Yeast in vitro susceptibility testing and impact of their phylogenetic relationship

A. Schmalreck

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Objectives: Clinical important ascomycetous yeasts isolates (CAYI) are grouped traditionally according to their species identification in anamorphic (ANA) form genera, which are polyphyletic (e.g. Candida, Geotrichum).The aim of this study was to evaluate whether grouping of CAYI in corresponding teleomorphic (TELE) genera or in phylogenetic (PHYLO) groups results in a more coherent susceptibility pattern when comparing with their placement in the current used ANA groups. Methods: In a recently performed German antifungal multicenter study the minimum inhibitory concentration (MIC) of voriconazole (VOR), itraconazole (ITR), fluconazole (FLC), and flucytosine (FCY) for 9.893 CAYI had been determined in parallel by microdilution (DIN 58940). Susceptibility pattern analysis was used for assessment of yeast susceptibility data when grouping anamorphic CYI in (i) corresponding TELE-groups (when existing) and in (ii) PHYLOgroups. The latter were arranged according to recently published studies on inferring phylogenetic clades based upon multi-gene analyses. Results: The CAYI studied had been classified into one anamorphic genus (Candida spp.) comprising 33 different species. These isolates could be re-classified into 8013 strains (81.5%) froming one ANA-group (Candida spp., with no known teleomorphic designations so far), which could be assigned to 7 major phylogenetic groups. The remaining 1820 strains (18.5%) with their corresponding teleomorphic affiliates comprised 21 species of 12 genera, and could also assigned to the mentioned 7 major phylogenetic groups. CAYI of TELE-groups showed significantly higher resistance rates than those which could not be regrouped into the TELEassociated isolates (43.0% , 26.1% , 6.0% versus 4.4% , 8.0% , 2.5% for FLC, ITR, and VOR, respectively). The susceptibility patterns of the PHYLO-G group differed significantly from the others. CAYI of PHYLO-G, B, and D (e.g. C. guilliermondii, C. intermedia, C. lusitaniae; C. inconspicua, C. krusei) showed consistently higher MICs for all triazoles tested when compared to those of clade A (C. albicans complex, C. parapsilosis complex, C. tropicalis), which encompasses the most frequently encountered clinical isolates, frequently exhibiting low MICs. Conclusions: Grouping of the ana-/telemorphic ascomycetous yeasts into their phylogenetic clades thus reflecting their natural relationship, is correlated with significant different MIC distributions, and results in reliably more coherent susceptibility patterns, when compared to grouping into anamorphic or teleomorphic genera, which may be also sometimes polyphyletic (e.g. Pichia). Thus PHYLO-grouping may enhance and facilitate prediction of parallel and/or cross resistant isolates and results in a more reliable MIC-assessment.

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4th Trends in Medical Mycology
    • TIMM 4th (2012)