections caused by carbapenem-resistant Enterobacteriaceae are an emerging problem associated with high rates of morbidity and mortality, particularly amongst critically ill patients [1], [2]. Carbapenem-resistant Enterobacteriaceae are usually resistant not only to β-lactam antimicrobials but also to most other classes of antimicrobial agents [2].

The most important mechanism of resistance to carbapenems is carbapenemase production. KPC and VIM are the most prevalent carbapenemases produced by Enterobacteriaceae not only in Greece but also in several other countries worldwide [3], [4], [5], [6], [7], [8].

Patients colonised with carbapenem-resistant Enterobacteriaceae are thought to be the source of transmission in healthcare settings [7], [9]. Surveillance cultures are useful in identifying such patients in order to implement infection control measures rapidly. Accurate detection of colonisation with carbapenem-resistant Enterobacteriaceae within a short time from sampling contributes to the effectiveness of the above measures as well as to the adequacy of the prescribed empirical antimicrobial treatment in severely ill patients [10], [11].

Commercially prepared media for isolation of vancomycin-resistant enterococci and meticillin-resistant Staphylococcus aureus (MRSA) have facilitated cultivation techniques for rapid evaluation of gastrointestinal colonisation. Until recently, detection of carbapenem-resistant Enterobacteriaceae has been troublesome and was performed using in-house-prepared selective media such as agar or tryptic soy broth containing a 10 μg disk of imipenem, meropenem or ertapenem [9], [11], [12] or with polymerase chain reaction (PCR)-based techniques [13], each with its own advantages and disadvantages. CHROMagar™ KPC (Hy-Labs, Rehovot, Israel) is a commercially prepared chromogenic solid medium supplemented with agents that inhibit the growth of carbapenem-sensitive bacteria. Following 24 h of incubation, carbapenem-resistant Enterobacteriaceae colonies appear with different colours according to their specific enzymatic properties: Escherichia coli appear as red colonies, Klebsiella spp., Enterobacter spp. and Citrobacter spp. as metallic blue and Pseudomonas spp. as translucent cream colonies [14].

In the Intensive Care Unit (ICU) of University General Hospital ‘Attikon’ (Athens, Greece), in order to achieve early detection of carbapenem-resistant Gram-negative bacteria, surveillance cultures (rectal swabs and bronchial secretions) are performed twice weekly in all ICU patients. Samples are plated on in-house-prepared McConkey agar supplemented with antibiotics. In the present study, the performance of CHROMagar KPC was compared with that of the in-house-daily prepared McConkey agar plates supplemented with imipenem for the detection of carbapenem-resistant Enterobacteriaceae.

Section snippets

Preliminary studies

To evaluate the reliability of the new CHROMagar KPC and its applicability for detecting carbapenem-resistant Gram-negative pathogens harbouring different resistance mechanisms, 17 genotypically characterised clinical isolates were studied, including 12 carbapenemase-producing Enterobacteriaceae, 3 carbapenemase-producing non-fermenters and 2 meropenem-resistant non-carbapenemase producing Pseudomonas aeruginosa isolates (Table 1). The colour and morphological characteristics of the colonies

Preliminary studies

All tested strains grew on CHROMagar KPC. Klebsiella pneumoniae, Enterobacter cloacae and Enterobacter aerogenes harbouring blaVIM and/or blaKPC yielded large steel blue colonies at 24 h. blaVIM- or blaKPC-harbouring E. coli and Citrobacter freundii grew as pink colonies or pink with darker centre colonies, respectively, whereas Proteus mirabilis grew as colonies with a brown halo. The two types of carbapenemases (VIM or KPC) could not be differentiated by the colony morphology of producing

Discussion

Rapid detection of antibiotic-resistant nosocomial pathogens in the gastrointestinal tracts of patients is considered a necessary step in successful infection control protocols [2], [22]. It provides useful information on the prevalence of colonisation by multiresistant pathogens and dictates strict implementation of additional infection control measures. Obtaining results in a short time interval and taking immediate action in terms of infection control prevents further dissemination of

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    Present address: 6th Department of Internal Medicine, Diagnostic and Therapeutic Center of Athens ‘Hygeia’, 4 Erythrou Stavrou Str. and Kifissias Ave., 151 23 Maroussi, Greece.

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