Previous Article | Next Article 
Journal of Clinical Microbiology, November 2002, p. 4308-4312, Vol. 40, No. 11
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.11.4308-4312.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Rapid Identification and Differentiation of Candida albicans and Candida dubliniensis by Capillary-Based Amplification and Fluorescent Probe Hybridization
Rangaraj Selvarangan,1 Ajit P. Limaye,1,2 and Brad T. Cookson1,3*
Departments of Laboratory Medicine,1
Medicine,2
Microbiology, University of Washington Medical Center, Seattle, Washington 98195-71103
Received 2 May 2002/
Returned for modification 2 July 2002/
Accepted 4 August 2002

ABSTRACT
We developed a rapid genotypic assay to differentiate the germ
tube-positive yeasts
Candida albicans and
Candida dubliniensis.
Fluorescently labeled nucleic acid probe binding and subsequent
denaturation from the target site in the PCR amplicons produced
characteristic peak melting temperatures (
Tm) that identified
each species. Peak
Tms of
C. albicans (
n = 69) and
C. dubliniensis (
n = 28) isolates produced in the presence of their respective
probes were 61.04 ± 0.64°C and 60.52 ± 1.01°C
(averages ± standard deviations). No signal was generated
when the
C. albicans or
C. dubliniensis probes were tested against
DNA from their counterparts. Both probes reacted with
Candida tropicalis DNA, but the
Tm was 51.85 ± 0.05°C with
the
C. albicans probe and 51.92 ± 0.10°C with the
C. dubliniensis probe, differentiating
C. tropicalis DNA from
C. albicans and
C. dubliniensis. A novel hybrid probe was designed
to identify both species in a single reaction based on a 4°C
difference in peak
Tms. Our assay is rapid (

2 h) and allows
reliable detection and differentiation of the two germ tube-positive
Candida spp.

TEXT
Candida albicans and
Candida dubliniensis are two closely related,
germ tube- and chlamydospore-positive
Candida species (
14,
30).
A careful analysis of the germ tube-positive
C. albicans from
AIDS patients in Dublin, Ireland, revealed that a subset of
them formed a homogenous cluster distinct from
C. albicans by
DNA fingerprinting analysis (
30). This distinct subset of
C. albicans was grouped into a new species and named
C. dubliniensis sp. nov. This report was followed by numerous others from the
United States and other parts of the world identifying
C. dubliniensis as a clinically important species in human immunodeficiency
virus (HIV)-seropositive patients (
2,
5,
6,
8,
16,
26,
29; D.
A. Stevens, Letter, J. Clin. Microbiol.
39:416, 2001) and immunosuppressed
patients (
1,
9-
12,
17,
24; D. Marriott, M. Laxton, and J. Harkness,
Letter, Emerg. Infect. Dis.
7:479, 2001).
C. dubliniensis has
been isolated from up to 30% of HIV-infected patients with oral
candidiasis (
28). Invasive infections have also been reported
from AIDS and cancer patients (
1,
27). It is important to understand
the epidemiology of
C. dubliniensis, as it is an emerging opportunistic
pathogen capable of rapidly expressing stable fluconazole resistance
in vitro and, in vivo, after prolonged therapy for oropharyngeal
candidiasis in HIV-infected patients (
19,
20,
25). To date,
invasive
C. dubliniensis isolates reported from the AIDS and
cancer patients have not demonstrated any fluconazole resistance.
C. dubliniensis is also found mixed with
C. albicans in oropharyngeal
samples from HIV-infected patients, leading to difficulty in
isolating
C. dubliniensis and understanding its contribution
to clinical disease.
