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Journal of Clinical Microbiology, June 2000, p. 2423-2426, Vol. 38, No. 6
Department of Oral
Medicine1 and Department of
OCBS,2 Dental School, University of
Maryland, and Department of Pathology, The Johns Hopkins
University,3 Baltimore, Maryland 21201
Received 11 February 2000/Returned for modification 22 March
2000/Accepted 7 April 2000
Fungal opportunistic infections, and in particular those caused by
the various Candida species, have gained considerable
significance as a cause of morbidity and, often, mortality. The newly
described species Candida dubliniensis phenotypically
resembles Candida albicans so closely that it is easily
misidentified as such. The present study was designed to determine the
frequency at which this new species is not recognized in the clinical
laboratory, to determine the patient populations with which C. dubliniensis is associated, to determine colonization versus
infection frequency, and to assess fluconazole resistance. Over a
2-year period, 1,251 isolates that were initially identified as
C. albicans by a hospital clinical laboratory were
reevaluated for C. dubliniensis by inability to grow at
45°C, colony color on CHROMagar Candida medium, coaggregation assay
with Fusobacterium nucleatum, and sugar assimilation
profiles (API 20C AUX yeast identification system). A total of 15 (1.2%) isolates from 12 patients were identified as C. dubliniensis. Ten of the patients were found to be
immunocompromised (these included patients with human immunodeficiency
virus infection or AIDS, cancer patients receiving chemotherapy, and
patients awaiting transplantation). Thirteen isolates were highly
susceptible to fluconazole (MIC, <0.5 µg/ml). Three isolates from
one patient, genotypically confirmed as the same strain, showed
variable susceptibility to fluconazole. The first isolate was
susceptible, whereas the other two isolates were dose-dependent
susceptible (MIC, 16.0 µg/ml). These data confirm the close
association of C. dubliniensis with immunocompromised
states and that increased fluconazole MICs may develop in vivo. This
study emphasizes the importance of screening germ-tube-positive yeasts
for the inability to grow at 45°C followed by confirmatory tests in
order to properly identify this species.
The continuing AIDS epidemic,
malignancies, and aggressive chemotherapeutic interventions have
created an extremely vulnerable population of immunocompromised
patients who are highly susceptible to a variety of microbial
infections, including fungal infections (1, 4, 5, 16, 28,
31). Antifungal drug resistance and the emergence of novel
species and species previously not associated with human disease as
potential pathogens have also greatly contributed to the drastic
increase in fungal infections (2, 24, 29). Although the true
clinical significance of the newly identified species Candida
dubliniensis is not yet fully known, it is already established as
an opportunistic pathogen due to its association with recurrent oral
infections as well as its implication in cases of superficial and
systemic disease in immunocompromised individuals (2, 11,
27). In addition to being a significant cause of candidosis,
C. dubliniensis was found to be easily induced to develop
resistance to fluconazole in vitro, a phenomenon which may have
resulted in its emergence (15, 16). Few studies have been
performed to investigate virulence factors of C. dubliniensis; however, preliminary data so far have suggested that
isolates of this species produce higher levels of proteinase and are
more adherent to buccal epithelial cells than Candida
albicans isolates (10, 29). One recently observed characteristic exhibited by C. dubliniensis is that cells
grown at 37°C on Sabouraud dextrose agar (SDA) coaggregate in vitro with the oral anaerobic bacterium Fusobacterium nucleatum
(ATCC 49256), whereas C. albicans cells grown under the same
conditions fail to do so (8). This binding of C. dubliniensis to F. nucleatum may play an important role
in the colonization ability of C. dubliniensis in the oral
cavity (8, 13, 14). In addition to its potential clinical
implications, this intergeneric coaggregation between F. nucleatum and C. dubliniensis has been used to develop
a rapid and specific assay to discriminate between C. dubliniensis and C. albicans isolates (8).
Most recently, electron microscopic studies comparing the cell walls of
C. dubliniensis and C. albicans revealed major
ultrastructural differences between these two species that allow
C. dubliniensis to express constant cell surface
hydrophobicity (7).
As C. dubliniensis continues to gain importance as a
significant opportunistic pathogen and in light of its predisposition for fluconazole resistance, it has become important for laboratories to
screen for this species in clinical specimens. Until this species is
fully accepted as a clinically important Candida species by clinical laboratories, C. dubliniensis isolates will
continue to be identified as C. albicans and the efforts to
investigate the epidemiology and the true clinical significance of this
new species will continue to be hampered (26, 29). To that
end, the investigation described here was initiated to document the prevalence of C. dubliniensis presumptively identified as
C. albicans in the clinical laboratory. In addition, the
study aimed to determine the presence of fluconazole resistance among
clinical isolates, to test for strain stability among multiple isolates
from the same patient, and to provide some epidemiological and clinical data regarding the patients from whom these C. dubliniensis
isolates were recovered.
