Molecular Epidemiology and Fluconazole Resistance Mechanisms in Candida tropicalis Isolates From Bloodstream Infections in Sichuan, China

Objective  To analyze the molecular epidemiology of Candida tropicalis (C. tropicalis) isolates from bloodstream infections, and to investigate preliminarily the underlying mechanisms of fluconazole resistance.

Methods  Clinical C. tropicalis isolates were collected from bloodstream infections at West China Hospital, Sichuan University and other hospitals in Sichuan Province. All the collected isolates were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and the antifungal susceptibility test was performed by the broth microdilution method. Multilocus sequence typing (MLST) was conducted using the first generation sequencing technique. Genes associated with drug-resistance were sequenced and the mutation sites were identified. The relative expression levels of genes associated with drug-resistance were analyzed using reverse transcription-quantitative polymerase chain reaction (RT-qPCR).

Results  A total of 39 C. tropicalis isolates from bloodstream infections were collected. The MLST phylogenetic tree showed that most of the strains clustered in the phylogenetic tree shared identical or similar minimal inhibitory concentrations (MICs). All MLST types were not matched with the isolates in the PubMLST database, representing new diploid sequence types (DSTs). A total of 3 sense mutations, including Ser154Phe, Tyr132Phe, and Tyr257His were identified in ERG11, and were present only in drug-resistant strains. UPC2 contained 4 sense mutations, including Ala251Thr, Gln289Leu, Ser279Glu, and Gln313His, which were also present only in drug-resistant strains. Ser279Glu and Gln313His were previously unreported. There were two sense mutations in ERG3, namely Ser112Gly and Ala48Ser, both being previously unreported, and Ser112Gly was found only in drug-resistant strains. The expression levels of ERG11 (14.48 vs 7.109, P = 0.003) and UPC2 (1.922 vs 0.832, P = 0.04) in non-sensitive group (MIC > 2 mg/L) were higher than those in sensitive group (MIC ≤ 2 mg/L), and the difference was statistically significant.

Conclusion  There may be a unique genetic relationship among C. tropicalis isolates from Sichuan province, and the MICs of fluconazole against C. tropicalis are associated with molecular typing. ERG11 mutations were the major mechanism underlying fluconazole resistance in C. tropicalis isolates. The upregulation of ERG11 and UPC2 was associated with fluconazole resistance in C. tropicalis.