Information on the prevalence of the C. dubliniensis infections is accumulating due to the interest in screening immunocompromised patients for this pathogen and differentiating it from the other germ tube-positive yeast, C. albicans (10, 27). Several phenotypic assays may help to differentiate C. dubliniensis from C. albicans. The differentiating phenotypes include the capacity to grow at 45°C, the capacity to assimilate xylose or
-methyl-D-glucoside, and the type of colony color produced on CHROMagar Candida medium (7, 13, 21; I. F. Salkin, W. R. Pruitt, A. A. Padhye, D. Sullivan, D. Coleman, and D. H. Pincus, Letter, J. Clin. Microbiol. 36:1467, 1998). These assays are time consuming and/or subjective in interpretation and therefore do not perform as optimal screening assays for the potentially fluconazole-resistant C. dubliniensis. Hence, several investigators have used genotypic rather than phenotypic assays to identify and differentiate these two closely related species. The genotypic assays have included ribosomal DNA analysis and PCR fingerprinting and molecular beacon assays (18, 22, 31), which produced reliable differentiation yet are labor intensive or time consuming.
Here we report a simple, rapid, and sensitive genotypic method to differentiate C. albicans from C. dubliniensis. We amplified the internal transcribed spacer region 2 (ITS2) DNA and identified Candida species by sequence-specific hybridization. The ITS2 is a spacer region flanked by the 5.8S and 28S rRNA gene. It was previously demonstrated that the sequence polymorphisms in the ITS regions 1 and 2 can be used to reliably identify medically important yeasts (3, 4). Our present methodology takes advantage of nucleotide differences between C. albicans and C. dubliniensis found in the ITS2 region. The assay involves rapid PCR thermal cycling using glass capillaries and real-time fluorimetric detection of the PCR products. DNA amplification was achieved by 45 thermal cycles in 45 min. The amplified products were identified by melting curve analysis (MCA) using various hybridization probes bound to target sequences in the ITS region. The 3' end of the donor probe is coupled to the donor fluorophore (fluorescein) dye, and the 5' end of the acceptor probe is coupled with acceptor fluorophore (LC-Red 640) dye. Fluorescence resonance energy transfer occurs when the two probes hybridize to the amplicon in close proximity, allowing the fluorescein to emit light that is absorbed by the LC-Red 640 dye (Fig. 1). MCA is performed after the completion of the amplification cycles to identify specific amplicons. The probe coupled with the LC-Red 640 hybridizes to conserved target sequence present in both Candida spp. and serves as the acceptor probe. The donor probes (C. albicans, C. dubliniensis, and Hybrid) coupled with the fluorescein dye hybridize to adjacent DNA containing species-identifying nucleotide polymorphisms. When the temperature is slowly increased, the donor probe melts off from the target sequence producing the characteristic melting peaks (Tm). The Tm is dependent on the length, G+C content, and degree of homology between the donor probe and the template. For our assay we designed one acceptor probe (CANC) and three donor probes: two probes were specific for either C. albicans (CALB) or C. dubliniensis (CDUB), and a third probe (CFLUC) included a single mismatch for each of the two species, allowing it to hybridize and produce characteristic Tms with each target in a single reaction.
We validated our assay by using 69
C. albicans and 28
C. dubliniensis isolates characterized by ITS2 sequence polymorphisms (
4). Eleven
C. dubliniensis isolates were prospectively collected from our
laboratory by examining the xylose assimilation-negative, germ
tube-positive
Candida species in the VITEK YBC system as suggested
by Pincus et al. (
23). These isolates were obtained from several
specimens including mouth washes of stem cell transplant patients,
vaginal culture samples, stool samples, and bloodstream infection.
DNA was extracted from yeast colonies grown after 48 h on Sabouraud's
agar by using the QIAspin Miniprep kit (Qiagen) following the
manufacturer's protocol. Amplification was performed in 7 µl
of total reaction volume consisting of 1X LightCycler master
hybridization mix (Roche Diagnostics), plus an additional 2.0
mM of MgCl
2 and a 1 µM concentration of the ITS3-1 forward
primer (5'-CATCGATGAAGAACGCAGC-3') and the ITS4 reverse primer
(5'-TCCTCCGCTTATTGATATGC-3'). Product identification was achieved
by adding a 0.1 µM concentration of one of the three donor
probes, CALB (5'-TCCCTCAAACCGCTGGGTTT-3'-FLU), CDUB (5'-TCCCTCAAACCCCTAGGGTTT-3'-FLU),
or CFLUC (5'-TCCCTCAAACCCCTGGGTTT-3'-FLU), along with a 0.2
µM concentration of the acceptor probe, CANC (5'-LC-Red640-TGTTGAGCAATACGACTTGGGTTTGp-3')
to the reaction mixture. Thermal cycling conditions were as
follows: 95°C initial denaturation for 30 s was followed
by 45 cycles of 95°C for 1 s, 55°C for 10 s, and 72°C
for 25 s. MCA with species probes was performed by using the
following thermal cycling profile: 95°C for 0 s, 45°C
for 30 s, and ramping to 85°C. The MCA profile for the hybrid
probe was 95°C for 10 s, 40°C for 30 s, and ramping
to 70°C.
