Evaluation of two commercial carbapenemase gene assays, the Rapidec Carba NP test and the in-house Rapid Carba NP test, on bacterial cultures

The Novel CarbaLux Test for Carbapenemases and Carbapenem Deactivating AmpC Beta-Lactamases

Objectives

To evaluate the rapid phenotypic CarbaLux test for routine diagnostics in the medical laboratory in a proof of concept study.

Methods

isolates of Gram-negative bacteria suspicious for carbapenem resistance including Enterobacterales (67), Pseudomonas (10), Acinetobacter (5), and Stenotrophomonas (1) species, collected between 2016 and 2018 from in-patients, were tested for carbapenemase activity using a novel fluorescent carbapenem. When subjected to extracted bacterial carbapenemases its fluorescence disappears. All bacteria to be tested were cultured on Columbia blood agar and few on other commercial media. MALDI TOF MS, molecular assays, automated MIC testing, and in part, agar diffusion tests served to characterize the isolates. For comparison, few selected bacteria were also investigated by prior phenotypic tests for carbapenemase detection.

Results

Under UV light, the CarbaLux test allowed a rapid detection of 39/39 carbapenemase-producing bacteria, including 15 isolates with OXA carbapenemases (e.g., OXA-23, OXA-24/40-like OXA-48-like or OXA-181). Several isolates had low MICs but still expressed carbapenemases. Among Enterobacter spp., it detected six strains with hyper-produced AmpC beta-lactamases, which deactivated carbapenems but were not detectable by prior rapid phenotypic assays. An unexpected high carbapenemase activity appeared with these enzymes. They were identified as AmpC variants by inhibition with cloxacillin.

Conclusion

Other than prior rapid phenotypic assessments for carbapenemases, which use secondary effects such as a change of pH, the inactivation of the fluorescent carbapenem substrate can be visualized directly under UV light. The new test works at 100 to 200-fold lower, therapy-like substrate concentrations. It takes advantage of the high substrate affinity to carbapenemases allowing also the detection of less reactive resistance enzymes via a trapping mechanism, even from bacteria, which might appear unsuspicious from initial antibiograms. The novel fluorescence method allows simple and safe handling, reliable readings, and documentation and is suitable for primary testing in the clinical laboratory.

Multicenter evaluation of the RAPIDEC® CARBA NP assay for the detection of carbapenemase production in clinical isolates of Enterobacterales and Pseudomonas aeruginosa

Carbapenem-resistant Gram-negative bacilli are a major public health problem. Accurate and rapid detection of carbapenemase-producing organisms can facilitate appropriate infection prevention measures. The objective was to evaluate the performance of the RAPIDEC® CARBA NP assay (RAPIDEC), a screening assay that utilizes a pH indicator to detect carbapenem hydrolysis within 2 h. A multicenter study evaluated 306 clinical bacterial strains of Enterobacterales (n = 257) and Pseudomonas aeruginosa (n = 49). The RAPIDEC was compared to a composite reference standard-the Clinical Laboratory Standards Institute (CLSI) Carba NP assay, PCR for specific carbapenemase genes (bla_KPC, bla_NDM, bla_OXA-48-like, bla_VIM and bla_IMP), and phenotypic carbapenem susceptibility testing. The assay was evaluated using two culture incubation times for the bacterial isolates: "routine"(cultures incubated 18-24 h) and "short" (cultures incubated 4-5 h). For the routine incubation, the overall percent agreement was 98.7% with a positive percent agreement (PPA) of 99.6% and a negative percent agreement (NPA) of 97.4%; there were five false positives and one false negative. For the short incubation, the overall percent agreement was 98.0% with a PPA of 98.5% and a NPA of 97.3%; there were five false positives and four false negatives. RAPIDEC results for the P. aeruginosa isolates were 100% concordant with the reference standard for both incubation times. The RAPIDEC assay is an accurate and rapid (≤ 2 h) assay for the detection of the most common carbapenemases in clinical isolates. Growth from a short incubation culture may be used to reliably detect carbapenemase production in clinical strains.

Carba NP test for the detection of carbapenemase-producing Pseudomonas aeruginosa

INTRODUCTION

The Carba NP test is a biochemical chromogenic assay developed to detect carbapenemase activity. Variable performance has been reported according to the type of carbapenemase and bacterial species involved. We aimed to describe the benefit of the Carba NP test and its commercial version, the RAPIDEC® CARBA NP, to detect carbapenemase-producing Pseudomonas aeruginosa.

METHODS

PubMed and ScienceDirect databases were searched. The following data was collected from each included study: research protocol, molecular profile of the tested strains, and sensitivity and specificity of the test used to detect carbapenemase-producing P. aeruginosa.

RESULTS

Thirty-four studies were included. The most frequently tested strains were metallo-beta-lactamase producers. The pooled sensitivity to detect carbapenemase-producing P. aeruginosa with the original Carba NP test, the Clinical and Laboratory Standards Institute (CLSI) Carba NP test, and the RAPIDEC® CARBA NP was 92%, 95%, and 96%, respectively. The pooled specificity was 99% with the original and the CLSI Carba NP tests, and 92% with the RAPIDEC® CARBA NP. Several studies evaluated modified versions of the Carba NP test to detect carbapenemase-producing P. aeruginosa, with reported sensitivity and specificity exceeding 90% in most cases.

CONCLUSION

The Carba NP test allows for fast screening and easy handling as well as optimal performance to detect carbapenemase-producing P. aeruginosa. These findings should be confirmed by further studies including a larger cohort of isolates and various types of carbapenemases, mainly non-metallo-beta-lactamases.

