Candida dubliniensis
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| Candida dubliniensis | ||||||||||||||
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| Candida dubliniensis |
Candida dubliniensis is an organism associated with AIDS patients. It is a germ-cell positive yeast of the genus Candida, similar to Candida albicans but it forms a different cluster upon DNA fingerprinting.
Prevalence and epidemiology
Candida dubliniensis is found all around the world. The species was only described in 1995, and it is thought to have been previously identified as Candida albicans. Retrospective studies support this, and have given an indication of the prevalence of C. dubliniensis as a laboratory pathogen.
The most useful test for distinguishing C. dubliniensis from C. albicans, is to culture them at 45°C. Most C. albicans will grow at this temperature, while most C. dubliniensis won’t.
A study done in Europe of 2,589 isolates, that were originally reported as C. albicans, revealed that 52 of them (2.0%) were actually C. dubliniensis. Most of these isolates were from oral or faecal specimens from HIV positive patients, though one vaginal and two oral isolates were from healthy volunteers. Another study done in the United States, used 1,251 yeasts previously identified as C. albicans, it found 15 (1.2%) were really C. dubliniensis. Most of these samples were from immunocompromised individuals: AIDS, chemotherapy, or organ transplant patients. The yeast was most often recovered from repiratory, urine and stool specimens.
The Memorial Sloan-Kettering Cancer Center also did several studies, both retrospective, and current. In all 974 germ-tube positive yeasts, 22 isolates (2.3%) from 16 patients were C. dubliniesis. Fifteen of these patients were adults, one a child. Nine were male, 7 were female. All were immunologically compromised with either malignancy or AIDS. The isolates came from a variety of different sites.
Candida dubliniensis is an opportunistic pathogen that can cause both superficial and invasive infections, (mostly in the immunocompromised). About 1-2% of isolates once identified as C. albicans, were subsequently found to be C. dubliniensis. This is most likely still the case now, 1-2% of GCT positive yeasts are probably C. dubliniensis.
Antifungal susceptibility
Most of the isolates of C. dubliniensis are from people who are immunocompromised. Antimicrobial susceptibility is important, as these patients often receive long-term treatment with various anti-fungal drugs. A 1997 report from Ireland found that all 20 isolates were susceptible to itraconazole, ketoconazole and amphotericin B. Sixteen of these were fluconazole sensitive, while four were more resistant.
Importantly, stable fluconazole resistance could be induced (in vitro) by subjecting sensitive strains to increasing concentrations of the antifungal. This resistance is mediated by a multidrug transporter that can be mobilized rapidly in vitro, on exposure to fluconazole.
An AIDS patient in Germany, who had been treated with fluconazole for 18 months, became unresponsive to fluconazole 400 mg/day.
Cases in America have also shown the emergence of fluconazole resistant C. dubliniensis. Three isolates were discovered in Texas, two were resistant (MIC, 64 µg/mL). And one had dose-dependant susceptibility (MIC, 16 µg/mL). In a test on C. dubliniensis in HIV+ patients in Maryland, most isolates were highly susceptible to fluconazole, though one was dose-dependant susceptible, (16 µg/mL), meaning a high dose of fluconazole given to the patient would halt the yeast.
A study of 71 isolates in Ireland, showed that both the fluconazole resistant and susceptible strains were susceptible to itraconazole, amphotericin B, and 5-fluorocytosine (microdilution). They were also susceptible to investigational triazoles and voriconazole, also echinocandin.
It seems C. dubliniensis is very prone to being resistant to fluconazole, or at least need a higher dose. It still retains susceptibility to the other common antifungals, and some investigational new antimicrobials, which can be used when fluconazole fails.