Comparative proteomic profiles of Aspergillus fumigatus and Aspergillus lentulus strains by surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF-MS)

Ribosomal subunit protein typing using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the identification and discrimination of Aspergillus species

Background

Accurate identification of Aspergillus species is a very important subject. Mass spectral fingerprinting using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) is generally employed for the rapid identification of fungal isolates. However, the results are based on simple mass spectral pattern-matching, with no peak assignment and no taxonomic input. We propose here a ribosomal subunit protein (RSP) typing technique using MALDI-TOF MS for the identification and discrimination of Aspergillus species. The results are concluded to be phylogenetic in that they reflect the molecular evolution of housekeeping RSPs.

Results

The amino acid sequences of RSPs of genome-sequenced strains of Aspergillus species were first verified and compared to compile a reliable biomarker list for the identification of Aspergillus species. In this process, we revealed that many amino acid sequences of RSPs (about 10–60%, depending on strain) registered in the public protein databases needed to be corrected or newly added. The verified RSPs were allocated to RSP types based on their mass. Peak assignments of RSPs of each sample strain as observed by MALDI-TOF MS were then performed to set RSP type profiles, which were then further processed by means of cluster analysis. The resulting dendrogram based on RSP types showed a relatively good concordance with the tree based on β-tubulin gene sequences. RSP typing was able to further discriminate the strains belonging to Aspergillus section Fumigati.

Conclusions

The RSP typing method could be applied to identify Aspergillus species, even for species within section Fumigati. The discrimination power of RSP typing appears to be comparable to conventional β-tubulin gene analysis. This method would therefore be suitable for species identification and discrimination at the strain to species level. Because RSP typing can characterize the strains within section Fumigati, this method has potential as a powerful and reliable tool in the field of clinical microbiology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-1009-3) contains supplementary material, which is available to authorized users.

Aspergillus fumigatus-Related Species in Clinical Practice

Aspergillus fumigatus is the main etiologic agent of invasive aspergillosis (IA). Other Aspergillus species belonging to the section Fumigati (A. fumigatus complex) may occasionally be the cause of IA. These strains are often misidentified, as they cannot be distinguished from A. fumigatus by conventional morphological analysis and sequencing methods. This lack of recognition may have important consequences as these A. fumigatus-related species often display some level of intrinsic resistance to azoles and other antifungal drugs. A. lentulus, A. udagawae, A. viridinutans, and A. thermomutatus (Neosartorya pseudofischeri) have been associated with refractory cases of IA. Microbiologists should be able to suspect the presence of these cryptic species behind a putative A. fumigatus isolate on the basis of some simple characteristics, such as defect in sporulation and/or unusual antifungal susceptibility profile. However, definitive species identification requires specific sequencing analyses of the beta-tubulin or calmodulin genes, which are not available in most laboratories. Multiplex PCR assays or matrix-assisted laser desorption ionization - time-of-flight mass spectrometry (MALDI-TOF MS) gave promising results for rapid and accurate distinction between A. fumigatus and other Aspergillus spp. of the section Fumigati in clinical practice. Improved diagnostic procedures and antifungal susceptibility testing may be helpful for the early detection and management of these particular IA cases.