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Dermatophytes, molds, fungal biomarkers and Pneumocystis

17 min

Skills you'll build4 skills
  • Confirm a dermatophyte from skin, hair, or nail by colony and microscopic features
  • Distinguish broad pauciseptate from narrow septate hyphal patterns
  • Read galactomannan and beta-D-glucan results against the fungi they miss
  • Report a quantitative Pneumocystis PCR without overcalling pneumonia or colonization

Try first

Try first

A sinus biopsy shows narrow, regularly septate hyaline hyphae with acute-angle, dichotomous branching invading tissue. What does the laboratory report from the direct examination?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Hyphal patterns in tissue

Hyphae invading viable tissue support invasive mold disease. Their morphology suggests a group of molds.1,2

FeatureMucorales patternAspergillus-like pattern
WidthBroad, ribbon-likeNarrow
SeptaFewRegular
BranchingIrregular, at wide anglesAcute, dichotomous

Folds from tissue processing can look like septa. Fusarium, Scedosporium and other hyaline molds look like Aspergillus in tissue. Culture on a medium that contains no cycloheximide, or molecular testing, identifies the mold.1,2

Dermatophyte culture and identification

Skin scale is collected from the active border of the lesion. KOH shows hyaline septate hyphae and arthroconidia and does not identify a species. A color change on dermatophyte test medium (DTM) is presumptive, because nondermatophyte molds can also alkalinize it. A dermatophyte is reported from colony and microscopic features or a validated identification method.3

Fungal biomarkers and their limitations

Galactomannan comes from the Aspergillus cell wall, and its sensitivity falls during mold-active therapy. (1→3)-β-D-glucan rises in several invasive fungal diseases and in Pneumocystis pneumonia, and it does not identify the organism. Hemodialysis materials, blood products and glucan-containing gauze can cause false elevations. Mucorales and Cryptococcus often produce little detectable β-D-glucan, and Mucorales produce no detectable galactomannan. Negative biomarkers do not exclude mucormycosis and other rare mold infections.2,4

Reading a Pneumocystis PCR

Pneumocystis jirovecii has no routine clinical culture, and lower respiratory specimens give the best yield. Quantitative PCR is more sensitive than staining. No universal cutoff separates pneumonia from colonization. A high burden supports disease in a compatible host. A low positive result can mean early infection, resolving infection or colonization, and burden is often lower in patients without HIV. A low serum β-D-glucan makes Pneumocystis pneumonia less likely.5

Follow the bench demonstration

Sources for this step
References
  1. Patterson TF, Thompson GR III, Denning DW, et al. Practice guidelines for the diagnosis and management of aspergillosis: 2016 update by the Infectious Diseases Society of America. Clin Infect Dis. 2016;63(4):e1-e60. doi:10.1093/cid/ciw326
  2. Cornely OA, Alastruey-Izquierdo A, Arenz D, et al. Global guideline for the diagnosis and management of mucormycosis: an initiative of the European Confederation of Medical Mycology in cooperation with the Mycoses Study Group Education and Research Consortium. Lancet Infect Dis. 2019;19(12):e405-e421. doi:10.1016/S1473-3099(19)30312-3
  3. Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2021.
  4. Donnelly JP, Chen SC, Kauffman CA, et al. Revision and update of the consensus definitions of invasive fungal disease from the European Organization for Research and Treatment of Cancer and the Mycoses Study Group Education and Research Consortium. Clin Infect Dis. 2020;71(6):1367-1376. doi:10.1093/cid/ciz1008
  5. Panel on Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents With HIV. Pneumocystis pneumonia. National Institutes of Health. Accessed September 27, 2026. https://clinicalinfo.hiv.gov/en/guidelines/hiv-clinical-guidelines-adult-and-adolescent-opportunistic-infections/pneumocystis

Watch one

Read each decision and its explanation.

Skin scale from the active border of a scaly foot lesion is cultured. The KOH-calcofluor preparation showed hyaline septate hyphae. On day 7 the DTM has turned red, and a white, cottony colony grows on the nonselective fungal medium. A tape mount made in the biological safety cabinet shows small teardrop-shaped microconidia along the sides of the hyphae and no macroconidia. What do you report?

  1. Check the specimen: scale from the active border.

    The advancing edge contains viable organisms, and the healing center contains few.

  2. Read the direct preparation: hyaline septate hyphae.

    Hyaline septate hyphae in skin support a dermatophyte. KOH cannot identify the species.

  3. Read the DTM: red, a presumptive finding.

    Nondermatophyte molds can also turn DTM red, so the color change is presumptive.

  4. Read the colony microscopy.

    Small pyriform microconidia along the hyphae fit the Trichophyton rubrum complex. The T. mentagrophytes/interdigitale complex forms round microconidia in clusters, and Epidermophyton floccosum forms club-shaped macroconidia and no microconidia.

  5. Report at the level the features support.

    Morphology supports identification to the species complex. Naming beyond what the features show risks a wrong identification.

Report Trichophyton rubrum complex, from the colony and microscopic features. The red DTM alone would not have supported a dermatophyte report.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A sinus biopsy shows broad, ribbon-like, irregularly branching hyphae with few septa invading viable tissue and vessel walls. What does the laboratory report from the direct examination?

