Identification of Dermatophyte and mold species by MALDI-TOF Mass Spectrometry

Routine procedures for dermatophyte and mold identification rely on an
examination of strain characteristics and microscopic morphology.
Conventional methods used to identify dermatophytes and molds are often
time-consuming and may be inconclusive because of atypical microscopic or
colony morphology. The present study evaluates a promising alternative based
on the analysis of clinical fungal isolates by MALDI-TOF mass spectrometry.
A total of 63 isolates belonging to 27 species of dermatophytes and molds were
analyzed. The isolates were obtained from reference samples
(„Ringversuchsproben“, INSTAND e.V., n= 17) as well as from reliable
identified clinical isolates from a variety of specimens including skin, nail
scrapings and hair fragments. The clinical isolates were obtained from the
University Hospital of Zurich (Department of Dermatology, n= 30) and from
the Synlab Laboratory Leinfelden (n=16). Because many fungi attached very
strong to agar, sample harvesting without agar contamination was not possible.
Therefore, the isolates were additionally inoculated in liquid culture media.
Almost 93% (59 of 63) of the MALDI-TOF identification results matched to
those of conventional methods respectively to the identity of the reference
samples. Four mismatches were obtained from clinical isolates. Trichophyton
mentagropytes var. Interdigitale was identified incorrectly as Trichophyton
tonsurans (n= 4). However these mismatches arise from database discrepancy
and could be eliminated by further development of the database.In Summary,
MALDI- TOF analyses of common dermatophytes and molds resulted in
reliable species identification. Furthermore, this procedure allowed very rapid
identification results. The isolates could be identified already after a growth
time of 4-14 days, when inoculated in liquid culture media.