Specificity of the probes was verified by testing other yeast species previously characterized by ITS length and/or sequence polymorphism (4). These included Candida tropicalis (ATCC750), Candida glabrata (ATCC2001), Candida krusei (ATCC6258), Candida lusitaniae (ATCC34449), Candida guilliermondii (ATCC6260), Candida parapsilosis (UWFP251), Cryptococcus neoformans (UWFP360), C. lambica (UWFP346), Cryptococcus albidus (ATCC10666), and Candida kefyr (UWFP208) (Table 1). Characteristic peak Tms for the species probes with their cognate candidal species were obtained by MCA. The C. albicans probe gave a peak melting temperature of 61.04 ± 0.64°C (average ± standard deviation) only in the presence of C. albicans DNA (Fig. 2). Likewise, the C. dubliniensis probe gave a peak Tm of 60.52 ± 1.01°C for all the C. dubliniensis isolates tested (data not shown). Both the probes identified DNA from the respective species with an analytical sensitivity of
1 fg of DNA. These species probes did not produce any peak Tms by MCA with any other species tested except for C. tropicalis. However, the Tm obtained with C. tropicalis DNA was approximately 52°C with either the C. albicans probe or the C. dubliniensis probe, and these values were readily distinguishable.
The hybrid probe, CFLUC, produced a peak
Tm of 49.94 ±
0.54°C for
C. albicans and 54.03 ± 1.01°C for
C. dubliniensis (Fig.
3). Interestingly, the hybrid probe also
produced a peak
Tm of 53.46 ± 0.09°C with
C. tropicalis and 47.81 ± 0.24°C for
C. parapsilosis. Notably,
the hybrid probe produces
Tms which may potentially overlap
between
C. dubliniensis and
C. tropicalis. Hence, the use of
the hybrid probe to differentiate between
C. albicans and
C. dubliniensis warrants pretesting and selection of germ tube-positive
species only, thereby eliminating any
C. tropicalis isolates.
Alternatively,
C. dubliniensis colonies can be preselected by
their dark green color when CHROMagar medium is used for primary
isolation. To identify the small proportion (up to 5%) of germ
tube-negative
C. albicans (
15) and/or avoid germ tube testing,
the
C. albicans probe should be used.
We report the use of a rapid screening assay for
C. dubliniensis.
By targeting a specific sequence in the ITS2 region of both
C. albicans and
C. dubliniensis, we can specifically identify
these two closely related species based on the characteristic
Tm produced with specific donor probes. Thermal cycling in glass
capillaries provides a rapid assay with real-time detection
using hybridization of probes. The MCA is a particularly useful
feature of the LightCycler system and adds specificity to identification
of PCR amplicons. Additionally, the MCA is capable of identifying
mutations by showing a shift in the peak
Tm. The uniformity
of the
Tms for 69
C. albicans and 28
C. dubliniensis isolates
indicates that the target site is well conserved among isolates
from the same species. Following DNA extraction, the amplification
and detection can be completed in less than 1 h, making it the
most rapid assay reported to date for reliably identifying these
species. The above-mentioned characteristics, i.e., rapidity,
specificity, reliability, and user friendliness, make this an
adaptable assay in a laboratory setting. Unlike the rRNA genes,
which evolve slowly and are relatively conserved, ITS region
DNA contains sufficient sequence variation to make it a suitable
molecular target for identification of fungi to the species
level (
3,
4). The availability of the rRNA genes in high copy
numbers provides an added advantage, because it theoretically
increases the sensitivity of the assay. Additionally, amplification
and subsequent detection by probe hybridization in a closed
system reduce the possibility of cross-contamination. Currently
we are using this technology to identify the commonly encountered
invasive
Candida species from blood culture systems in our clinical
laboratory. Doing so permits identification of the common
Candida species on the same day that blood cultures become positive
and eliminates the 2- to 3-day period for confirmatory identification
by traditional methods. Such rapid reporting can have an impact
on clinical care by assisting the clinician in making appropriate
drug choices and patient management decisions.

