Genetic mechanisms of in vitro-induced flucytosine resistance in Candidozyma auris

Thu, 20 Aug 2026 15:19:56 +0000

Objective.

To develop an in vitro model for the development of Candidozyma auris resistance to flucytosine (5-FC) and characterize its associated mutational profile.

Materials and Methods.

An adaptive laboratory evolution study was performed on seven clinical C. auris isolates of the South Asian clade obtained from independent epidemiological foci in Russian hospitals. Isolates underwent stepwise selection with increasing 5-FC concentrations, with molecular verification at defined checkpoints. Minimum inhibitory concentrations (MICs) were determined by broth microdilution (EUCAST). The coding sequences of FUR1, FCY1 and FCY2 were Sanger-sequenced and compared with the reference strain B8441. The fitness cost was additionally assessed as the area under the growth curve (AUC) under standard and subinhibitory conditions.

Results.

In all seven isolates resistance emerged rapidly and reproducibly: by passage P10 the 5-FC MIC rose to > 64 mg/L and persisted through P30. The FCY1 S70R substitution was present in all parental isolates and was not considered adaptive. Resistance was associated mainly with mutations in FUR1 (G68, and also L168F, E182Q) and FCY2 (Y353, G317*); in some lineages high MICs occurred without detectable changes in these genes. Evolved variants (P30) showed reduced growth activity (lower AUC) in the absence of the drug but outgrew parental isolates at the subinhibitory 5-FC concentration.

Conclusions.

Experimental selection of C. auris in the presence of flucytosine resulted in the rapid and reproducible emergence of high-level resistance, with MICs exceeding 64 mg/L. The resistant phenotype arose independently in several lineages and was associated with distinct alterations in genes involved in 5-FC metabolism, indicating genetic heterogeneity in the underlying adaptive mechanisms. In the absence of the drug, acquired resistance was accompanied by reduced growth, consistent with a fitness cost. However, exposure to a subinhibitory concentration of 5-FC shifted relative fitness in favor of the resistant variants. Thus, sustained exposure to low concentrations of flucytosine may promote the selection of resistant C. auris subpopulations despite their reduced baseline growth capacity.

Gusarova K.V. on КМАХ

Genetic mechanisms of *in vitro*-induced flucytosine resistance in Candidozyma auris

Objective.

Materials and Methods.

Results.

Conclusions.

Genetic mechanisms of in vitro-induced flucytosine resistance in Candidozyma auris