STEPTRACKERMICROBIOLOGY + INFECTIOUS DISEASE | OPPORTUNISTIC FUNGI | HIGH-YIELD REVIEW

Opportunistic Fungi

Fungi that rarely harm healthy people but exploit specific defects — neutropenia, T-cell loss, diabetic ketoacidosis, catheters and broad-spectrum antibiotics.

FOCUS ON

Candida albicans: yeast + pseudohyphae, germ tube ⁺ at 37 °C → thrush, vulvovaginitis, esophagitis, diaper rash, catheter fungemia, endocarditis (IV drugs), endophthalmitis. Aspergillus fumigatus: septate hyphae branching at acute (45°) angles → aspergilloma in old cavities, ABPA in asthma/CF, invasive disease in neutropenia (hemoptysis, halo sign). Cryptococcus neoformans: encapsulated yeast (India ink, mucicarmine, antigen), pigeon droppings → meningitis in AIDS ("soap bubble" lesions). Mucor/Rhizopus: broad non-septate hyphae at right angles → rhinocerebral mucormycosis in DKA. Pneumocystis jirovecii: PCP in AIDS.

FUNGUS · DISEASE

RISK · MECHANISM

CLINICAL PICTURE

DIAGNOSIS · TREATMENT

Oral candidiasis (thrush)

Infants, inhaled steroids, antibiotics, dentures, HIV

White plaques that scrape off to red base

Nystatin or clotrimazole; fluconazole

Candida vulvovaginitis

Antibiotics, diabetes, pregnancy, OCPs

Itching, thick white "cottage cheese" discharge, normal pH

KOH: pseudohyphae · fluconazole once

Candida esophagitis

HIV (CD4 < 100), immunosuppression

Odynophagia, white plaques

Fluconazole

Invasive candidiasis

Central lines, TPN, ICU, neutropenia, abdominal surgery; IV drugs → endocarditis

Fungemia, endophthalmitis, hepatosplenic abscesses

Blood culture · echinocandin (caspofungin), remove line · eye exam

Chronic mucocutaneous candidiasis

T-cell (Th17/IL-17) defects, AIRE mutation (APS-1)

Persistent skin, nail, mucosal Candida

Immune workup

Aspergilloma

Fungus ball grows in an old cavity (TB, sarcoid)

Hemoptysis; mobile mass with air crescent

Surgery if bleeding

Allergic bronchopulmonary aspergillosis

Type I + III hypersensitivity in asthma, CF

Worsening wheeze, brown mucus plugs, central bronchiectasis, eosinophilia, ↑ IgE

Steroids + itraconazole

Invasive aspergillosis

Neutropenia, high-dose steroids, transplant, CGD; hyphae invade vessels

Fever, pleuritic pain, hemoptysis; nodules with halo sign, infarction

Galactomannan antigen · voriconazole

Cryptococcal meningitis

AIDS (CD4 < 100); inhaled from pigeon droppings; polysaccharide capsule (antiphagocytic)

Subacute headache, fever, ↑ intracranial pressure; skin papules

CSF antigen (latex), India ink halos, mucicarmine · amphotericin + flucytosine → fluconazole; serial LPs

Mucormycosis

DKA (acidosis + high glucose + free iron), neutropenia, deferoxamine; hyphae invade vessels

Facial pain, black necrotic eschar of nose/palate, sinusitis → orbit, brain, cavernous sinus

Surgical debridement + amphotericin B · correct DKA

Pneumocystis pneumonia

CD4 < 200, steroids, transplant; atypical fungus (no ergosterol)

Dyspnea, dry cough, hypoxemia on exertion, bilateral ground-glass, ↑ LDH

Silver stain of BAL (cup-shaped cysts) · TMP-SMX (+ steroids)

Picture it — opportunistic fungi under the microscope

CandidaBudding yeast with chains of elongated cells (pseudohyphae); germ tubes at 37 °C.

AspergillusThin septate hyphae with acute, dichotomous branching.

Mucor / RhizopusWide, irregular, non-septate hyphae branching at right angles.

CryptococcusYeast with a thick polysaccharide capsule that excludes India ink; narrow-based budding.

Schematic diagrams, not to scale.

Mechanism — host defect predicts the fungus

Neutropenia

→

Aspergillus, Candida, MucorHyphae invade vessels

Low CD4 / T cells

→

Cryptococcus, Pneumocystis, mucosal Candida

DKA, iron overload

→

Mucor/RhizopusThrive on acid, glucose, iron

Hyphae compared

FUNGUS

SEPTA

BRANCHING

HOST

Aspergillus

Septate

Acute (~45°)

Neutropenia, asthma, cavity

Mucor/Rhizopus

Non-septate, ribbon-like

Wide (~90°)

DKA

Candida

Pseudohyphae + budding yeast

—

Mucosa, lines

How it's tested

COMMON CONFUSIONAspergillus (septate, acute-angle branching, voriconazole) vs Mucor (non-septate, right-angle branching, amphotericin — voriconazole doesn't cover it).

THE TRAPFluconazole doesn't cover Aspergillus or Mucor, and many Candida glabrata/krusei strains are resistant.

YOUR APPROACHHost defect → likely fungus → microscopic form (yeast, pseudohyphae, septate vs non-septate hyphae, capsule) → drug that covers it.

If you see… think…

Neutropenic patient with nodule and surrounding halo on CT → invasive Aspergillus

Asthmatic with brown plugs, eosinophilia, central bronchiectasis → ABPA

Mass moving inside an old TB cavity → aspergilloma

DKA patient with black necrotic nasal eschar → mucormycosis

AIDS, headache, high opening pressure, India ink halos → Cryptococcus

Germ tube ⁺ yeast forming pseudohyphae → Candida albicans

Fungemia from a central line → Candida (echinocandin, pull line)

Persistent thrush and nail candidiasis with hypoparathyroidism → APS-1 (AIRE)

Cup-shaped cysts on silver stain of BAL → Pneumocystis

Antigen test for invasive aspergillosis → galactomannan