Perhaps the most significant development in clinical bacteriology over the last decade has been the global proliferation of Enterobacterales with acquired carbapenemase enzymes (CPE).1 Available data suggest that the vast majority of carbapenemases in Enterobacterales belong to one of the five major families: IMP, NDM, and VIM metallo-enzymes, and the Klebsiella pneumoniae carbapenemase (KPC) and OXA-48-like enzymes.2 CPE are frequently resistant to virtually all β-lactam antibiotics (including carbapenems) and carbapenemase genes are frequently transmissible via plasmids. Concomitant resistance to several other antimicrobial classes is common in CPE, dramatically reducing treatment options for infected patients.
Of particular concern among CPE are producers of OXA-48-like carbapenemase (hereafter referred to as ‘OXA-48’) that are already dominant among CPE in many European countries and are threatening to become dominant in the UK. Producers of OXA-48 can be particularly difficult to detect as they do not always confer resistance to carbapenems (particularly meropenem), and there is no specific inhibitor to assist in their detection. The MASTDISCS® Combi OXA (Mast Group Ltd., Bootle, UK) set for OXA-48 detection and carbapenemase screening is a kit comprising three discs. Disc A contains temocillin with KPC and metallo-β-lactamase (MBL) inhibitors, Disc B contains temocillin plus avibactam, and Disc C contains a penem antibiotic. The kit is intended to detect carbapenemase-producing Enterobacterales (CPE) and differentiate isolates with OXA-48-like enzymes from those with KPC or MBL. The authors describe here the first evaluation of this assay using standard European Committee on Antimicrobial Susceptibility Testing (EUCAST)/Clinical and Laboratory Standards Institute
MATERIAL AND METHODS
The kit was evaluated using a diverse collection of 208 well characterised Enterobacterales including carbapenemase-producers (CPE: n=159), isolates with extended spectrum β-lactamase (ESBL) and/or AmpC β-lactamase (n=47), and two control strains. Susceptibility testing was performed using EUCAST methodology on Mueller–Hinton agar. After overnight incubation, inhibition zone diameters were measured and the presence of carbapenemases was inferred following the
All of the CPE isolates (n=159) were correctly assigned as being carbapenemase-producers (sensitivity: 100%; specificity: 92%). Four out of 49 other isolates were incorrectly assigned as carbapenemase-producers, including isolates with TEM-10, LAT, and two isolates with DHA-1. Of the isolates with OXA-48-like enzymes, 61 out of 62 were correctly assigned as OXA-48-like producers with one isolate inferred to be a producer of KPC or MBL. Of 97 isolates with KPC or MBL, all but one were correctly assigned as KPC/MBL producers. A single isolate with a combination of carbapenemases (VIM and OXA-48) was assigned as an OXA-48 producer. Finally, one isolate of CPE with non-metallo-carbapenemase Class A (NMC-A) was falsely assigned as KPC/MBL.
The kit performed very well as a screening test with a challenging set of bacterial isolates. Most importantly, the presence of a carbapenemase was predicted with absolute sensitivity (100%) and high specificity (91.8%). Only 4 out of 49 non-CPE would require additional investigation as possible CPE, and the vast majority of these 49 isolates expressed ESBL or AmpC activity. This combination of three discs could be included with other antimicrobials for routine testing of Enterobacterales in clinical laboratories using EUCAST methodology.