Evaluating the Need for Full Identification of Yeast

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The page below is a sample from the LabCE course Mycology: Yeasts and Dimorphic Pathogens. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Evaluating the Need for Full Identification of Yeast

Full identification of every yeast that is grown in culture may not be necessary, depending on the laboratory's protocols and the patient population that it serves.
Yeasts that are isolated from sterile body sites, tissue biopsies, and blood cultures usually require full identification to the species level, if possible. Some laboratories may also require species identification if yeast is isolated from a catheter tip -- either from an indwelling intravenous catheter, an intraventricular shunt, or the catheter used in continuous peritoneal dialysis. If orange-pink yeast colonies are observed in blood cultures, they can be identified as Rhodotorula rubra. Fungemia has been the most common form of R. rubra infection, almost always associated with a catheter.
Only a limited identification may be required for other body sites. Orange-pink yeast colonies from non-critical sites can be reported as "probable Rhodotorula species. Rhodotorula species may be found in hospital water sources and have caused nosocomial outbreaks in cases where bronchoscopes, brushes and other devices have been washed with contaminated water. Rhodotorula species may also be present on the skin, particularly of hospitalized patients, partially explaining the high association of fungemia with intracutaneous catheters.
For non-pigmented yeasts, most laboratories will perform a germ tube test and report either "Candida albicans" or "Yeast isolated, not Candida albicans" on that basis, unless the colony and/or wet mount morphology suggests Cryptococcus species. However, in genital specimens, a report of "Candida albicans" or "Yeast isolated, not Candida albicans" may not be adequate; increased incidences of C. glabrata in vaginal infections have recently been reported. The concern is that many strains of C. glabrata are showing reduced susceptibility to fluconazole, a commonly used anti-fungal agent. One of the key characteristics in the identification of C. glabrata is the selective assimilation of glucose and trehalose.
In respiratory specimens, such as sputum or bronchoscopy specimens, the presence of yeast is not usually indicative of an acute pulmonary infection, but is probably the result of oropharyngeal infection or colonization. The principle pathogen of concern in the lower respiratory tract is Cryptococcus neoformans; isolates showing characteristics of this organism should be identified completely. Respiratory isolates of yeast other than Cryptococcus may be significant in cystic fibrosis, oncology, neonatal, or other immunocompromised hosts.