FOOTNOTES
* Corresponding author. Mailing address: Department of Laboratory Medicine, Box 357110, 1959 NE Pacific St., University of Washington Medical Center, Seattle, WA 98195-7110. Phone: (206) 598-6131. Fax: (206) 598-6189. E-mail:
cookson{at}u.washington.edu.


REFERENCES
1 - Brandt, M. E., L. H. Harrison, M. Pass, A. N. Sofair, S. Huie, R. K. Li, C. J. Morrison, D. W. Warnock, and R. A. Hajjeh. 2000. Candida dubliniensis fungemia: the first four cases in North America. Emerg. Infect. Dis. 6:46-49.[Medline]
2 - Brown, D. M., M. A. Jabra-Rizk, W. A. Falkler, Jr., A. A. Baqui, and T. F. Meiller. 2000. Identification of Candida dubliniensis in a study of HIV-seropositive pediatric dental patients. Pediatr. Dent. 22:234-238.[Medline]
3 - Chen, Y. C., J. D. Eisner, M. M. Kattar, S. L. Rassoulian-Barrett, K. LaFe, U. Bui, A. P. Limaye, and B. T. Cookson. 2001. Polymorphic internal transcribed spacer region 1 DNA sequences identify medically important yeasts. J. Clin. Microbiol. 39:4042-4051.[Abstract/Free Full Text]
4 - Chen, Y. C., J. D. Eisner, M. M. Kattar, S. L. Rassoulian-Barrett, K. LaFe, S. L. Yarfitz, A. P. Limaye, and B. T. Cookson. 2000. Identification of medically important yeasts using PCR-based detection of DNA sequence polymorphisms in the internal transcribed spacer 2 region of the rRNA genes. J. Clin. Microbiol. 38:2302-2310.[Abstract/Free Full Text]
5 - Coleman, D., D. Sullivan, B. Harrington, K. Haynes, M. Henman, D. Shanley, D. Bennett, G. Moran, C. McCreary, and L. O'Neill. 1997. Molecular and phenotypic analysis of Candida dubliniensis: a recently identified species linked with oral candidosis in HIV-infected and AIDS patients. Oral Dis. 3(Suppl. 1):S96-S101.
6 - Diaz-Guerra, T. M., E. Mellado, E. M. Cuenca, F. Laguna, and J. L. Rodriguez-Tudela. 1999. Molecular characterization by PCR-fingerprinting of Candida dubliniensis strains isolated from two HIV-positive patients in Spain. Diagn. Microbiol. Infect. Dis. 35:113-119.[CrossRef][Medline]
7 - Gales, A. C., M. A. Pfaller, A. K. Houston, S. Joly, D. J. Sullivan, D. C. Coleman, and D. R. Soll. 1999. Identification of Candida dubliniensis based on temperature and utilization of xylose and
-methyl-D-glucoside as determined with the API 20C AUX and Vitek YBC systems. J. Clin. Microbiol. 37:3804-3808.[Abstract/Free Full Text]
8 - Giammanco, G. M., S. Pignato, S. Salvo, and G. Giammanco. 2000. Carbohydrate assimilation profiles of the first Italian Candida dubliniensis clinical isolates recovered from an HIV-infected individual. Res. Microbiol. 151:889-891.[Medline]
9 - Gottlieb, G. S., A. P. Limaye, Y. C. Chen, W. C. Van Voorhis. 2001. Candida dubliniensis fungemia in a solid organ transplant patient: case report and review of the literature. Med. Mycol. 39:483-485.[Medline]
10 - Jabra-Rizk, M. A., A. A. Baqui, J. I. Kelley, W. A. Falkler, Jr., W. G. Merz, and T. F. Meiller. 1999. Identification of Candida dubliniensis in a prospective study of patients in the United States. J. Clin. Microbiol. 37:321-326.[Abstract/Free Full Text]
