Improved diagnostic modalities to accommodate phenotypic variation among Malassezia pachydermatis clinical isolates

This study investigated strain variation that affects traditional diagnostic Malassezia pachydermatis phenotypes and identification of the yeast using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In the context of characterizing M. pachydermatis isolates from dogs with clinically diagnosed otitis externa, we noticed that isolates assigned to Clade II within a 3-clade phylogeny were phenotypically different from the others including the Clade I type strain (ATCC 14522/CBS 1879). On Sabouraud Dextrose Agar, Clade II isolates grew as tan-colored, flat colonies with a wet texture that was distinct from the opaque, cream-to-white, dry-to-waxy colonies for Clade I and Clade III strains. On CHROMagar Malassezia, Clade II colonies were dark pink or purple with a matte surface appearance which differed from the pale pink, shinier colonies of the type strain. Although Clade I and Clade III isolates were successfully identified to the species level using MALDI-TOF MS with existing main spectra profiles (MSPs) in the Bruker BDAL library, Clade II isolates could not be identified. To improve this diagnostic niche, a custom Clade II MSP was created that produced confident species-level identification for all Clade II isolates across culture conditions relevant to diagnostic laboratory processes. This effort also identified a reproducible mass spectral peak (8,700 ─ 8,800 m/z) that was diagnostic for clade assignment. These results, taken together, expand the phenotypic description for M. pachydermatis and facilitate rapid and reliable identification of the yeast in a diagnostic setting.