One System for All: Is Mass Spectrometry a Future Alternative for Conventional Antibiotic Susceptibility Testing?

The two main pillars of clinical microbiological diagnostics are the identification of potentially pathogenic microorganisms from patient samples and the testing for antibiotic susceptibility (AST) to allow efficient treatment with active antimicrobial agents. While routine microbial species identification is increasingly performed with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), routine AST still largely relies on conventional and molecular techniques such as broth microdilution or disk and gradient diffusion tests, PCR and automated variants thereof. However, shortly after the introduction of MALDI-TOF MS based routine identification, first attempts to perform AST on the same instruments were reported. Today, a number of different approaches to perform AST with MALDI-TOF MS and other MS techniques have been proposed, some restricted to particular microbial taxa and resistance mechanisms while others being more generic. Further, while some of the methods are in a stage of proof of principles, others are already commercialized. In this review we discuss the different principal approaches of mass spectrometry based AST and evaluate the advantages and disadvantages compared to conventional and molecular techniques. At present, the possibility that MS will soon become a routine tool for AST seems unlikely – still, the same was true for routine microbial identification a mere 15 years ago.

Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry for the Rapid Detection of Antimicrobial Resistance Mechanisms and Beyond

Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has been successfully applied in recent years for first-line identification of pathogens in clinical microbiology because it is simple to use, rapid, and accurate and has economic benefits in hospital management. The range of clinical applications of MALDI-TOF MS for bacterial isolates is increasing constantly, from species identification to the two most promising applications in the near future: detection of antimicrobial resistance and strain typing for epidemiological studies. The aim of this review is to outline the contribution of previous MALDI-TOF MS studies in relation to detection of antimicrobial resistance and to discuss potential future challenges in this field. Three main approaches are ready (or almost ready) for clinical use, including the detection of antibiotic modifications due to the enzymatic activity of bacteria, the detection of antimicrobial resistance by analysis of the peak patterns of bacteria or mass peak profiles, and the detection of resistance by semiquantification of bacterial growth in the presence of a given antibiotic. This review provides an expert guide for MALDI-TOF MS users to new approaches in the field of antimicrobial resistance detection, especially possible applications as a routine diagnostic tool in microbiology laboratories.

A novel GoldNano Carb test for rapid phenotypic detection of carbapenemases, particularly OXA type, in Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter spp

Objectives

To develop a simple gold nanoparticle (AuNP)-based colorimetric test, GoldNano Carb (GoldC), for detecting carbapenemase production in Gram-negative bacteria, compared with updated Carba NP (CNP) and CarbAcineto NP (CAcNP) tests by using PCR methods as gold standard.

Methods

Ninety-nine carbapenemase-producing Enterobacteriaceae (CPE), Pseudomonas spp. and Acinetobacter spp. isolates and 89 non-CPE isolates were tested by the GoldC and CNP. Additionally, the CAcNP was performed in the Acinetobacter spp. isolates. The final imipenem (imipenem/cilastatin form) concentration was 5 mg/mL for all three tests. For the GoldC, the imipenem powder was added directly to bacterial cell suspension in distilled water prior to detection of acid product by the citrate-capped AuNP solution. An AuNP change from red to purple, blue or green indicates carbapenemase activity.

Results

The GoldC detected all carbapenemase producers except one OXA-23-like producer (99.0% sensitivity), whereas 11 carbapenemase producers (10 Acinetobacter and 1 P. aeruginosa) were CNP negative (88.9% sensitivity). However, the GoldC and CNP provided 100% and 98.6% sensitivity, respectively, for the CPE and Pseudomonas spp. Both tests gave one false positive from CTX-M-1-like-producing Enterobacter spp. (98.9% specificity). The GoldC and CAcNP detected 96.7% and 93.3% of the Acinetobacter spp. isolates, respectively. Interestingly, times to positivity by the GoldC were markedly shorter than those by the CNP (76.8% versus 36.2% positive at 5 min) and CAcNP (43.3% at 5 min versus 20% within 30 min).

Conclusions

The GoldC is fast, easy, highly sensitive and inexpensive (∼$0.25 per test), suggesting that it may be suitable for routine carbapenemase detection in low-resource settings for infection control or epidemiological purposes.

Multicenter evaluation of the RAPIDEC® CARBA NP test for rapid screening of carbapenemase-producing Enterobacteriaceae and Gram-negative nonfermenters from clinical specimens

The rapid diagnosis of carbapenemase-producing (CP) bacteria is essential for the management of therapy and infection control. In this study, RAPIDEC® CARBA NP (RCNP) was evaluated for the rapid screening of CP Enterobacteriaceae, Acinetobacter baumannii complex, and Pseudomonas aeruginosa from clinical specimens collected at five Italian hospitals. Firstly, each site tested 20 well-characterized strains in a blinded fashion. Secondly, each center prospectively tested 25 isolates from blood cultures processed with a rapid workflow (6h after subculture) and 25 isolates from other specimens processed after an overnight culture. The presence of carbapenemases was confirmed by multiplex real-timePCRs targeting carbapenemase genes. RCNP presented an overall sensitivity, specificity, positive predictive value, and negative predictive value of 70%, 94%, 82%, and 89%, respectively, with a higher performance in detection of CP Enterobacteriaceae and a poorer performance in detection of CP A. baumannii complex. With isolates from blood cultures, RCNP could significantly reduce the time required for identification of CP Enterobacteriaceae (less than 9h since the positivization of blood cultures).