A total of 1,251 isolates of C. albicans, recovered
between January 1998 and January 2000 by the Clinical Microbiology
Laboratories at the University of Maryland Hospital, were evaluated.
The isolates were originally identified by their ability to produce
germ tubes and chlamydospores on Tween 80-oxgall-caffeic acid agar
(Remel, Lenexa, Kans.). For this study, all isolates were screened for their ability to grow on SDA (Difco Laboratories, Detroit, Mich.) at
45°C for 48 h. Isolates that failed to grow at 45°C were
retested for germ tube production in serum after 3 h of incubation
at 37°C. Germ-tube-positive isolates negative for growth at 45°C
were then incubated at 37°C on SDA for 24 h for the F. nucleatum coaggregation (CoAg) assay, were streaked on the
chromogenic medium CHROMagar Candida (CHROMagar, Paris, France) for
evaluation of colony color after 48 h of incubation at 37°C, and
were tested for sugar assimilation. Isolates confirmed as C. dubliniensis were then evaluated for fluconazole susceptibility.
The CoAg ability of the presumptive C. dubliniensis isolates
with the anaerobic oral bacterium F. nucleatum ATCC 49256 was tested as described previously (8). C. dubliniensis type strain CD36 and C. albicans ATCC
18804 were used as positive and negative controls, respectively.
Assimilation patterns with a variety of carbohydrate substrates were
determined by using the API 20C system (bioMerieux Vitek, Inc.,
Hazelwood, Mo.) according to the manufacturer's instructions.
In vitro fluconazole susceptibility was determined by a macrobroth
dilution susceptibility assay carried out according to the method
outlined in The National Committee for Clinical Laboratory Standards
document M27-A (17). The MIC was recorded as the highest concentration of drug demonstrating at least 80% reduction in turbidity when compared to the positive control tube. Interpretation of
results was performed according to the guidelines of Rex et al.
(22). A MIC at 48 h of In order to test for strain stability in patients with multiple
isolates, EK using pulsed-field gel electrophoresis was used. The
preparation of intact DNA in agarose plugs was performed by the method
of King et al. (9). All isolates meeting the phenotypic criteria of C. dubliniensis, a C. albicans
control, and the C. dubliniensis type strain (CD36) were
tested as previously described (6).
Epidemiological and clinical information for patients from whom
C. dubliniensis isolates were recovered was obtained by
review of patients' hospital records, with the approval of the
University of Maryland.
Of the 1,251 yeast isolates initially identified as C. albicans, 15 failed to grow at 45°C. All 15 isolates were
confirmed as germ tube positive by repeat testing. In addition, they
all produced abundant chlamydospores on corn meal agar; chlamydospores were often attached in triplet or pair arrangement at the end of short,
hyperbranching pseudohyphae. All of the 15 isolates produced the
characteristic dark green colony color on CHROMagar Candida medium,
identical to the color produced by the C. dubliniensis type
strain CD36 (courtesy of Derek Sullivan) used for
comparison.
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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
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8 µg/ml was taken to
indicate a sensitive organism. A MIC of
16 to 32 µg/ml was taken to
indicate a dose-dependent susceptible organism. A MIC of
64 µg/ml
was taken to indicate a resistant organism.
TABLE 1.
Characteristics of the C. dubliniensis
clinical isolates
Sugar-assimilation profiles obtained for 14 of the 15 isolates
generated an API 20C biocode (6172130), which corresponded to a very
good identification of C. dubliniensis. One isolate gave a
biocode of 6152130, which also corresponded to a very good identification for C. dubliniensis. Multiple isolates
recovered from the same patients had identical biocodes. None of the 15 isolates of C. dubliniensis assimilated
-methyl-D-glucoside or xylose, the two key sugars that
have been used to differentiate C. dubliniensis from
C. albicans (19, 23).
A 4+ visual coaggregation reaction with F. nucleatum was observed with all 15 C. dubliniensis patient isolates grown at 37°C. No coaggregation occurred with the 37°C-grown C. albicans ATCC strain used as a negative control.
Thirteen of the 15 isolates were sensitive (MIC,
0.5 µg/ml),
whereas two isolates recovered from the same patient were
dose-dependent susceptible (MIC, 16.0 µg/ml) (Table 1).
Eight to nine DNA bands, including a chromosome-sized band of <1 Mb,
were obtained for the five C. dubliniensis isolates
recovered from two patients by pulsed-field gel electrophoresis,
consistent with the reference strain CD36. The variation in the
mobilities of the DNA molecules yielded three different EK patterns
with the five isolates. Two isolates from the same patient (patient 8;
Table 3) had very similar EK patterns, differing only by one band.
However, three isolates from the second patient (patient 5; Table
2) had an identical EK pattern which was
different from that of the first patient.
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Chart review of the patients from whom C. dubliniensis
isolates were recovered revealed that 10 of the 12 patients were
immunocompromised (Tables 2 and 3). Five
patients had human immunodeficiency virus (HIV) infection or AIDS,
whereas seven patients were HIV negative. C. dubliniensis
was most commonly isolated from oral or respiratory specimens. None of
the patients had evidence of candidal infection at the time of
collection.
