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Antimicrobial Agents and Chemotherapy, May 2007, p. 1855-1858, Vol. 51, No. 5
0066-4804/07/$08.00+0 doi:10.1128/AAC.00182-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
A Candida albicans Petite Mutant Strain with Uncoupled Oxidative Phosphorylation Overexpresses MDR1 and Has Diminished Susceptibility to Fluconazole and Voriconazole
Shaoji Cheng,1,
Cornelius J. Clancy,1,2,
Katherine T. Nguyen,1
William Clapp,1,2 and
M. Hong Nguyen1,2*
University of Florida College of Medicine,1
North Florida/South Georgia Randall Veterans Health System, Gainesville, Florida2
Received 7 February 2007/
Accepted 11 February 2007

ABSTRACT
We showed that a
Candida albicans petite mutant in which oxidative
phosphorylation is uncoupled was eightfold more resistant to
fluconazole and voriconazole than SC5314 but equally susceptible
to ketoconazole, itraconazole, and amphotericin B. Strain P5
significantly overexpressed
MDR1, which likely accounts for
the decreased drug susceptibility.

TEXT
The azole antifungal agents are widely used to treat diverse
Candida albicans infections. Resistance to these drugs can contribute
to treatment failures (
28,
29). The molecular bases of resistance
are best characterized for fluconazole (
12,
23,
42). Mechanisms
include drug efflux due to overexpression of ATP-binding cassette
(ABC) transporters encoded by
CDR1 and
CDR2 (
10,
30,
32) and
major facilitator transporters encoded by
MDR1 and
FLU1 (
5,
15,
30); increased expression of
ERG11, which encodes the target
enzyme 14-

-demethylase; mutations in
ERG11 (
21,
38); and mutations
in
ERG3 that inactivate
5,6 desaturase and cause accumulation
of growth-supporting 14

-methylfecosterol (
19,
20,
27). More
recently, aneuploidy, particularly of chromosome 5, has been
implicated and considered likely due to increased copy numbers
of
ERG11 and other genes (
36).
Petite mutant strains of Candida glabrata and Saccharomyces cerevisiae are resistant to fluconazole and other azoles through mechanisms that are not fully defined (1-4, 34, 42). Although a C. albicans petite mutant was tolerant to amphotericin B (14), susceptibility of such strains to azoles is unknown. In a previous study, we serially passed C. albicans strain SC5314 through the spleens of mice and recovered a petite mutant (called P5) in which oxidative phosphorylation was uncoupled (7). The primary objectives of this study were to determine the susceptibility of SC5314 and strain P5 to azoles and, in the event of differences, to study previously characterized mechanisms of resistance.
Using a standard broth macrodilution method (24), we found the MICs of fluconazole and voriconazole to be eightfold higher against strain P5 than SC5314 but still within susceptible ranges (Table 1). The MICs of ketoconazole and itraconazole did not differ between the strains, nor did MICs of amphotericin B (Table 1). Moreover, amphotericin B time-kill curves (8) were similar at drug concentrations 0.25x and 1x MIC (Fig. 1).
We used a broth microdilution method to measure the MICs of
several drugs that are structurally unrelated to fluconazole
but subjected to efflux by ABC and/or Mdr1p transporters (Table
1). MICs were identical against both strains for all drugs except
brefeldin A and cerulenin. MICs of these agents were consistently
4- and

