Reply to “Matrix-Assisted Laser Desorption Ionization-Time of Flight identification of Schizophyllum commune: Perspectives on the review by Chowdhary et al.”

Abstract

SIR: We acknowledge the observations by Antoinea et al.1 regarding rapid identification of filamentous basidiomycetes (FBM) using Matrix-Assisted Laser Desorption/Ionisation-Time of Flight Mass Spectrophotometry (MALDI-TOF MS) in response to our review.2 Today, based on protein mass spectral profiles, MALDI-TOF MS is adapted for use in microbiology laboratories for accurate microbial identification.3 However, in the present scenario it will remain an ongoing challenge to identify filamentous and nonsporulating fungi using this technique due to various protocols employed depending on the taxa studied.4,5 Recently, few studies have reported reproducible protocols for the identification of filamentous fungi such as species of Aspergillus, Penicillium, Fusarium, Trichophyton and Mucorales.5,6 Furthermore, proposed methods were applied to limited number of isolates and majority of isolates belong to Aspergillus species.6 Although, a recent comprehensive study by Ranque et al. analyzed 625 nondermatophyte filamentous fungi isolated from five teaching hospitals of France by MALDI and conventional methods.3 The authors concluded that MS compared with conventional method significantly improves species identification but the main limitation is that MS identification is possible only if the species is included in the reference spectra library.3 In this respect attention is called for the fact that for non-sporulating moulds especially basidiomycetes database in the latest version of Bruker V3.2.1.1 is only limited to three isolates of single species, that is, Schizophyllum commune. We had summarized earlier that many species of FBM such as Ceriporia lacerata,7Hormographiella aspergillata, Bjerkandera adusta, Volvariella volvacea, Inonotus tropicalis, Oxyporus corticola, Irpex lacteus, Perenniporia species8 are associated with pulmonary and fatal deep-seated mycoses.2,9 Notably, only a few previous studies had characterized FBM by MALDI-TOF MS.5,6,10 Schmidt and Kallow identified basidiomycetes associated with indoor wood decayed fungi including species of Serpula, Coniophora and Antrodia, none of which has been implicated in human infections.10 Also, Lau et al. from National Institute of Health (NIH), developed a comprehensive database for identification of clinically relevant molds using solid media by MALDI found that of the 7.8% (n = 33) isolates which yielded no identification of MALDI-TOF MS, 25 were basidiomycetes not associated with clinical disease.5 We noted that Antonia et al. also identified a solitary isolate of S. commune cultured from a case of sinusitis using MALDI-TOF MS.1 In our experience creation of an in-house library of mass spectra profile (MSP) of 10 S. commune isolates from cases of allergic bronchopulmonary mycoses and invasive pulmonary mycoses yielded identification of another 10 molecularly characterized S. commune with ≥2 score values. In contrast with Bruker database low score of 1.7–2 was observed for these newly identified 10 S. commune isolates. Also, it is pertinent to mention that a modified ethanol formic acid extraction protocol was used based on freshly grown cultures of S. commune on Sabouraud dextrose agar, as previously described by Ranque et al.3 for filamentous fungi. Notably, Lau et al. developed NIH mold database of 294 mold isolates and blindly challenged 421 clinical isolates. The NIH mold database provided species-level (score ≥2) identification for 370 (88.9%) while the most updated Bruker library alone (August 2012) identified only 3 isolates (0.7%).5 Although the Bruker database has been upgraded in latest version but the necessity of the development of in house database specifically of the isolates, which are locally epidemiologically important, is hardly emphasized.11 Finally, so far there is paucity of studies on application of MALDI-TOF MS for identification of clinically relevant broad-spectrum basidiomycetes, therefore, DNA based methods are still warranted for their classification and identification. Future studies on characterization of MSP of basidiomycetes from different geographical regions including sizeable number of species may establish MALDI-TOF as a robust method for their identification.