Histopathology and Mycology of Fungal Infections
Moderators: Michel Huerre, Gary W. Procop, Mary Klassen-Fischer, Randall T. Hayden, Glenn D. Roberts
Section 2 -
Yeast and Yeast-Like Forms
Gary W. Procop
Case 1 - Clinical History:
The patient was a 53 year old man with sarcoidosis on mycophenolate, prednisone, plaquenil and had
recently received his first dose of remicade for steroid refractory disease. He had chronic erythematous
papules without ulceration on the ankles bilaterally and onychomycosis. The papules were biopsied at an
outside hospital and read as "deep seated fungal infection, cryptococcus vs. blastomyces, and submitted
to the Cleveland Clinic for further evaluation. The culture isolate recovered from the biopsy, and the
biopsy materials were reviewed (biopsy presented here). The patient had no systemic symptoms and no
evidence of pulmonary disease.
Case 1 - Figure 1
Case 1 - Figure 2
Case 2 - Clinical History:
The patient was a 63 year old man with chronic lymphocytic leukemia, associated thrombocytopenia, and
pneumonia. His past medical history consisted of neutropenic fever five months prior to admission for
which he finished a course of augmentin, voriconazole and ciprofloxacin. Due to residual pneumonia,
thought possibly to be caused by a fungus based on a previous CT scan, the patient continued voriconazole
and levaquin for another 14 days. This seemed to resolve by CT by three months prior to admission, but
the Infectious Diseases clinician decided to continue with acyclovir and levaquin prophylaxis. Patient
also continued chemotherapy cycles during this time period as tolerated.
Case 2 - Figure 1
Case 2 - Figure 2
The patient had a subjective fever and chills for a two days prior to admission. He had a mildly
productive cough with clear sputum for approximately one week that had increased over the past two days.
He has been fatigued since his last chemotherapy cycle, two weeks prior. He had no shortness of breath,
nausea, dysuria, diaphoresis, or diarrhea. Chest x-ray at admission showed bilateral mid and lower lung
nodules that were thought to be progressive compared with previous studies. A diagnostic transbronchial
biopsy was performed.
A wide variety of fungi that infect humans produce yeast or yeast-like forms in tissue. These may
actually be yeast organisms, such as C. albicans and other Candida species, the yeast or "parasitic" form of dimorphic fungal pathogens, or
other, less commonly encountered organisms, such as Prototheca – which is
actually an achlorophyllic algae. It may be difficult to definitively identify the fungus present in
some instances, but often an accurate identification is possible given enough forms to examine and
One of the most important features for differentiating these organisms is the inflammatory response.
For the most part, the type of inflammatory response generated is typical for particular organisms.
However, these typical responses may be alternated or attenuated by immunosuppression, whether inherited,
contracted (infectious) or iatrogenic. For example, Candida species in an
immunocompetent host usually generates a neutrophilic response, whereas the yeast form of H. capsulatum usually is associated with granulomas. In addition to learning the
typical inflammatory responses to these and other pathogens, the Infectious Diseases Pathologist must
learn the attenuated responses of the immunocompromised host, which vary by degree and type of
After characterizing the inflammatory response, a careful morphologic study of the fungal forms
present should be made. Particular attention should be given to size, variability of size, type of
reproduction- budding vs. other, the presence of hyphae, pseudohyphae, or both, and other "special
features". The special feature to recognize are capsules – which should be confirmed using histochemical
stains, the character of the budding (e.g., the pathognomonic broad-based bud of Blastomyces dermatitidis), the spherule of Coccidioides, and the sporangium of
Prototheca, amongst others.
The actual size of the yeast forms present may be determined by measuring many yeast and averaging the
results, but this is only occasionally done in a busy surgical pathology practice. Yeasts and yeast-like
fungi can generally be categorized as small (2-5 mM), intermediate (>5 - <10 m M) or large (>10
m M). For example, the yeast forms of H. capsulatum and Candida glabrata are almost always in the small size category, whereas Blastomyces dermatitidis is almost never in this category (i.e. it is either in
the intermediate or large yeast category).
Similarly, some yeasts or yeast-like forms, such as those produced by Histoplasma capsulatum and Pneumocystis jiroveci, are
relatively consistent in size, whereas the yeasts of Cryptococcus neoformans
are typically variable in size. When a spherule of Coccidioides bursts,
variable sized endospores and enlarged, immature spherules may be seen; this variation in size is typical
of C. immitis.
Attention to the details of asexual reproduction is also helpful in achieving an accurate
identification. The most common form of reproduction is blastoconidiation or budding. If budding is
seen the number of connected buds or daughter cells should be observed, as well as the thickness of the
bud connection or "neck". Histoplasma, Cryptococcus, Malassezia, and Blastomyces usually have a single bud, but occasionally two may be seen. The buds
of Histoplasma and Cryptococcus are more
"narrow necked", whereas the broad based buds are seen with Blastomyces and
Malassezia. These latter two are effectively differentiated based on size,
whereas the former two are differentiated based on the presence/absence of a capsule and as mentioned
differences in size variability, amongst other features. The yeast of Paracoccidioides brasiliensis are typically surrounded by numerous, narrow-necked
buds, which is a pathognomonic finding for this fungus.
The presence of hyphae and/or pseudohyphae suggests the possibility of a Candida species other than C. glabrata, but other
fungi may demonstrate this morphology. Other possibilities include Trichosporum species, Blastoschizomyces capitatus,
and Exophiala jeanselmei in a phaeohyphomycotic cyst. It is important when
true hyphae are seen that the presence of a hyaline septate mold is not necessarily assumed, but rather a
search is made for the possibility of a budding yeast component, since several Candida species may produce true hyphae. Clinicians may opt for quite different
therapies for patients with an invasive yeast infection than for those with an infection by an invasive
hyaline septate mold.
It is also necessary to become familiar with "special features" or special forms that are associated
with particular fungi. Likely the capsule of Cryptococcus neoformans is the
"special feature" of yeast and yeast-like fungi that is most commonly recognized. However, other yeast
forms, such as Histoplasma capsulatum may appear to have a capsule secondary
to retraction artifact caused by tissue processing. Therefore the use of special stains, such as
muricarmine or alcian blue, to confirm the nature of the polysaccharide capsule are recommended. The
Fontana Masson stain highlights melanin and melanin-like precursors present in the cell wall of C. neoformans, although these yeast do not appear brown or dermatiaceous using
typical light microscopy. Several other special features such as the broad-based bud of B. dermatitidis and reproduction via endogenous fission planes seen in C. immitis, Penicillium marneffei, and Prototheca are
important to note.
In summary, the basic morphology of yeast and yeast-like fungi in histologic sections, and the
inflammatory response they generate affords the identification of many of these fungal pathogens. When
it does not, traditional culture and advanced molecular techniques may be employed to achieve a