Mycology III: zygomycetes and yeasts

zygomycetes

• large ribbon like hyphae that contain the occasional septa

  • may not be apparent

  • grow rapidly (lid lifters)

• identification is based upon the sporangia and the appearance and placement of rhizoids

• found worldwide

Zygomycetes genus

• rhizopus

• mucor

• syncephalastrum

• cunninghamella

Zygomycetes infections

• important cause of morbidity and mortality in immunocompromised patients

• infection typically is acquired through inhalation of spores

• have an affinity for vascular invasion

• causes necrosis of surrounding tissue

• may also invade the brain through the sinuses

rhizopus

• rapid growing

• colonial growth is wooly with a grayish appearance

• rhizoid opposite sporangiospore

Mucor

• rarely a cause of human disease

• rhizoids are not present

• rapidly growing

Absidia

• rapid growing

• rhizoids alternate with conidiophores

cunninghamella

• produces single-spored sporangioles supported on small denticles projecting from the vesicle

Syncephalastrum

• rarely associated with human disease

• produces tubular sporangial heads

• rhizoids are present

Yeasts

• most common fungi isolated from patients

• a part of the normal flora (except crypto)

• significance of isolation depends on body site

• yeast are unicellular budding cells that are round to oval in shape

• cultures are moist creamy or glabrous

Candida info

• most frequently encountered opportunistic fungal infections

• found everywhere in the environment

• normal flora of skin, mouth, vagina and stool

• if is is recovered from several body sited at the same time it is a good indicator of disseminated infection

Clinically significant yeasts

• C. albicans

• C. tropicalis

• C. parapsilosis

• C. glabrata

• C. krusei

• Cry. neoformans

• malassezia furfur

on direct exam..

• simple budding yeast cells with or without pseudohyphae are observed

• Colonial morphology is of little  value in identification.

        Helpful in determining what tests for  identification should be performed.

• Identification is typically based  upon biochemical reactions and  or microscopic examination

Cryptococcus neoformans

• widely found in nature

• associated with bird droppings especially pigeons

• aerosolization is a requirement for most infections

• infections may be acute, subacute or chronic

• in the immunocompromised patient it is common to see disseminated disease

Cryptococcus neoformans ID

• Traditionally india ink prep was used for rapid detection

• II positive

• now the cryptococcal antigen is utilized due to the higher sensitivity

• colony morphology: small white to tan colonies that are mucoid to creamy

• Growth is inhibited by cycloheximide

• colonies appear tan on bird seed (niger seed) agar (definitive ID)

Malassezia furfur

• causes tinea versicolor

• skin infection characterized by superficial brownish scaly area on light skin and lighter areas on dark skin.

• can cause disseminated infection in infants on lipid replacement therapy

• requires olive oil for growth invitro

• colonies are small creamy to white

• direct exam small hyphal fragments with oval shaped cells are observed

• “Spaghetti and meatballs’’

Germ tube

• Quick- results in about 3 hours easily perfromes and relatively inexpensive

• appear as hyphal like extensions that are produced without constriction at the point of origin

• C. albicans +

• other candida species =

Cornmeal agar

• morphological observations of yeast arrangement of blastoconidia

• presence or absence of pseudohyphae and chlamydospores seen in C. albicans

• take 72 hours

Other ID of yeasts

• commercially available ID systems

• rapid results in 18 to 72 hours

• ID is based upon database of thousands of biotypes

• should be used in conjunction with cornmeal morphology

Susceptibility Testing of Yeast

• performed at physician request

• some species are innately resistant to some antifungals

• C. glabrata to itraconazole, and some emerging resistance to Amphotericin B

• C. krusei to fluconazole