Mycology –Yeast

Student Lab

Division of Medical Technology

Carol Larson MSEd, MT(ASCP)

Basic Characteristics

• Unicellular

• Generally normal flora

Basic Characteristics

• Asexual reproduction– Blastoconidia– Pseudohyphae– Arthroconidia

Direct Examination

• Observe for reproductive structures

• Gram stain– Gram positive

• India ink stain– Capsules

Growth Requirements

• Growth in 2-3 days

• Temperature– 25-37ºC– Best at 30ºC

Colony Morphology

• Appearance– Similar to bacteria colony

• Texture– Glabrous– May be filamentous

• Pigmentation

• Confuse with Staphylococcus

Yeast ID Methods

• Germ Tube Production– First test performed when yeast– Procedure– Interpretation

Yeast ID Methods

• Cornmeal-Tween 80 agar– Procedure– Observe microscopically for

• Chlamydospores• Blastoconidia• Pseudohyphae• Arthrospores

Yeast ID Methods

• Niger Seed Agar (Birdseed agar)– Procedure– Observe for brown pigment caused by

phenol oxidase activity– Cryptococcus

neoformans

Yeast ID Methods

• Urease– Procedure– Positive:

• Cryptococcus sp. (rapid)• Trichosporon sp.• Rhodotorula sp.

– Negative:• Candida sp.• Geotrichum sp.

Yeast ID Methods

• Carbohydrate assimilation– Utilize carbohydrate as sole source of

carbon in the presence of oxygen– Positive = growth

• Carbohydrate fermentation– Utilize a carbohydrate anaerobically– Positive = gas

Yeast ID Methods

• Commercial identification kits– Manual methods

– Automated methods

Candida species

• Normal flora

• Leading opportunistic fungal infection in man

• Candidiasis– Mucocutaneous– Cutaneous– Systemic

Factors predisposing to infection

• Immunocompromised

• Malignancy

• Prolonged antibiotic therapy

• Lupus, Diabetes

• Pregnancy and use of birth control pills

• Age

• Damaged skin barrier

Candida albicans

• #1 fungal isolate in laboratory

• Germ tube: positive

• CMT: clustered blastoconidia at septa of pseudohyphae, terminal chlamydospores

• CHO: sucrose positive

Candida albicans

Candida stellatoidea

• Germ tube: positive

• CMT: clustered blastoconidia at septa of pseudohyphae, terminal chlamydospores

• CHO: sucrose negative

Candida tropicalis

• Germ tube: negative

• CMT: sparse single or short-chained blastoconidia anywhere along pseudohyphae, rare chlamydospores

• CHO: sucrose positive

Candida glabrata

• Causes UTI

• Germ tube: negative

• CMT: only blastoconidia, no pseudohyphae

• CHO: only glucose and trehalose positive

Candida krusei

• Germ tube: negative

• CMT: tree-like branching of abundant blastoconidia from the septa of elongated pseudohyphae (“cross-matchsticks” appearance)

Candida pseudotropicalis

• Germ tube: negative

• CMT: branched pseudohyphae with chains of elongated blastoconidia – logs in a stream arrangement of broken up blastoconidia positioned parallel to each other

Candida parapsilosis

• Germ tube: negative

• CMT: few single or small clustered blastoconidia at or between septa of thin curved pseudohyphae. Sometimes giant pseudohyphae may be observed

Cryptococcus neoformans

• Cryptococcosis– Pulmonary– Meningitis

• Pigeon droppings

Cryptococcus neoformans

• Gram stain

• India ink stain

Cryptococcus neoformans

• Germ tube: negative

• CMT: large, round blastoconidia, no pseudohyphae

• Urease: positive (within 3 hours)

Cryptococcus neoformans

• Niger seed agar: brown pigment

Cryptococcus neoformans

• Cryptococcal antigen test

Geotrichum candidum

• Geotrichosis– Mimics Candida infections– Infection in debilitated people

• Considered a mold

• Germ tube: negative

• CMT: arthroconidia andtrue hyphae

• Urease: negative

Trichosporon beigelii

• Causes White Piedra

• Germ tube: negative

• CMT: arthroconidia, true hyphae, rare blastoconidia

• Urease: positive

Rhodotorula species

• Saprophytic yeast found in soil

• Rarely a pathogen

• Often encapsulated

• Urease: positive

• Colony pigmentation: reddish-orange

Saccharomyces species

• Rarely a pathogen

• Ascospores (acid fast positive)

Malassezia furfur

• Causes Tinea Versicolor

• Septicemia in patients receiving intravenous lipid therapy

• KOH: clusters of thick-walled, round budding yeast and short, straight or angular mycelial fragments (spaghetti and meatball appearance)

• Lipophilic

In Summary …

• Key characteristics of yeast

• Identification methods– Germ tube– CMT– Biochemical tests

• Clinically significant yeast– Candida albicans– Cryptococcus neoformans