MALDI-TOF mass spectrometry applied to identifying species of insect-pathogenic fungi from the Metarhizium anisopliae complex

Abstract

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has proven to be a powerful tool for taxonomic resolution of microorganisms. In this proof-of-concept study, we assessed the effectiveness of this technique to track the current gene sequence-based phylogenetic classification of species in the Metarhizium anisopliae complex. Initially the phylogenetic analysis of 5′ strains by sequencing of the 59′ end of the TEF-1α gene region revealed seven species within M. anisopliae sensu lato and two varieties outside this complex. Because initial studies on MS profiles from different cell types showed that mycelial fragments or conidia produced on nutrient-poor medium may yield too much background noise, all subsequent spectrometric analyses were performed with acidhydrolyzed conidia from 10–12 d old PDA cultures. The initial MALDI-TOF reference library included protein spectral profiles from nine taxonomically distinct, molecularly identified isolates sharing high genetic homology with the ex-type or ex-epitype isolates of these taxa in Metarhizium. A second reference library added one isolate each for M. anisopliae sensu stricto and M. robertsii. The second, larger reference library (including 11 taxa) allowed nearly perfect MALDI-TOF matching of DNA-based species identification for the 40 remaining isolates molecularly recognized as M. anisopliae sensu stricto (n = 19), M. robertsii (n = 6), M. majus (n = 3), M. lepidiotae (n = 1), M. acridum (n = 3), M. flavoviride var. pemphigi (n = 1), plus seven unidentified strains (six of them phylogenetically close to M. anisopliae sensu stricto and one outside the Metarhizium pingshaense-anisopliae-robertsii-brunneum clade). Due to the increasing frequency of phylogenetically (genomically) based taxonomic revisions of fungi, this approach is especially useful for culture collections, because once the protein profiles of Metarhizium isolates are obtained taxonomic updating of MALDI-TOF library data is easily accomplished by comparing stored profiles with those of newly proposed taxa.