Abstract
Production of chlamydospores on casein agar at 24°C for 48 h provides a simple means for differentiating Candida dubliniensis from Candida albicans based on chlamydospore production. Of 109 C. dubliniensis isolates tested on this medium, 106 (97.2%) produced abundant chlamydospores and three produced few chlamydospores. In contrast, of the 120 C. albicans isolates tested, 111 (92.5%) failed to produce any chlamydospores, whereas the remaining nine isolates produced few chlamydospores. These findings indicate that abundant chlamydospore production on casein agar is a useful test for discriminating between C. dubliniensis and C. albicans.
Since its first description in 1995, Candida dubliniensis has been isolated from a variety of specimens from humans in countries all over the world (6, 13, 15, 18-20). As a consequence of the increasing number of reports on the isolation of C. dubliniensis, it is important to be able to rapidly and accurately identify this species in most clinical mycology laboratories. However, identification of C. dubliniensis is hampered by its close relationship with Candida albicans, a situation that has sometimes led to the misidentification of isolates of C. dubliniensis as C. albicans (19). At present, the most accurate differentiation between isolates of the two species is performed in reference laboratories with the use of molecule-based techniques such as PCR or DNA fingerprinting with repetitive sequence-containing DNA probes (5, 17, 19). However, these sophisticated techniques are not suitable and often not readily applicable for use in small clinical mycology laboratories, where simple and rapid methods are needed. Reliable phenotypic methods for the identification of C. dubliniensis isolates include carbohydrate assimilation profile analysis by using commercially available yeast identification systems and detection of differential antigen expression by immunofluorescence microscopy (2, 3, 11, 12, 20). Furthermore, a variety of other ancillary tests have been developed for discriminating between C. dubliniensis and C. albicans isolates, including the inability of C. dubliniensis to grow at 45°C (12). However, whereas these tests are useful for the presumptive identification of C. dubliniensis, they are not definitive. One of the key features employed in the initial description of C. dubliniensis was its ability to produce abundant chlamydospores on cornmeal agar and rice-agar-Tween-agar (20). Chlamydospore production by C. dubliniensis on Staib agar and caffeic acid-ferric citrate agar has also been used recently for the differentiation of C. dubliniensis from C. albicans (1, 17). In the present study, the production of chlamydospores by C. dubliniensis and C. albicans on casein agar was investigated as an additional means for differentiating the two species.
The reference and clinical isolates used in this study are shown in Table 1. Conventional morphological and physiologic methods, as well as molecular techniques, were employed to confirm the identity of all isolates (1, 2, 4, 5, 11, 19). All yeasts studied were initially grown for 48 h at 30°C on Sabouraud glucose agar (Difco, Detroit, Mich.). Casein agar was prepared as described by Larone (7). Briefly, 10 g of skim milk (Sveltesse dried skimmed milk; Nestlé España SA, Esplugues de Llobregat, Barcelona, Spain; and Marvel dried skimmed milk; Premier Brands, Merseyside, United Kingdom) was dissolved in 90 ml of distilled water, and 3 g of agar was dissolved in 97 ml of distilled water. After autoclaving of both solutions separately at 121°C for 15 min, they were allowed to cool to 45 to 50°C and were then mixed together and dispensed in 25-ml amounts into 90 mm-diameter petri dishes. Caesin agar plates were inoculated with culture growth from a 48-h-old Sabouraud agar by cutting several shallow parallel groves in the agar with a wire loop, followed by incubation at 24 C for 48 h. Following incubation, samples of culture growth were stained with lactophenol cotton blue and were examined for chlamydospore production by light microscopy (20). Yeast isolates were also tested for growth at 45°C for 48 h on Sabouraud glucose agar as described by Pinjon et al. (12).
TABLE 1.
Yeast isolates used in this study
| Organism and source or country of isolation | No. of isolates | Specimen | Source or reference(s) |
|---|---|---|---|
| C. dubliniensis | |||
| Argentina | 1 | Oral | 5, 18 |
| Australia | 3 | Oral | 5, 20 |
| Canada | 5 | Oral | 5, 12 |
| Finland | 1 | Oral | 5, 12 |
| Germany | 4 | Oral | 5, 12 |
| Greece | 1 | Oral | 5, 12 |
| Ireland | 24 | Oral | 5, 18, 20 |
| 4 | Blood | 5; this study | |
| 1 | Vaginal | 5, 10 | |
| Italy | 3 | Oral | This study |
| Israel | 2 | Res. tr.a | 5, 13 |
| 2 | Sputum | 5, 13 | |
| 1 | Urinary | 5, 13 | |
| 2 | Vaginal | 5 | |
| 1 | Wound | 5 | |
| The Netherlands | 2 | Blood | 5, 8 |
| 1 | Sputum | 5, 8 | |
| Spain | 33 | Oral | 5, 12, 14; this study |
| 1 | Blood | 15 | |
| 1 | Vaginal | This study | |
| Switzerland | 7 | Oral | 3, 5 |
| United Kingdom | 6 | Oral | 5, 12, 16 |
| 1 | Blood | 5, 12, 16 | |
| 1 | Fecal | 5, 12 | |
| Culture collection | NCPF 3108 | N/A | 20 |
| C. albicans | |||
| Argentina | 9 | Oral | This study |
| Greece | 4 | Oral | This study |
| Ireland | 6 | Oral | 1; this study |
| Hong Kong | 1 | Oral | 12 |
| Spain | 45 | Oral | This study |
| 36 | Vaginal | This study | |
| 1 | Urinary | This study | |
| 1 | Penile | This study | |
| United States | 9 | Oral | 1, 11 |
| Culture collection | NCPF 3153 and 3156 and ATCC 26555, 64385, 64548, 64550, 90028, and 90029 | N/A | |
| C. glabrata | |||
| Spain | 9 | Oral | This study |
| Culture collection | NCPF 3203 | N/A | |
| C. guilliermondii | |||
| Spain | 6 | Oral | This study |
| 1 | Vaginal | This study | |
| Culture collection | NCPF 3099 | N/A | |
| C. krusei | |||
| Spain | 3 | Oral | This study |
| 1 | Vaginal | This study | |
| Culture collection | ATCC 6258 | N/A | |
| C. lusitaniae | |||
| Spain | 1 | Bronchial aspirate | This study |
| 1 | Fecal | This study | |
| 1 | Urinary | This study | |
| C. parapsilosis | |||
| Spain | 5 | Oral | This study |
| C. rugosa | |||
| Bulgaria | 6 | Blood | This study |
| 1 | Urinary | This study | |
| 1 | Prosthetic valve | This study | |
| C. stellatoidea type I | |||
| Culture collection | ATCC 11006 | N/A | |
| C. stellatoidea type II | |||
| Culture collection | ATCC 20408 | N/A | |
| C. tropicalis | |||
| Ireland | 5 | Oral | This study |
| Spain | 10 | Oral | This study |
| Culture collection | NCPF 3111 | N/A |
Res. tr., respiratory tract; N/A, not applicable.
