Clinical Pathology: General Principles, Microbiology

Candida is the most frequently isolated fungus from blood cultures, and candidemia is the fourth commonest cause of hospital-associated bloodstream infections in the U.S. Candida albicans is the predominant isolate; Candida glabrata is common among the elderly and Candida parapsilosis among children. Associated risk factors for candidemia include the use of broad-spectrum antibiotics, central venous catheters, and length of stay in intensive care units.

• Differentiation between C. albicans and C. glabrata impacts patient care by influencing the empiric selection of antifungal therapy. Generally, C. albicans is susceptible to fluconazole, whereas C. glabrata may be resistant.

• Peptide nucleic acid fluorescent in situ hybridization (PNA FISH) assay uses multi-color labeled fluorescent PNA probes to target specific 26S rRNA sequences of clinically important yeasts, such as C. albicans and C. glabrata, directly from positive blood culture bottles. When PNA probes hybridize to specific targets, the species is identified by microscopic visualization of the fluorescent colored cells (e.g., green for C. albicans).

In vitro susceptibility testing for yeast uses broth microdilution technology and requires incubation for 24-48 hours for Candida and 72 hours for Cryptococcus neoformans. Interpretative guidelines for this purpose were established by the Clinical and Laboratory Standards Institute for fluconazole, itraconazole, voriconazole, anidulafungin, caspofungin, micafungin, and flucytosine.

• Invasive Candida infections can be treated with lipid formulations of amphotericin B, azoles (e.g., fluconazole), or echinocandins (e.g., caspofungin). C. glabrata infections often require echinocandins or amphotericin in lieu of treatment with azoles, which are fungistatic.

Heil EL, Daniels LM, Long DM, et al: Impact of a rapid peptide nucleic acid fluorescence in situ hybridization assay on treatment of Candida infections. Am J Health Syst Pharm. 2012;69:1910-1914.

Shepard JR, Addison RM, Alexander BD, et al: Multicenter evaluation of the Candida albicans/Candida glabrata peptide nucleic acid fluorescent in situ hybridization method for simultaneous dual-color identification of C. albicans and C. glabrata directly from blood culture bottles. J Clin Microbiol. 2008;46:50-55.

Clinical and Laboratory Standards Institute. Reference method for broth dilution antifungal susceptibility testing of yeasts, M27-A3, vol 28. Wayne, PA, 2008.

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