Preliminary Identification Methods for Various Yeasts

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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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Preliminary Identification Methods for Various Yeasts

A preliminary microscopic examination of a clinical specimen for the presence of yeast can often provide a rapid means of detecting infections so that treatment can begin.
Wet mounts and potassium hydroxide (KOH) preps
Wet mounts and KOH preps are often performed in physicians' offices or clinic laboratories, typically on vaginal secretions. Yeast may be present in the form of blastospores (budding cells-- indicated by the blue arrows) or pseudohyphae (a chain of elongated yeast cells resembling hyphae-- indicated by the red arrow).
KOH is able to dissolve keratinaceous material; in thick discharges, yeasts may be detected more easily with this preparation. KOH preps are also useful for examining skin, hair, nails, tissue, and respiratory specimens.

India ink prep
The India ink prep can provide both rapid detection and presumptive identification of Cryptococcus neoformans. The capsule of Cryptococcus will exclude the ink particles, so that a characteristic halo forms around the yeast cell, as indicated by the arrows in the image below. Budding forms should be observed to confirm a positive smear. However, this test is not as sensitive as the cryptococcal antigen test. India ink preparations are negative in at least half the cases of cryptococcal meningoencephalitis, and early in disease, inflammatory cells may not be present.

Germ tubes
In many laboratories, the germ tube is the starting point in the identification of a yeast. The form seen below is a yeast cell giving rise to a germ tube. The germ tube is about half the width and three to four times the length of the cell. It is a straight-sided extension from the yeast cell. If a constriction is present, this indicates the formation of pseudohyphae, rather than a germ tube.
In the classic germ tube test (where fetal bovine serum is lightly inoculated with the yeast isolate and incubated at 35 - 37° C for 2.5 - 3 hours), the production of germ tubes is usually diagnostic of Candida albicans. However, Candida dubliniensis also produces germ tubes, although the incidence of this organism is much less than C. albicans. Still, laboratories may consider reporting germ tube positive yeasts as "Candida albicans/dubliniensis." It is recommended that a minimum of five well-defined germ tubes are observed before the isolate is called positive.