11 - Jabra-Rizk, M. A., W. A. Falkler, Jr., W. G. Merz, A. A. Baqui, J. I. Kelley, and T. F. Meiller. 2000. Retrospective identification and characterization of Candida dubliniensis isolates among Candida albicans clinical laboratory isolates from human immunodeficiency virus (HIV)-infected and non-HIV-infected individuals. J. Clin. Microbiol. 38:2423-2426.[Abstract/Free Full Text]
12 - Kamei, K., M. J. McCullough, and D. A. Stevens. 2000. Initial case of Candida dubliniensis infection from Asia: non-mucosal infection. Med. Mycol. 38:81-83.[Medline]
13 - Kurzai, O., H. C. Korting, D. Harmsen, W. Bautsch, M. Molitor, M. Frosch, and F. A. Muhlschlegel. 2000. Molecular and phenotypic identification of the yeast pathogen Candida dubliniensis. J. Mol. Med. 78:521-529.[CrossRef][Medline]
14 - McCullough, M. J., K. V. Clemons, and D. A. Stevens. 1999. Molecular and phenotypic characterization of genotypic Candida albicans subgroups and comparison with Candida dubliniensis and Candida stellatoidea. J. Clin. Microbiol. 37:417-421.[Abstract/Free Full Text]
15 - McGinnis, M. R. 1980. Yeast identification, p. 364-367. In M. R. McGinnis (ed.), Laboratory handbook of medical mycology. Academic Press, New York, N.Y.
16 - Meiller, T. F., M. A. Jabra-Rizk, A. Baqui, J. I. Kelley, V. I. Meeks, W. G. Merz, and W. A. Falkler. 1999. Oral Candida dubliniensis as a clinically important species in HIV-seropositive patients in the United States. Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 88:573-580.[CrossRef][Medline]
17 - Meis, J. F., M. Ruhnke, B. E. De Pauw, F. C. Odds, W. Siegert, and P. E. Verweij. 1999. Candida dubliniensis candidemia in patients with chemotherapy-induced neutropenia and bone marrow transplantation. Emerg. Infect. Dis. 5:150-153.[Medline]
18 - Meyer, W., K. Maszewska, and T. C. Sorrell. 2001. PCR fingerprinting: a convenient molecular tool to distinguish between Candida dubliniensis and Candida albicans. Med. Mycol. 39:185-193.[Medline]
19 - Moran, G. P., D. Sanglard, S. M. Donnelly, D. B. Shanley, D. J. Sullivan, and D. C. Coleman. 1998. Identification and expression of multidrug transporters responsible for fluconazole resistance in Candida dubliniensis. Antimicrob. Agents Chemother. 42:1819-1830.[Abstract/Free Full Text]
20 - Moran, G. P., D. J. Sullivan, M. C. Henman, C. E. McCreary, B. J. Harrington, D. B. Shanley, and D. C. Coleman. 1997. Antifungal drug susceptibilities of oral Candida dubliniensis isolates from human immunodeficiency virus (HIV)-infected and non-HIV-infected subjects and generation of stable fluconazole-resistant derivatives in vitro. Antimicrob. Agents Chemother. 41:617-623.[Abstract]
21 - Niimi, K., M. G. Shepherd, and R. D. Cannon. 2001. Distinguishing Candida species by ß-N-acetylhexosaminidase activity. J. Clin. Microbiol. 39:2089-2097.[Abstract/Free Full Text]
22 - Park, S., M. Wong, S. A. Marras, E. W. Cross, T. E. Kiehn, V. Chaturvedi, S. Tyagi, and D. S. Perlin. 2000. Rapid identification of Candida dubliniensis using a species-specific molecular beacon. J. Clin. Microbiol. 38:2829-2836.[Abstract/Free Full Text]
23 - Pincus, D. H., D. C. Coleman, W. R. Pruitt, A. A. Padhye, I. F. Salkin, M. Geimer, A. Bassel, D. J. Sullivan, M. Clarke, and V. Hearn. 1999. Rapid identification of Candida dubliniensis with commercial yeast identification systems. J. Clin. Microbiol. 37:3533-3539.[Abstract/Free Full Text]
