Evaluation of a MALDI-TOF MS cephalosporinase assay for blood culture isolates

Abstract

Objectives: Rapid detection of beta-lactamases has the potential to improve antimicrobial stewardship and sepsis outcomes. The aim of this study was to evaluate a recently TGA-approved cephalosporinase detection method utilising matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) on blood culture isolates. This was compared to standard antimicrobial susceptibility testing and phenotypic cephalosporinase detection. Methods: A total of 114 gram negative blood culture isolates were retrospectively analysed. Isolates were selected to either be ceftriaxone non-susceptible by Vitek 2 semi-automated system (version 08.01; bioMerieux, USA, AST-N246 card) (n=31) and/or demonstrate an inducible cephalosporinase or ESBL on disc approximation testing (n=32). This cohort was compared to an equal number of ceftriaxone-susceptible isolates by Vitek 2 (n=57). The range of organisms included Escherichia coli (n=62), followed by Serratia marcescens (n=12), Pseudomonas aeruginosa (n=11), Enterobacter cloacae complex (n=10), Klebsiella aerogenes (n=5), Klebsiella pneumoniae (n=3), Klebsiella oxytoca (n=3), Morganella morganii (n=3), Acinetobacter calcoaceticus-A. baumannii complex (n=3), and Citrobacter freundii (n=2). Sub-cultured colonies were prepared with the MBT STAR-Cepha IVD Kit and third-generation cephalosporin hydrolysis was detected using the MBT STAR-BL IVD Module (Bruker Daltronics) on MALDI-TOF MS after 30 min incubation time. Results: The MALDI-TOF MS assay sensitivity and specificity for third-generation cephalosporinase activity was 61.3% and 77.1% respectively. The assay detected 11/11 (100%) ESBL producers and 5/6 (83.3%) AmpC type third-generation cephalosporinase producers. Additionally, third generation cephalosporinase activity was detected by the MALDI-TOF MS assay in 17 of 26 inducible cephalosporinase positive isolates (65.4%) that tested susceptible to ceftriaxone by Vitek 2. Of all species included, Pseudomonas aeruginosa had the lowest concordance of MALDI-TOF MS and Vitek 2 results, with only 1/11 isolates (9%) positive by MALDI-TOF MS. Conclusions: Overall the MALDI-TOF MS cephalosporinase assay lacked sensitivity in detection of third generation cephalosporinases compared to antimicrobial susceptibility testing by Vitek 2. The risk for false negative results in screening for resistance in blood culture isolates is too high to consider its implementation in the standard workflow, although there may be possible benefit in detecting ESBL and ampC type cephalosporinase activity following species identification as an alternative to standard disc approximation testing.