OXA-900, a Novel OXA Sub-Family Carbapenemase Identified in Citrobacter freundii, Evades Detection by Commercial Molecular Diagnostics Tests.

Using whole-genome sequencing and cloning of the target gene, we identified blaOXA-900 carbapenemase, a novel blaOXA belonging to a distant and distinct sub-family of blaOXA-48-like. The plasmid-mediated gene was identified in a C. freundii isolate with elevated carbapenem MICs that evaded detection by commercial DNA-based methods. The novel gene, an OXA-48 family carbapenem-hydrolyzing class D ß-lactamase, OXA-900, likely originates from marine environmental Shewanella. Since this plasmid-mediated gene has entered a member of the Enterobacterales and evades detection by commonly used tests, it may gain wide dissemination among Enterobacterales.

Multiplex lateral flow immunochromatographic assay is an effective method to detect carbapenemases without risk of OXA-48-like cross reactivity.

BACKGROUND: It is essential to detect carriers of carbapenemase-producing Enterobacterales in order to implement infection control measures. The objectives of this study was to evaluate the NG-Test® CARBA 5 (CARBA 5) assay for detection of five carbapenemases and to assess the cross reactivity of other OXA-type carbapenemases with the OXA-48-like specific antibodies. METHODS: A total of 197 Enterobacterales isolates were tested. To evaluate the cross reactivity, 73 carbapenem-resistant A. baumannii, harboring OXA-type variants, were tested. Polymerase chain reaction (PCR) served as gold standard for carbapenemase identification. RESULTS: Excellent agreement was found between PCR and CARBA 5, for all but one isolate. The single false positive result (a blaSME positive S. marcescens isolate) was incorrectly positive for blaOXA-48 by CARBA 5. No cross reactivity was observed. The sensitivity and specificity were 100.0% and 98.0%, respectively. CONCLUSIONS: The CARBA 5 assay is highly sensitive and specific and is recommended as a tool for the detection of the main carbapenemases of interest in clinical microbiology laboratories.

Performance of Rapid Polymyxin™ NP and Rapid Polymyxin™ Acinetobacter for the detection of polymyxin resistance in carbapenem-resistant Acinetobacter baumannii and Enterobacterales.

BACKGROUND: The global spread of carbapenem-resistant Enterobacterales (CRE) and Acinetobacter baumannii (CRAB) has prompted the reintroduction of colistin as a last-resort treatment. Although the recommended method for colistin susceptibility testing is broth microdilution (BMD), methods that are more rapid and easy to use are needed. OBJECTIVES: To evaluate the performance of two commercial kits for colistin susceptibility testing: Rapid Polymyxin™ NP (RP-NP) for CRE and Rapid Polymyxin™ Acinetobacter (RP-AB) for CRAB. METHODS: A total of 76 CRE and 87 CRAB isolates were collected from hospitalized patients in Europe and Israel. The isolates were subcultured twice on 5% sheep blood in tryptic soy agar. We tested colistin susceptibility using the RP-NP and RP-AB kits and compared the results with those from BMD. RESULTS: Of the CRE isolates, 25% (19/76) were resistant to colistin using BMD. Categorical agreement between RP-NP and BMD was 93.4% (71/76), major errors 1.8% (1/57) and very major errors 21.1% (4/19). Sensitivity was 78.9% and specificity was 98.2%. Of the CRAB isolates, 58.6% (51/87) were resistant to colistin by BMD. Categorical agreement between RP-AB and BMD was 59.8% (52/87), major errors 13.9% (5/36) and very major errors 58.8% (30/51). Sensitivity of RP-AB was 41.2% and specificity was 86.1%. CONCLUSIONS: In many of the tested isolates, weak or inconclusive colour changes in the test wells caused difficulty in interpretation, resulting in an unacceptable rate of very major errors.