In the lab, Candida albicans is the only common Candida species that will test positive for a germ tube test. In this test, yeast colonies are added to sheep or bovine serum and incubated at 37C for a few hours.

Candida glabrata is a small, oval yeast with single terminal buds. C. glabrata does not produce pseudohyphae; whereas, Candida tropicalis will produce long pseudohyphae.

Candida can also be distinguished by a special plate in the lab called a CHROM agar plate that will distinguish Candida species by color.

Cryptococcus neoformans:
Cryptococcus neoformans is a circular/ovoid encapsulated budding yeast that can be found in soil, decaying wood, and bird poop. When inhaled, it can disseminate from the lungs throughout the body or even to the central nervous system (CNS); however most healthy people will not get sick from inhaling it. It mostly infects immunocompromised people. In the lab, it will have brown colonies on bird seed agar which is unique to C. neoformans, can be visualized with an India ink stain, and will test urease positive.

Malassezia:
Malassezia furfur is a common normal flora yeast but can cause skin infections including tinea versicolor. It does not have pseudohyphae. They are small ovoid yeast with a characteristic “collaret.” In the lab it typically requires oil for growth.

Dimorphic fungi:
Dimorphic fungi are fungi which can exist as mold/hyphal/filamentous form or as yeast. Oftentimes dimorphic fungi will grow as a mold at room temperature (20-25C) and a yeast in the body (37C). Blastomyces, Coccidioides, Histoplasma, and Sporothrix are all examples of dimorphic fungi.

Blastomyces dermatitidis:
B. dermatitidis causes blastomycosis, a potentially serious condition if not treated. It is found mostly in decaying wood and leaves. Blastomyces infection is most common through inhalation. Like most other fungi, Blastomyces will not infect most people but will infect immunocompromised people. It is common in the Ohio and Mississippi River Valleys as well as the Great Lakes area.

Morphology at 25C, it forms septate hyphae with short or long conidiophores and round or pear shaped conidia on the apex of the conidiophore that make the structure look like a lolli-pop. At 37C they form yeast like cells with broad based budding.

Coccidioides:
Coccidioides species can cause coccidiodomycosis, also known as valley fever. It is common to the environment in the southwestern United States and parts of Mexico and South America. Coccidioides will not infect most people but certain subgroups are more susceptible.

Morphology at 25C, it forms barrel shaped arthroconidia. At 37C, it forms large spherules with endospores.

Histoplasma capsulatum:
Histoplasma capsulatum is typically found in soil that has been heavily soiled with bat and bird poop. Transmission is via inhalation, and most people who are exposed will not get sick. Geographically it is associated with the Ohio and Mississippi River Valleys.

Morphology at 25C are septate hyphae with round to pear shaped microconidia and tuberculate macroconidia. Morphology at 37C are small round or oval budding cells.

Sporothrix scheckii:
Sporothrix scheckii can cause a rare condition called sporotrichosis, also known as “rose gardeners’ disease.” People can become infected by contacting spores on plant material like rose bushes, hay, and sphagnum moss. Typically the spores will enter through a cut or abrasion and cause a local cutaneous infection; however, in some cases and in immunocompromised people it can spread to the joints, bones, and central nervous system (CNS).

Morphology at 25C are narrow, septate, branching hyphae with slender conidiophores at right angles.
Morphology at 37C are long, slender, cigar shaped cells.

Dermatophytes:
Dermatophytes are fungi that infect the skin, hair, and nails. Remember Digger the dermatophyte from that old toenail fungus commercial?

They typically do not infect living tissues and require keratin for growth which is present in high amounts on skin, hair, and nails. Dermatophytes have a unique ability to breakdown the compact protein keratin and use it for nutrition. Dermatophytes can be transferred by contact, like athletes foot.

Dermatophytes consist of three genera: Epidermophyton, Microsporum, and Trichophyton. Dermatophytes typically produce two different asexual reproductive cells, microconidia and macroconidia.

Epidermophyton:
Epidermophyton floccosum will infect skin and nails but not hair. Microscopically, they will have septate hyphae with club shaped macroconidia. They do not produce microconidia.

Microsporum:
Microsporum audouinii can cause scalp ringworm especially in children, it rarely effects adults. It has a spindle shaped macroconidia, can have comb like hyphae, and terminal chlamydoconidia.

Microsporum canis typically infects cats and dogs but rarely humans. It has a pointy shaped macroconidia.

Trichophyton:
Trichophyton mentagrophyte is the main cause of athletes’ foot! This dermatophyte has made a lot of people scratch their feet. Microscopically, they will have microconidia that are round and clustered on branched conidiophores. They will have cigar shaped macroconidia and can have coiled, spiral hyphae.

 
Zygomycetes:
Zygomycetes characteristically have non-septate, broad, ribbon-like hyphae. They also have sac like structures called sporangium which give rise to sporangiospores, and root like structures called rhizoids produced along the hyphae (Mucor will not have rhizoids). Zygomycetes can undergo sexual reproduction when two hyphae fuse which eventually leads to the creation of a diploid zygospore.

Examples of zygomycetes are Absidia, Rhizopus, and Mucor genera. They are ubiquitous in the environment and can cause mucormycosis in immunocompromised individuals.

 
Other opportunistic molds:

Aspergillus:
Aspergillus species are ubiquitous in our environment and most people breathe them in every day. They can cause problems in immunocompromised people. Microscopically they have septate hyphae, unbranched conidiophores arising from specialized foot cells, and conidiophores are enlarged at the tip. Vesicles are completely or partially covered with flask shaped phialides which may be uniseriate or biseriate

Penicillium:
Penicillium species are ubiquitous in nature and some members of the genus are the source of the widely used antibiotic penicillin. They can cause problems in immunocompromised people. Penicillium species have septate hyphae, branched or unbranched conidiophores with secondary branches known as metulae. The metulae have whorls or flask-shaped phialides with unbranched chains of smooth or rough round conidia. They have a brush like or skeleton hand like appearance.