Performance Evaluation of the MBT STAR®-Carba IVD Assay for the Detection of Carbapenemases With MALDI-TOF MS

Abstract

Objectives: The increasing rate of carbapenem resistance in Gram-negative bacteria is a major public health problem and rapid detection is essential for infection management. We evaluated the performances of the MBT STAR^®-Carba IVD assay (Bruker Daltonics) to detect carbapenemase-producing organisms (CPO) from bacterial colonies and directly from positive blood culture bottles with MALDI-TOF MS. Methods: We analyzed 130 strains with a reduced susceptibility to at least one carbapenem including 109 CPO (6 KPC, 27 NDM, 21 VIM, 1 IMP, 41 OXA-48-like, 8 OXA-23, 2 OXA-24/-40, and 2 OXA-58) and 21 non-CPO. The assay on colonies was performed with all 130 strains while the assay on spiked blood cultures was performed with 45 strains. Samples were prepared with the MBT STAR^®-CARBA IVD kit and imipenem hydrolysis by the potential carbapenemase was analyzed with the MBT STAR^®-BL module (Bruker Daltonics) on MALDI-TOF MS. Results: Performed on colonies, the assay detected all carbapenemase-producing Enterobacteriaceae (n = 78), Pseudomonas spp. (n = 19) and Acinetobacter spp. (n = 12). All 21 tested non-CPO remained negative resulting in sensitivity and specificity of 100%. Performed on positive blood cultures, the assay detected all carbapenemase-producing Enterobacteriaceae (n = 23) and Pseudomonas spp. (n = 4) but missed 9/12 carbapenemase-producing Acinetobacter spp. However, a prolonged imipenem-incubation time of the strain pellet improved carbapenemase detection. Non-CPO from positive blood culture bottles remained negative (n = 5) with the assay with the exception of one Klebsiella pneumoniae isolate. Conclusion: The MBT STAR^®-Carba IVD assay is a highly reliable method for the detection of carbapenemase activity in Gram-negative bacteria. However, time-consuming sample preparation steps and reagent costs need to be considered before implementation in a routine clinical microbiology laboratory.

Keywords: Gram-negative bacteria; MALDI-TOF MS; carbapenemase detection; hydrolysis assay; performances; positive blood cultures.

FIGURE 1

Scheme illustrating the MBT STAR^®-Carba IVD workflow from colonies and from spiked blood cultures. One to five colonies from overnight cultures or the pellet obtained from positive blood cultures by the MBT Sepsityper kit were mixed with the reconstituted MBT STAR^®-Carba Antibiotic Reagent containing imipenem. After incubation and centrifugation, cell-free supernatant is spotted onto a MALDI target plate and overlaid with matrix. Spectra are then acquired using the MALDI Biotyper^®smart system and analyzed by the MBT STAR^®-BL IVD module.

FIGURE 2

Results overview in the MBT STAR^®-BL Software Module. The software monitors the carbapenemase activity on acquired mass spectra (4 mass spectra per isolate) by automatic calculation of the intact Imipenem intensity and corresponding ratio hydrolysed / non-hydrolysed. Results are normalized to signal intensities obtained from positive and negative control strains (normalized logRQ value). Color-coded plot indicates presence (orange) or absence (blue) of carbapenemase activity in bacterial strains with the horizontal lines representing the limit for positive and negative results.