Evaluation of MALDI-TOF MS for molecular typing of Acinetobacter baumannii in comparison with orthogonal methods

Colonisation and subsequent healthcare associated infection (HCAI) with Acinetobacter baumannii is a concern for vulnerable patient groups within the hospital setting. Outbreaks involving multi drug-resistant strains are associated with increased patient morbidity, mortality and poorer overall outcomes. Reliable molecular typing methods can help to trace transmission routes and manage outbreaks. In addition to methods deployed by reference laboratories, Matrix Assisted Laser Desorption/Ionisation-Time of Flight Mass Spectrometry (MALDI-TOF MS) may assist by making initial in-house judgements on strain relatedness. However, limited studies on method reproducibility exist for this application. We applied MALDI-TOF MS typing to A. baumannii isolates associated with a nosocomial outbreak and evaluated different methods for data analysis. In addition, we compared MALDI-TOF MS with whole genome sequencing (WGS) and Fourier-transform infrared spectroscopy (FTIR) as orthogonal methods to further explore their resolution for bacterial strain typing. A related sub-group of isolates consistently clustered separately to the main outbreak group by all investigated methods. This finding, combined with epidemiological data from the outbreak, indicates that these methods identified a separate transmission event unrelated to the main outbreak. However, the MALDI-TOF MS upstream approach introduced measurement variability impacting upon method reproducibility and limiting its reliability as a stand-alone typing method. Emerging typing methods which offer different degrees of resolution, and target different biomolecules, could be utilised in-house to more quickly confirm or rule out transmission events. This work highlights some of the steps to be improved before such tools can be fully integrated into routine diagnostic service workflows for strain typing.