Evaluation of phenotypic detection of carbapenemase-producing Pseudomonas spp. from clinical isolates

Carbapenems are considered last-resort antibiotics for the treatment of infections caused by multidrug-resistant Gram-negative bacteria. Although the main mechanism of carbapenem-resistance in Pseudomonas aeruginosa is the loss of OprD porin, carbapenemases continue to be a problem worldwide. The aim of this study was to evaluate the performance of phenotypic tests (Carba NP, Blue Carba, and mCIM/eCIM) for detection of carbapenemase-producing Pseudomonas spp. in Brazil. One hundred twenty-seven Pseudomonas spp. clinical isolates from different Brazilian states were submitted to phenotypic and molecular carbapenemase detection. A total of 90 carbapenemase-producing P. aeruginosa and 5 Pseudomonas putida (35, blaVIM-2; 17, blaSPM-1; 2, blaIMP-10; 1, blaVIM-24; 1, blaNDM-1; 39, blaKPC-2). The phenotypic Carba NP, Blue Carba, and mCIM/eCIM showed sensitivity of 94.7%, 93.6%, and 93.6%, and specificity of 90.6%, 100%, and 96.8%, respectively. However, only the Carba NP presented the highest sensitivity and showed the ability in differentiating the carbapenemases between class A and class B using EDTA. Blue Carba failed to detect most of the class B carbapenemases, having the worst performance using EDTA. Our results show changes in the epidemiology of the spread of carbapenemases and the importance of their detection by phenotypic and genotypic tests. Such, it is essential to use analytical tools that faithfully detect bacterial resistance in vitro in a simple, sensitive, rapid, and cost-effective way. Much effort must be done to improve the current tests and for the development of new ones.

Carba NP as a simpler, rapid, cost-effective, and a more sensitive alternative to other phenotypic tests for detection of carbapenem resistance in routine diagnostic laboratories

PURPOSE:

Resistance to carbapenems due to carbapenemases has been increasingly noticed in Enterobacteriaceae. Clinical and Laboratory Standards Institute (CLSI) has recommended the latest Carba NP (CNP) test as a confirmatory test for carbapenemase production in Enterobacteriaceae. Low sensitivity of disk diffusion (DD) and modified Hodge test (MHT) may result in missing out of resistant strains which can adversely affect clinical management. The present study compares three phenotypic tests - CNP test, DD, and MHT for detection of carbapenemase production.

MATERIALS AND METHODS:

Four hundred consecutive, nonduplicate Enterobacteriaceae isolates were tested for carbapenem resistance using ertapenem disc (10 μg) by Kirby–Bauer DD method, MHT, and CNP. These tests were performed and interpreted as per the CLSI standards. CNP was considered to be the reference test for comparison. Sensitivity, specificity, and accuracy rates for ertapenem DD and MHT were calculated.

RESULTS:

One hundred and six out of 400 strains were positive by CNP test. Of the 294 CNP-negative strains, 28 were resistant by DD and 18 were resistant by MHT. Of the 106 CNP-positive strains, 82 were resistant and 16 were intermediate by DD while 76 were positive by MHT ertapenem DD had a sensitivity and specificity of 66.04% and 90.48%, respectively. Sensitivity and specificity of MHT were 54.72% and 93.88%, respectively. There was considerable discordance between all the three tests.

CONCLUSION:

As a rapid, simple, and cost-effective test with a greater capability greater to detect carbapenemase producers, CNP can be implemented in routine diagnostic laboratories, thereby benefiting patient care and antimicrobial stewardship.

Performance of distinct phenotypic methods for carbapenemase detection: The influence of culture media

We evaluated the performance of five phenotypic tests [Modified Hodge Test (MHT); combined-disk test (CDT) using phenylboronic acid, EDTA, and cloxacillin; CarbaNP and CarbAcinetoNP; Blue-Carba, Carbapenembac™ and Carbapenembac Metallo™] for carbapenemase detection in Gram-negative bacilli (GNB). A total of 73 carbapenemase producers and 27 non-carbapenemase producers were tested. All GNB were subcultured onto Müeller-Hinton agar (MHA), MacConkey agar (MAC), and sheep blood agar (SBA). High sensitivity (100%) and specificity (100%) was observed for MHA using CarbaNP, Blue-Carba, and Carbapenembac™. The sensitivity and specificity of CarbaNP (98.6%/100%), Blue-Carba (97.1%/91.0%), and Carbapenembac™ (100%/96.5%) were slightly lower for SBA. In contrast, unacceptable sensitivity rates of CarbaNP (71.1%) and Blue-Carba (66.6%), but not Carbapenembac™ (97.3%), were observed for MAC. The colorimetric methods showed high sensitivity and specificity to detect carbapenemase production from isolates grown on MHA or SBA. However, colonies obtained from MAC must not be tested for carbapenemase detection by colorimetric methods.

