Molecular Mechanisms of Fluconazole Resistance in Candida parapsilosis Isolates from a U.S. Surveillance System

Abstract

ABSTRACTCandida parapsilosisis the second or third most common cause of candidemia in many countries. The Infectious Diseases Society of America recommends fluconazole as the primary therapy forC. parapsilosiscandidemia. Although the rate of fluconazole resistance amongC. parapsilosisisolates is low in most U.S. 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 U.S. hospitals. We sequenced theERG11andMRR1genes of 122C. parapsilosisisolates with resistant (30 isolates; 4.2%), susceptible dose-dependent (37 isolates; 5.2%), and susceptible (55 isolates) fluconazole MIC values and used real-time PCR of RNA from 17 isolates to investigate the regulation ofMDR1. 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 geneERG11and overexpression of the efflux pumpMDR1, possibly due to point mutations in theMRR1transcription factor that regulatesMDR1. TheERG11single nucleotide polymorphism (SNP) was found in 57% of the fluconazole-resistant isolates and in no susceptible isolates. TheMRR1SNPs were more difficult to characterize, as not all resulted in overexpression ofMDR1and not allMDR1overexpression was associated with an SNP inMRR1. Further work to characterize theMRR1SNPs and search for overexpression of other efflux pumps is needed.