2-fold higher, respectively, against strain P5. As a
further measurement of susceptibility, we diluted overnight
cultures of the strains (yeast extract-peptone-dextrose at 30°C)
to an optical density at 599 nm (OD
599) of 0.1 in fresh yeast
extract-peptone-dextrose and coincubated them at sub-MIC concentrations
of brefeldin A and cerulenin (Fig.
2). As shown, survival of
strain P5 was greater at all concentrations.
Using semiquantitative reverse transcription-PCR (RT-PCR) (Table
2), we demonstrated that
MDR1 expression was up-regulated in
strain P5 and
CDR1 was up-regulated in SC5314 (data not shown).
Levels of
CDR2 expression were very low in both strains. The
expression of
ERG11, ERG3, and
FLU1 did not differ between the
strains. To corroborate and better quantify our findings for
MDR1, CDR1, and
CDR2, we performed real-time RT-PCR (Table
2).
We demonstrated that expression of
MDR1 by strain P5 was 8.3-
to 12.5-fold greater than by SC5314, whereas expression of
CDR1 was 3.1- to 5.7-fold lower (Table
3). The expression of
CDR2 by strain P5 was slightly higher than by SC5314 (Table
3).
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TABLE 3. Differences in expression of CDR1, CDR2, and MDR1 by strain P5 compared to SC5314, as measured by real-time RT-PCR
|
The sequence of
ERG11 in strain P5 differed at two residues
from that of our SC5314 strain and the published sequence (
http://www.candidagenome.org/):
F14S and A409T.
ERG3 sequences did not differ from the published
sequence. Using whole-genome single-nucleotide polymorphism
(SNP) microarrays to analyze the strains' genotypes (
11), we
found that the SNP profiles were identical. These results effectively
excluded aneuploidy (A. Forche, personal communication).
To our knowledge, this is the first study of azole susceptibility in a C. albicans petite mutant strain. Similar to strain P5, previously described C. glabrata petite mutants have shown diminished susceptibility to fluconazole. Our results differed in several ways from earlier reports, however. Most notably, the C. glabrata petite mutants were fully resistant to both fluconazole and itraconazole, which was attributed to up-regulation of CDR1 with a lesser contribution from CDR2 (2, 3, 34). In strain P5, the diminished susceptibilities to fluconazole and voriconazole were most likely mediated by increased drug efflux due to overexpression of MDR1 and, to a lesser extent, CDR2.
Several pieces of evidence are consistent with a role for MDR1 overexpression. First, MDR1 was found by real-time RT-PCR to be significantly up-regulated in strain P5. Previous studies have shown that activation of MDR1 is associated with diminished fluconazole susceptibility but not necessarily full resistance (40). Second, strain P5 exhibited decreased susceptibility to brefeldin A and cerulenin, drugs that are also effluxed from cells by Mdr1p and by Mdr1p and Cdr2p, respectively (18, 41). Strain P5 and SC5314 were equally susceptible to ketoconazole, itraconazole, benomyl, sulfoneturon, and crystal violet, drugs that are not substrates for Mdr1p (17, 26, 30, 31, 37, 41), cycloheximide, which is pumped by both Mdr1p and ABC transporters, and rhodamine 6G, which is pumped by ABC transporters only. Finally, we excluded other molecular causes of resistance, including overexpression of ERG11, mutations in ERG3, or aneuploidy. The ERG11 mutations in strain P5 have not been previously associated with diminished azole susceptibility (33). Nevertheless, we cannot definitively conclude that they did not contribute to our results, particularly since the levels of drug resistance were low. Indeed, we must acknowledge that C. albicans is likely to possess as-yet-uncharacterized resistance mechanisms (39) which might also have influenced our findings.
Whereas petite mutant yeasts have generally demonstrated diminished azole susceptibility, amphotericin B results have been less consistent. In addition to our findings, amphotericin B-tolerant C. albicans and hypersusceptible C. glabrata strains have been previously reported (2, 3, 9, 13, 14). The diverse types of mitochondrial damage that cause petite mutant phenotypes are known to alter sterol synthesis in different ways (13, 14). Results to date suggest that the alterations to synthetic pathways have more consistent effects on susceptibility to azoles than amphotericin B. Along these lines, strain P5 is potentially a unique tool with which to study relationships between mitochondrial function, oxidative phosphorylation, sterol synthesis, and mechanisms of azole resistance.

ACKNOWLEDGMENTS
Experiments were performed in C. Clancy and K. Nguyen's labs
at the North Florida/South Georgia Veterans Health System.
This study was supported by the Medical Research Service of the Department of Veterans Affairs and the National Institute of Allergy and Infectious Diseases (NIH PO1 AI061537-01).
We thank Anja Forche and Judith Berman of the University of Minnesota for performing SNP microarray experiments.

FOOTNOTES
* Corresponding author. Mailing address: University of Florida College of Medicine, P.O. Box 100277, JHMHC, Gainesville, FL 32610. Phone: (352) 379-4027. Fax: (352) 379-4015. E-mail:
nguyemt{at}medicine.ufl.edu 
Published ahead of print on 26 February 2007. 
S.C. and C.J.C. contributed equally to the manuscript. 

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Antimicrobial Agents and Chemotherapy, May 2007, p. 1855-1858, Vol. 51, No. 5
0066-4804/07/$08.00+0 doi:10.1128/AAC.00182-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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