Molecular mechanisms of fluconazole resistance in Candida parapsilosis isolates from a US surveillance system.

Grossman NT1, Pham CD1, Cleveland AA1, Lockhart SR2.

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Abstract

Candida parapsilosis is the second or third most common cause of candidemia in many countries. The Infectious Disease Society of America recommends fluconazole as primary therapy for C. parapsilosis candidemia. Although fluconazole resistance among C. parapsilosis isolates is low in most US institutions, the resistance rate can be as high as 7.5%. This study was designed to assess the mechanisms of fluconazole resistance in 706 incident bloodstream isolates from US hospitals. We sequenced the ERG11 and MRR1 genes of 122 C. parapsilosis isolates with resistant (30 isolates; 4.2%), susceptible dose-dependent (37 isolates; 5.2%) and susceptible (55 isolates) fluconazole MIC values, and used RT-PCR on RNA from 17 isolates to investigate the regulation of MDR1. By comparing these isolates to fully fluconazole susceptible isolates we detected at least two mechanisms of fluconazole resistance: an amino acid substitution in the 14-α-demethylase gene ERG11, and overexpression of the efflux pump MDR1, possibly due to point mutations in the MRR1 transcription factor that regulates MDR1. The ERG11 single nucleotide polymorphism (snp) was found in 57% of the fluconazole resistant isolates and in no susceptible isolates. The MRR1 snps were more difficult to characterize, as not all resulted in overexpression of MDR1 and not all MDR1 overexpression was associated with a snp in MRR1. Further work to characterize the MRR1 snps and search for overexpression of other efflux pumps is needed.