24 - Polacheck, I., J. Strahilevitz, D. Sullivan, S. Donnelly, I. F. Salkin, and D. C. Coleman. 2000. Recovery of Candida dubliniensis from non-human immunodeficiency virus-infected patients in Israel. J. Clin. Microbiol. 38:170-174.[Abstract/Free Full Text]
25 - Ruhnke, M., A. Schmidt-Westhausen, and J. Morschhauser. 2000. Development of simultaneous resistance to fluconazole in Candida albicans and Candida dubliniensis in a patient with AIDS. J. Antimicrob. Chemother. 46:291-295.[Abstract/Free Full Text]
26 - Schorling, S. R., H. C. Kortinga, M. Froschb, and F. A. Muhlschlegel. 2000. The role of Candida dubliniensis in oral candidiasis in human immunodeficiency virus-infected individuals. Crit. Rev. Microbiol. 26:59-68.[CrossRef][Medline]
27 - Sebti, A., T. E. Kiehn, D. Perlin, V. Chaturvedi, M. Wong, A. Doney, S. Park, and K. A. Sepkowitz. 2001. Candida dubliniensis at a Cancer Center. Clin. Infect. Dis. 32:1034-1038.[CrossRef][Medline]
28 - Sullivan, D., and D. Coleman. 1997. Candida dubliniensis: an emerging opportunistic pathogen. Curr. Top. Med. Mycol. 8:15-25.[Medline]
29 - Sullivan, D., K. Haynes, J. Bille, P. Boerlin, L. Rodero, S. Lloyd, M. Henman, and D. Coleman. 1997. Widespread geographic distribution of oral Candida dubliniensis strains in human immunodeficiency virus-infected individuals. J. Clin. Microbiol. 35:960-964.[Abstract]
30 - Sullivan, D. J., T. J. Westerneng, K. A. Haynes, D. E. Bennett, and D. C. Coleman. 1995. Candida dubliniensis sp. nov.: phenotypic and molecular characterization of a novel species associated with oral candidosis in HIV-infected individuals. Microbiology 141:1507-1521.[Abstract/Free Full Text]
31 - Williams, D. W., W. A. Coulter, M. J. Wilson, A. J. Potts, and M. A. Lewis. 2001. Identification of Candida dubliniensis, based on ribosomal DNA sequence analysis. Br. J. Biomed. Sci. 58:11-16.[Medline]
Journal of Clinical Microbiology, November 2002, p. 4308-4312, Vol. 40, No. 11
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.11.4308-4312.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
This article has been cited by other articles:
-
Rakeman, J. L., Bui, U., LaFe, K., Chen, Y.-C., Honeycutt, R. J., Cookson, B. T.
(2005). Multilocus DNA Sequence Comparisons Rapidly Identify Pathogenic Molds. J. Clin. Microbiol.
43: 3324-3333
[Abstract]
[Full Text]
-
Gutzmer, R., Mommert, S., Kuttler, U., Werfel, T., Kapp, A.
(2004). Rapid identification and differentiation of fungal DNA in dermatological specimens by LightCycler PCR. J Med Microbiol
53: 1207-1214
[Abstract]
[Full Text]
-
Frade, J. P., Warnock, D. W., Arthington-Skaggs, B. A.
(2004). Rapid Quantification of Drug Resistance Gene Expression in Candida albicans by Reverse Transcriptase LightCycler PCR and Fluorescent Probe Hybridization. J. Clin. Microbiol.
42: 2085-2093
[Abstract]
[Full Text]
-
Selvarangan, R., Bui, U., Limaye, A. P., Cookson, B. T.
(2003). Rapid Identification of Commonly Encountered Candida Species Directly from Blood Culture Bottles. J. Clin. Microbiol.
41: 5660-5664
[Abstract]
[Full Text]