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In this investigation, 15 of 1,251 (1.2%) isolates initially identified as C. albicans by the University of Maryland Clinical Microbiology Laboratory were found to be C. dubliniensis. The inability of isolates to grow at 45°C proved to be a simple and reliable method for presumptive identification of C. dubliniensis (20). However, for proper identification of this species in the clinical laboratory, we propose that all germ-tube-positive yeast isolates that fail to grow at 45°C on SDA be confirmed by a 3-min CoAg assay or by carbohydrate assimilation profiles with the API yeast identification system.
Ten of the 12 patients were confirmed to be clinically immunocompromised; six patients were HIV positive (three of whom had AIDS) and three were receiving chemotherapy for treatment of cancer. Although there was no evidence of candidemia present in these patients, the recovery of C. dubliniensis from the cancer patients confirms the report of Meis et al. (12) describing the presence of this species among non-HIV-positive immunocompromised individuals. Two patients were awaiting organ transplantation, one had undergone abdominal surgery and was intubated, and one, other than having been recently hospitalized for pneumonia and having hypoalbuminemia (which could be an indication of malnourishment), had no evident cause for immunosuppression.
Thirteen of the 15 isolates, including two from the same patient
(patient 8; Table 3), were highly susceptible to fluconazole (MICs,
0.5 µg/ml) (Table 1). Interestingly, although the first of the
three isolates recovered from the other patient (patient 5; Table 2)
with multiple isolates was highly susceptible to fluconazole (MIC,
<0.125 µg/ml), the MICs for the remaining two isolates obtained 3 months later, 1 week apart, were higher, and both isolates were
dose-dependent susceptible (MIC, 16.0 µg/ml). The patient's chart
indicated prior fluconazole therapy. This observation supports the in
vitro development of resistance seen in C. dubliniensis
isolates following exposure to fluconazole, as reported by Moran et al.
(15, 16). This is also supported by strain typing data with
electrophoretic karyotyping. Isolates from the two patients with
multiple isolates had two EK patterns, each pattern identical for all
isolates from the same patient and not found in other patients,
demonstrating strain stability among isolates from the same patient.
The 15 isolates came from a variety of body sites; however, the majority were recovered from oral or respiratory sources and none were recovered from blood or deep tissue. In addition, none of the patients had clinical evidence of infection, indicating that the 12 patients were most likely colonized, although infections have been documented by other studies.
C. dubliniensis has been most frequently implicated in cases of recurrent mucosal candidosis in HIV-infected individuals and most recently in an unusual form of linear gingival erythematous candidosis (3, 24-27, 30). The role of C. dubliniensis, however, is not limited to oral candidosis. In 1999, a report by Meis et al. (12) described three cases of candidemia due to C. dubliniensis in HIV-negative patients with chemotherapy-induced immunosuppression. Although these cases constitute the first documented evidence of systemic involvement by C. dubliniensis, prior cases of candidemia may have been mistakingly attributed to C. albicans, the result of the close phenotypic resemblance between these two species. This is well illustrated by a recent survey by Odds et al. (18) that examined the prevalence of C. dubliniensis among a large stock collection of yeast isolates originally identified as C. albicans, dating back to the early 1970s. Among the 2,589 yeast isolates examined in that survey, 52 isolates were reidentified as C. dubliniensis. These findings show that C. dubliniensis isolates from clinical material predate the AIDS epidemic and that this species has been isolated in the past from HIV-negative subjects.
In a clinical review of six cases of C. dubliniensis in HIV-positive patients, Meiller et al. (11) associated C. dubliniensis with AIDS progression, as demonstrated by high viral loads. Most recently, 11 C. dubliniensis isolates were recovered from 11 HIV-negative hospitalized patients, confirming the association of this species with colonization and infection in populations other than the HIV infected (21).
Despite the close phenotypic resemblance between C. dubliniensis and C. albicans, discriminating between these two species in the clinical laboratory should no longer be a difficult task due to the availability of simple and rapid tests that provide reliable identification of C. dubliniensis.
As the immunocompromised population continues to grow in numbers, choice of proper medical management and adequate therapy becomes all the more paramount. The recognition of C. dubliniensis is necessary not only for therapeutic purposes, but also as a means to determine the incidence and prevalence of this species among the various susceptible populations, as well as to determine its role in disease, especially in invasive and systemic infections.
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ACKNOWLEDGMENTS |
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We thank the staff of the University of Maryland and the Johns Hopkins Hospital clinical microbiology laboratories for their cooperation and for providing the clinical samples.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Oral Medicine, Dental School, UMAB, 666 W. Baltimore St., Baltimore, MD 21201. Phone: (410) 708-7628. Fax: (410) 706-0519. E-mail: mrizk{at}umaryland.edu.
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