Microbial typing by matrix-assisted laser desorption ionization-time of flight mass spectrometry: do we need guidance for data interpretation?

Abstract

The integration of matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) in clinical microbiology has revolutionized species identification of bacteria, yeasts, and molds. However, beyond straightforward identification, the method has also been suggested to have the potential for subspecies-level or even type-level epidemiological analyses. This minireview explores MALDI-TOF MS-based typing, which has already been performed on many clinically relevant species. We discuss the limits of the method's resolution and we suggest interpretative criteria allowing valid comparison of strain-specific data. We conclude that guidelines for MALDI-TOF MS-based typing can be developed along the same lines as those used for the interpretation of data from pulsed-field gel electrophoresis (PFGE).

FIG 1

(A) MALDI-TOF MS spectra obtained for a single strain of E. coli grown on different media. The top spectrum shows results obtained after culture on Columbia–5% sheep blood agar, whereas the data shown below were from cells grown on Drigalski agar. (B) Theoretical representation of a PFGE gel after staining. Restriction fragments are visible, and differences between the individual lanes can be explained on the basis of presumed genetic events which are indicated on the right. Based on theoretical exercises such as the one presented, adapted from Tenover et al. (3), rules suggesting that up to three fragment differences can be ignored in order to still define clear epidemiological relatedness were developed. (C) The MALDI-TOF MS analogs for the PFGE data displayed in panel B. Fragments have been translated into peaks, and the events potentially leading to changes from the reference spectrum in row A are proposed (left). This relates to the emergence and disappearance of protein peaks and distinct shifts in protein peaks for various (epi)genetic reasons.