Performance of CarbaNP and CIM tests in OXA-48 carbapenemase-producing Enterobacteriaceae

This study applied two phenotypic tests, namely "Carbapenemase Nordmann-Poirel" (CarbaNP) test and "Carbapenem Inactivation Method" (CIM), against the isolates carrying the carbapenem resistance genes. The study included 83 carbapenem-resistant Enterobacteriaceae isolates producing oxacillinase-48 (OXA-48) and 30 carbapenem-sensitive Enterobacteriaceae isolates. Out of the total isolates studied, 77 isolates (92.77%) were identified as Klebsiella pneumoniae and six isolates (7.23%) were identified as Escherichia coli by Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry. Polymerase chain reaction (PCR) method used to detect resistance genes found that 74 isolates (89.16%) produced OXA-48 carbapenemase, whereas nine isolates (10.84%) produced both OXA-48 and New Delhi metallo-beta-lactamase-1 (NDM-1). The isolates producing both OXA-48 and NDM-1 were found to be positive by both phenotypic tests. Among isolates carrying only bla_OXA-48 gene alone, nine isolates (13.04%) for CarbaNP test and two isolates for CIM test (2.90%) displayed false negative results, respectively. The sensitivity of CarbaNP and CIM tests was found to be 89.16% and 97.59%, respectively, whereas the specificity was determined to be 100% for both tests. These findings suggest that CarbaNP and CIM tests are useful tools to identify the carbapenemase producers. Molecular methods like PCR are recommended to verify false negative tests predicted to have OXA-48 activity.

A Pilot Study on Carbapenemase Detection: Do We See the Same Level of Agreement as with the CLSI Observations

INTRODUCTION

Rapid identification of carbapenemase producing organisms is of great importance for timely detection, treatment and implementation of control measures to prevent the spread. The Modified Hodge Test (MHT) and Carba NP test is recommended by CLSI for the detection of carbapenemases in Enterobacteriaceae. However, MHT may give false positive results or fail to detect metallo β-lactamases (MBLs). In the US, MHT is the most widely used test for detection of carbapenemases and has been found to have a sensitivity and specificity of >90% for bla KPC producers. However, in India, the prevalence of bla NDM is higher than bla KPC producers.

AIM

To evaluate the usefulness of CarbaNP in an Indian setting.

MATERIALS AND METHODS

A total of 260 isolates of carbapenem resistant E.coli (n=57), Klebsiella spp. (n=85), Pseudomonas aeruginosa (n=60), and Acinetobacter baumannii (58) isolated from clinical specimens between 2012-2014 at the Christian Medical College, Vellore were included in the study. All the carbapenem resistant isolates were subjected to CarbaNP, MHT and multiplex PCR for detection of carbapenemase genes.

RESULTS

CarbaNP was found to be positive in 88% (n=50/57), 81% (n=69/51), 38% (n=23/60) and 81% (n=47/58) for E.coli, Klebsiella spp., P. aeruginosa, and A. baumannii respectively. While in MHT it showed, 89% (n=51/57) and 81 % (n=69/85) for E.coli and Klebsiella spp. respectively. In P.aeruginosa, synergy testing of imipenem plus cloxacillin showed that, 65% of CarbaNP negatives were ampC producers. Overall, the sensitivity and specificity of CarbaNP was found to be 94% and 100 for bla NDM; 77% and 100 % for bla OXA-48 like producers and 81% and 100% for CarbAcinetoNP respectively.

CONCLUSION

This observation was more than what was reported in CLSI guidelines. Therefore, it is advisable to evaluate an assay for better laboratory diagnosis at respective regions.