Genus-level identification of dermatophytes by MALDI-TOF MS after 2 days of colony growth

Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) is becoming a popular technology in clinical microbiology. It is a fast and highly specific method for the routine identification of micro-organisms. In this study, we evaluated the suitability of dermatophyte identification after only 2 days of colony growth using MALDI-TOF MS. Two protein extraction protocols were also evaluated consisting of either formic acid alone or of ethanol-formic acid-acetonitrile to achieve a complete protein extraction. Morphology-based techniques were used as the diagnostic standard methods and MALDI-TOF MS results were obtained using the manufacturer's spectral library. Using the formic acid protein extraction protocol after 2 days of colony growth, 70 and 46% of dermatophytes were properly identified at the genus and species-level respectively. The addition of ethanol-formic acid-acetonitrile extraction protocol increased the identification to 90 and 62%. Based on our observations, we propose a two-step workflow for the fast and reliable identification of dermatophytes after only 2 days of colony growth. This flow chart consists of a first direct deposition procedure with the addition of formic acid, followed by a complete protein extraction when dermatophyte identification is not successful.

In this study, a two-step workflow for the identification of clinical dermatophytes using MALDI-TOF analysis and commercially available spectral library was developed. The workflow consists of an initial direct deposition of the sample on the MALDI plate and formic acid protein extraction at 2 days of growth culture; if dermatophyte identification is not successful, a complete protein extraction using ethanol-formic acid-acetonitrile is subsequently performed. Using this workflow, the correct isolate identifications increase up to 90%; of these, 27% are identified at the genus-level, providing sufficient information to start an antifungal treatment. The method here proposed represents a fast and useful approach to differentiate dermatophytes grown in culture.