Incorrect. Width and branch angle vary, and processing folds can create false septa or misleading angles, so no genus is identified from tissue geometry.

Incorrect. Aspergillus-like hyphae are narrow, regularly septate, and branch at acute angles. Broad, ribbon-like hyphae with few septa and haphazard branching are the Mucorales pattern.

Correct. Ribbon-like hyphae with few septa and irregular branching are the Mucorales pattern, and invasion of viable tissue supports invasive mold disease. Culture on medium without cycloheximide and, when culture fails, molecular testing identify the genus.

Hint
  1. Look at width, septa and branching together.
  2. Consider how far hyphal morphology in tissue can go toward an identification.

Review Mucorales

Problem 2 of 3 · Hints available

Serum galactomannan and (1→3)-β-D-glucan are both negative in a patient with diabetic ketoacidosis and suspected rhino-orbital mucormycosis. How are these results interpreted?

Correct. Mucorales produce little galactomannan or β-D-glucan, so negative biomarkers leave mucormycosis possible. Direct examination, histopathology, and culture of involved tissue are needed to establish the diagnosis.

Incorrect. Mucorales typically produce little of either analyte, so negative results leave mucormycosis and other rare mold infections possible.

Incorrect. A negative galactomannan leaves invasive aspergillosis possible too, particularly during mold-active therapy.

Hint
  1. Consider which fungi release each analyte.
  2. Ask how much galactomannan and β-D-glucan Mucorales release into serum.

Review Fungal biomarkers and antigen tests

Problem 3 of 3 · On your own

A toenail culture turns DTM red on day 4. The colony is gray-green and powdery. A tape mount made in the cabinet shows conidiophores ending in swollen vesicles with chains of conidia. What do you report?

Nondermatophyte molds can also turn DTM red. The colony and microscopy here suggest a different mold.

Reported a dermatophyte from a DTM color change alone

A color change on dermatophyte test medium is presumptive, because nondermatophyte molds can also alkalinize the medium. Colony and microscopic features or a validated identification method are needed before a dermatophyte is reported.

Vesicles bearing chains of conidia are Aspergillus conidial heads. Its significance in a nail depends on repeated recovery and direct findings.

T. rubrum forms small pyriform microconidia along the hyphae and no vesicles.

E. floccosum forms smooth, club-shaped macroconidia in clusters.

Review Dermatophytes

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Beatrix Washburn, 58, MRN 2074386, is a retired choir director receiving chemotherapy for lymphoma.
  • She takes a mold-active azole as prophylaxis and has no HIV infection.
  • She now has fever, a dry cough and falling oxygen saturation.
  • Bronchoalveolar lavage (BAL) fluid goes for a quantitative Pneumocystis jirovecii PCR, and serum goes for fungal biomarkers.
TestResultReference intervalFlag
Galactomannan0.2Less than 0.5Less than 0.5
(1→3)-β-D-glucan240 pg/mL HighLess than 60 pg/mLLess than 60High
P. jirovecii PCR, BALDetected, low positiveNot detectedNot detected

Specimen: H 1, L 2, I 1. Serum. BAL fluid sent separately for PCR.

Decision 1 of 3

How is the negative galactomannan interpreted?

Galactomannan sensitivity falls during mold-active therapy, and she takes a mold-active azole.

Mucorales produce no detectable galactomannan, and the azole lowers sensitivity for Aspergillus.

Reported mucormycosis excluded on negative biomarkers

Mucorales produce no detectable galactomannan, Mucorales and Cryptococcus produce little beta-D-glucan, and galactomannan sensitivity falls during mold-active therapy. Negative results therefore leave mucormycosis and other rare molds possible, and tissue examination and culture are used to look for them.

During mold-active therapy a negative galactomannan does not exclude aspergillosis, and galactomannan does not detect Mucorales.

Review Fungal biomarkers and antigen tests

Decision 2 of 3

What does the β-D-glucan of 240 pg/mL add?

β-D-glucan is a panfungal marker that often rises in Pneumocystis pneumonia. It does not identify the fungus, and blood products or dialysis materials can raise it.

β-D-glucan comes from many fungi. The PCR is the organism-specific result here.

An elevated β-D-glucan is not specific to Aspergillus.

Review Fungal biomarkers and antigen tests

Decision 3 of 3

How is the low-positive PCR reported?

A low positive can mean early or resolving infection or colonization. The PCR value alone does not establish pneumonia.

Reported a low-positive Pneumocystis PCR as pneumonia

A low-positive quantitative PCR can mean early or resolving infection or colonization, and no universal cutoff separates them. Calling it pneumonia claims more than the result shows, and dismissing it as colonization can miss disease in a patient without HIV, whose organism burden is often lower.

Burden is often lower in patients without HIV, so a low positive can still represent pneumonia. Dismissing it can miss disease.

No universal cutoff separates pneumonia from colonization. The care team interprets the value with her immune status, imaging and the elevated β-D-glucan.

Review Pneumocystis

Each result is interpreted according to the host. Mold-active prophylaxis lowers galactomannan sensitivity, so the negative result does not exclude aspergillosis. Without HIV infection, Pneumocystis burden is lower, so the low-positive PCR is reported as a quantitative value for interpretation with her elevated β-D-glucan and her risk.

For reference

Sources checked