Yeast Culture Examination

How are yeasts macroscopically recognized?

1. Creamy colony texture

2. Spicules

What are spicules?

Small projections around the colony that are actually hyphae or pseudohyphae

How are yeasts confirmed microscopically?

1. Wet prep

2. Gram stain

Are yeasts larger or smaller than bacteria?

Larger

What method does McLendon Labs use to ID yeasts?

MALDI-TOF

What method is used if MALDI can’t ID yeast?

Fungal sequencing

What is the principle of India Ink preparation?

A suspension of particles that’s displaced by yeast capsules

What species was India Ink historically used to ID?

Cryptococcus neoformans

What specimen was historically used in India Ink?

Spinal fluid

What are the steps involved in India Ink?

1. A drop of india ink is placed on a glass slide with a loopful of centrifuged spinal fluid sediment and stirred.

2. A coverslip is added and then examine for yeast capsules

All species from which two genuses produce a capsule?

1. Cryptococcus

2. Rhodotorula

What is the most common yeast found in human infections?

Candida albicans

What is a hallmark feature of C. albicans?

The ability to produce germ tubes - the beginning of true hyphae

What two species will produce germ tubes within two hours?

1. Candida albicans

2. C. stellatoidea

How are Candida albicans and C. stellatoidea differentiated?

Sucrose Assimilation Test

What are the steps of the germ tube test?

1. Make a light suspension of yeast-like organism in 0.5 mLs of sterile serum

2. Incubate at 37 degrees for two hours

3. Place one drop of yeast serum on slide with a coverslip

4. Examine microscopically under 100X and 400X

What must you differentiate from a germ tube?

Pseudohyphae

How big are yeasts?

4-6 micrometers in diameter

What media was historically used to stimulate production of other structures over blastoconidia?

Cornmeal agar

What are blastoconidia?

Budding yeasts

What yeasts only produce blastoconidia?

1. Cryptococcus

2. Rhodotorula

3. Candida glabrata

4. Saccharomyces spp.

What are pseudohyphae?

Chains of elongated budding yeast that are constricted at the septa

What species produce pseudohyphae?

All other Candida species except Candida glabrata

What are hyphae?

Filamentous growth that’s not constricted at the septa

What organisms can produce hyphae?

1. Candida spp

2. Trichosporon spp

What are chlamydospores? What species produces these?

1. Thick-walled spores

2. Candida albicans

What are arthroconidia?

Spores produced by fragmentation of true hyphae

What species produce arthroconidia?

1. Geotrichum

2. Trichosporon