There is limited experience using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry for the typing of isolates in outbreak investigations. The performance of MALDI-TOF typing was evaluated against two different reference methods in the context of a neonatal intensive care unit (NICU) outbreak of invasive Serratia marcescens.
Outbreak-related patient and environmental isolates and frozen isolates of S. marcescens within the past ten years were sub-cultured and subjected to full protein extraction. Each extracted isolate was applied to eight target spots and run in triplicate on the MALDI-TOF. A master spectrum was compiled for each isolate using MALDI Biotyper 3 software. A dendrogram was created using this collection of master spectra and compared to results obtained from two traditional typing methods, pulse field gel electrophoresis (PFGE) and random amplification of polymorphic DNA (RAPD).
Overall, 19 invasive and non-invasive samples from 14 patients and 11 environmental samples were included. Four blood cultures from 3 different patients were found to harbor identical S. marcesens strains using PFGE and RAPD typing methods. These isolates appeared as a cluster on the MALDI-TOF dendrogram, with no distance between 3 of them and minimal distance separating the fourth one from the triad. The environmental isolates were demonstrated to be unrelated to all patient isolates using PFGE. Likewise, the MALDI-TOF dendrogram displayed two major clades, which predominantly separated the patient and environmental isolates.
In a small invasive S. marcescens NICU outbreak, typing by MALDI-TOF performed well compared to traditional methods. A particular attraction of this method is that it is increasingly available at clinical laboratories and can provide rapid results to inform infection control measures in real time. Further characterization of the strain discriminatory ability of the MALDI-TOF could expand its utility as a rapid way of typing in an outbreak setting as an adjunct to slower and less readily accessible typing methods.
J. Osowicki, None
D. Goldfarb, None
P. Tilley, None
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