Yeasts

Yeast infections predisposing factors

Immunosuppression, antibiotics, diabetes, indwelling catheters, steroid use, pregnancy, hospitalization, disrupt normal flora, allowing, opportunistic infection

Virulence factors yeasts

Capsule, melanin, thermotolerance, adherence, phenotypic switching, enzyme production, allow immune evasion, and survival

Capsule function yeast

Prevents phagocytosis especially in Cryptococcus neoformans

Melanin yeast

Protects against oxidative damage and host defenses

Thermotolerance yeast

Ability to grow at 37°C allowing infection in humans

Adherence yeast

Attachment to host tissues especially Candida species

Phenotypic switching yeast

Ability to change form to evade immune system

Budding yeast

Round cells reproducing by budding forming blastoconidia

Pseudohyphae

Elongated yeast cells attached with constrictions at septa

True hyphae

Long filamentous structures without constrictions and parallel walls

Germ tube test

Test for Candida albicans showing tube formation without constriction in serum at 37°C

Chromagar

Differential media using chromogenic substrates to identify yeast by colony color

Chromagar Candida albicans

Green colonies

Chromagar Candida glabrata

Dark violet colonies

Chromagar Candida krusei

Pink colonies with pale border

Chromagar Candida tropicalis

Blue gray colonies

Chromagar Candida parapsilosis

Pink to lavender colonies

Cornmeal agar yeast

Used to observe morphology including pseudohyphae and chlamydospores

Birdseed agar

Selective media detecting Cryptococcus neoformans producing brown black colonies due to melanin

India ink test

Negative stain showing capsule as halo around Cryptococcus cells

Antigen detection yeast

Detects fungal components such as cryptococcal antigen or beta D glucan in serum or CSF (AD)

Cryptococcal antigen test

Detects capsule polysaccharide in CSF or serum for rapid diagnosis (cryp)

Beta D glucan test

Detects fungal cell wall component indicating invasive fungal infection (BD)

Commercial yeast identification

Automated systems using biochemical or protein profiles for rapid identification

API 20C AUX

Manual biochemical test based on carbohydrate assimilation

VITEK yeast identification

Automated system using biochemical reactions for identification

MALDI TOF yeast

Mass spectrometry identifying organisms by protein fingerprinting

Malassezia furfur growth

Lipid dependent yeast requiring fatty acids for growth

Malassezia furfur patients

Common in adolescents immunocompromised and patients on IV lipids

Malassezia furfur infections

Causes tinea versicolor and catheter related infections

Malassezia furfur morphology

Spaghetti and meatballs appearance with short hyphae and round yeast

Candida albicans

Germ tube- positive, green colonies pseudohyphae and chlamydospores; normal flora & opportunistic pathogen

Candida krusei

Pink colonies, pseudohyphae present, intrinsically fluconazole resistant

Cryptococcus neoformans

Encapsulated yeast, India ink positive, birdseed agar, black colonie- causes meningitis

Rhodotorula

Produces red or orange pigmented colonies, environmental yeast

Geotrichum

Forms arthroconidia and resembles mold like yeast G

Trichosporon

Produces arthroconidia and pseudohyphae opportunistic pathogen T

Prototheca

Algae like organism not true yeast no budding causes rare infections P

Saccharomyces

Baker yeast occasionally opportunistic pathogen

Candida parapsilosis

Associated with catheters forms pseudohyphae

Candida glabrata

Small yeast cells no pseudohyphae increased antifungal resistance

Candida tropicalis

Pseudohyphae- present, blue colonies on chromagar, common in immunocompromised patients

Yeast identification clues

Use colony color, morphology, biochemical tests, growth requirements, and clinical context for identification