All 109 C. dubliniensis isolates tested produced chlamydospores on casein agar after 48 h of incubation at 24°C. Chlamydospores were stained dark blue by lactophenol cotton blue and were very abundant and arranged in groups around pseudomycelial growth (Fig. 1A), but in some cases isolated chlamydospores were also observed. The vast majority (111 of 120 [92.5%]) of the C. albicans isolates tested did not produce chlamydospores on casein agar after 48 h of incubation. In these isolates, only yeast-like cells of different sizes stained a light blue color by lactophenol cotton blue were observed (Fig. 1B). However, 9 of 120 (7.5%) of the C. albicans isolates tested produced chlamydospores on casein agar. These chlamydospores, although indistinguishable from those produced by C. dubliniensis, were difficult to observe due to their low number. The identity of these isolates as C. albicans was reconfirmed by carbohydrate assimilation profile analysis with the API ID32 system, by lack of reactivity with a C. dubliniensis antiserum, and in some cases by PCR (2, 4, 11). The production of chlamydospores on casein agar by Candida species other than C. dubliniensis and C. albicans was also investigated (Table 1). No chlamydospores were produced by the Candida tropicalis, Candida krusei, Candida parapsilosis, Candida guilliermondii, Candida glabrata, Candida lusitaniae, Candida rugosa, Candida stellatoidea type I, and C. stellatoidea type II isolates tested (Table 1). For all of the Candida strains included in the study, results similar to those described above were obtained in separate experiments with three different batches of casein agar.
FIG. 1.
Abundant chlamydospore production on casein agar by C. dubliniensis isolate 00131 (A) and absence of chlamydospore production by C. albicans isolate 00160 (B) incubated for 48 h at 24°C. Magnification, ×400.
Casein agar seems also to be suitable for studying chlamydospore production by fresh isolates, since, when 8 fresh C. dubliniensis oral isolates and 10 fresh C. albicans oral isolates were tested, all of the C. dubliniensis isolates produced abundant chlamydospores within 48 h, whereas none of the C. albicans isolates did.
The ability to grow at 45°C was studied in an attempt to differentiate the C. albicans isolates producing chlamydospores on casein agar from the C. dubliniensis isolates. Previous studies demonstrated that C. dubliniensis isolates do not grow at 45°C, whereas the majority of C. albicans do (12). While no growth was found with any of the 109 C. dubliniensis isolates at 24 and 48 h on Sabouraud dextrose agar at 45°C, all 120 of the C. albicans isolates grew well at 45°C.
Casein agar has been traditionally used to study the decomposition of casein by aerobic actinomycetes and dematiaceous fungi (7). However, results presented in this study show, for the first time, that casein agar is a good medium to induce the production of chlamydospores by C. dubliniensis isolates, a feature that can be used to differentiate C. dubliniensis from C. albicans. Although 106 of 109 (97.2%) of the C. dubliniensis isolates tested produced abundant chlamydospores on casein agar, nine isolates of C. albicans isolates also produced very few chlamydospores. Attempts to improve discrimination between the two species by decreasing the temperature of incubation or by modifying the composition of casein agar by varying the amount of skim milk added or by incorporating 1% Tween 80 were unsuccessful (data not shown). Discrimination between C. dubliniensis isolates and the nine C. albicans isolates that produced chlamydospores on casein agar was achieved by incubating the isolates at 45°C, since all nine C. albicans isolates grew well at that temperature, whereas the 109 C. dubliniensis isolates included in the study did not. Although growth at 45°C alone permitted discrimination between C. albicans and C. dubliniensis isolates in this study, it has been reported that some C. albicans isolates are not able to grow at 45°C (6, 9, 12).
Attempts to differentiate C. dubliniensis from C. albicans on the basis of chlamydospore production in media such as Staib agar and caffeic acid-ferric citrate agar have been made (1, 17). However, casein agar is less expensive and simpler to prepare than these media and casein agar could easily be prepared in routine clinical mycology laboratories. In conclusion, casein agar provides a simple and inexpensive means of differentiating isolates of C. dubliniensis and C. albicans. In the vast majority of cases, isolates producing abundant chlamydospores on casein agar can be presumptively identified as C. dubliniensis.
Acknowledgments
We thank all of our colleagues throughout the world who have sent us some of the strains tested in this study.
This investigation was supported by grants 9/UPV 0093.327-13550/2001 from the Universidad del País Vasco and PM99-0033 from the Dirección General de Enseñanza Superior e Investigación Científica from the Spanish Ministerio de Educación y Cultura.
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