17.1 Yeasts: Candida, Cryptococcus, and Malassezia

17.1 Yeasts: Candida, Cryptococcus, and Malassezia

Quick Answer: Match the yeast to the specimen and the one test that names it. Candida from mucosa or blood: germ tube historically for C. albicans, then CHROMagar, PNA-FISH, or MALDI; treat C. auris as an infection-control emergency. Cryptococcus from CSF or blood: narrow-based budding capsule (India ink/mucicarmine) and cryptococcal antigen (CrAg). Malassezia from skin or lipid-TPN blood: spaghetti-and-meatballs KOH and olive-oil overlay because the organism is lipid-dependent.

Official outline III.D Mycology (guideline 2025-09-25) tests yeasts as a core M topic. Dematiaceous molds, Microsporidium, and antifungal AST are SM-only — do not spend M study time on black-mold species lists or MIC interpretation. What the M exam does expect is that you can place a yeast in the correct clinical compartment, pick the right rapid test, and not confuse Candida, Cryptococcus, and Malassezia with one another or with the dimorphic yeasts in the next section.

Specimen source comes first

Yeasts are unicellular fungi that reproduce by budding (blastoconidia). Some species also form pseudohyphae (constricted filaments of elongated buds) or true hyphae. The bench question is not "is yeast present?" — Gram stains and KOH answer that — but what does this source mean?

Superficial sources — skin scrapings, nail clippings, oral swabs, and vaginal secretions — recover Candida and Malassezia as agents of mucocutaneous disease or as colonizers. A vaginal wet prep with clue cells and a fishy odor points to bacterial vaginosis, not yeast; budding yeasts and pseudohyphae on KOH support candidal vulvovaginitis. Skin KOH that shows short hyphae plus spores is tinea versicolor (Malassezia), not dermatophyte hyphae.

Deep and systemic sources — tissue, abscess aspirates, BAL, CSF, and other sterile fluids — change the meaning of the same morphology. Yeast in CSF is Cryptococcus until proven otherwise. Yeast in a liver or spleen biopsy in a neutropenic patient is invasive candidiasis. Isolation from a sterile site is never reported as "probable contaminant" without a documented clinical conversation.

Blood is its own compartment. Yeast in a blood-culture bottle is a critical value. Candida spp. cause most candidemia; C. auris, Cryptococcus, and Malassezia (neonates on lipid TPN) are the exam-relevant exceptions. Note the bottle type (standard aerobic vs mycosis/F or lysis-centrifugation), time to positivity, and the Gram-stain picture — round buds, elongated buds with pseudohyphae, or tiny oval yeasts — before a colony exists. Aerobic bottles recover most Candida; some Malassezia and Cryptococcus isolates prefer the fungal bottle or prolonged incubation.

Candida species

C. albicans is still the most common clinical isolate, but non-albicans species have risen with azoles, ICU catheters, and TPN. Disease states you must map to the organism:

• Mucocutaneous: thrush, denture stomatitis, esophagitis (especially HIV), vulvovaginitis, diaper dermatitis, chronic mucocutaneous candidiasis.
• Invasive: candidemia, endophthalmitis, hepatosplenic (chronic disseminated) candidiasis after neutrophil recovery, prosthetic-valve endocarditis, catheter UTI.
• Species clues: C. parapsilosis and biofilms on TPN catheters; C. glabrata in older and azole-exposed patients (no true hyphae or pseudohyphae on cornmeal Tween 80 — only tiny blastoconidia); C. krusei intrinsically fluconazole-resistant, so the identification drives therapy even though full antifungal AST is SM-only; C. tropicalis in neutropenia; C. auris in healthcare outbreaks.

Germ tube (historic, still tested). A light inoculum of colony is incubated in serum (or plasma) at 37°C for 2–3 hours. A true germ tube of C. albicans (and C. dubliniensis) is a filament that leaves the mother cell without a constriction. C. tropicalis can throw a "pseudogerm tube" with a constriction at the junction — a classic M trap. Over-incubation past 3 hours produces false filaments. Many laboratories have replaced daily germ tubes with CHROMagar and MALDI-TOF, but the outline still expects the principle. C. dubliniensis is germ-tube positive; it often fails to grow at 42°C, whereas C. albicans grows — MALDI now ends that argument.

CHROMagar Candida uses chromogenic substrates so mixed yeasts declare themselves by color on one plate: C. albicans green ( C. dubliniensis often darker green), C. tropicalis metallic blue, C. krusei pink, rough, and spreading, C. glabrata smaller mauve colonies. C. auris is not reliably identified by color — colonies may be cream, pink, or pale. Never rule out C. auris because the plate "doesn't look like it."

PNA-FISH and MALDI-TOF. Peptide nucleic acid FISH probes applied to a drop of positive blood-culture broth can separate C. albicans from a C. glabrata group in hours and shorten time to targeted therapy. MALDI-TOF MS is the workhorse species ID from a colony and, in some workflows, from a blood-culture pellet. Older MALDI libraries misidentified C. auris as C. haemulonii, Saccharomyces, or another rare yeast. If the isolate is from a healthcare facility, fails to ID cleanly, or is unexpectedly hardy, use a current C. auris-capable library or send molecular confirmation and notify infection prevention the same shift.

C. auris infection control is tested as laboratory practice, not as an MIC table. Place the patient on contact precautions in a single room. Clean the environment with a sporicidal (bleach-class / EPA List P or C. difficile List K) agent; ordinary quaternary ammonium products fail. Screen axilla and groin of contacts and unit neighbors. Report to public health. Do not wait for antifungal susceptibility results — those methods are SM-only on the 2025-09-25 outline — to start isolation.

Cornmeal agar with Tween 80 (Dalmau plate) still earns exam items: C. albicans shows terminal chlamydospores and clustered blastoconidia along pseudohyphae; C. glabrata shows only small blastoconidia; C. parapsilosis shows sage-brush or giant-cell morphology; C. krusei shows elongated cells that look like a "crossed matchstick." Biochemical panels (API 20C AUX, Vitek yeast cards) and urease (negative for most Candida, positive for Cryptococcus and Trichosporon) complete conventional ID when MALDI is down.

Cryptococcus species

C. neoformans (worldwide, aged pigeon droppings, AIDS-defining meningitis) and C. gattii (soil and eucalyptus; more often immunocompetent hosts; Pacific Northwest and tropics) are round to oval yeasts with a polysaccharide capsule. On Gram stain they may look like poorly staining "ghost" cells. India ink or nigrosin on CSF shows a clear halo around a narrow-based budding yeast. Mucicarmine stains the capsule red in tissue. Narrow-based budding is the contrast with Blastomyces (broad-based budding) in the next section.

Cryptococcal antigen (CrAg) on serum or CSF is more sensitive than India ink (ink misses many low-burden cases). Lateral-flow assays are the current bench standard; latex agglutination is the historic method. Titers can be followed. False positives are uncommon; rheumatoid factor and Trichosporon cross-reactivity are the classic caveats, and very high titers can prozone — dilute and repeat if the picture is cryptococcal but the undiluted CrAg is negative. A positive CSF CrAg with lymphocytic pleocytosis, high protein, and low-normal glucose is diagnostic while culture is still incubating.

Culture: creamy colonies on Sabouraud; bird-seed (niger seed) agar turns brown to black because phenoloxidase makes melanin. Urease positive. Inositol assimilation positive. Capsule is enhanced on chocolate agar or in CO2; poorly encapsulated strains still antigen-positive. Disease states: subacute meningitis/meningoencephalitis, cryptococcoma, pulmonary nodules, cutaneous lesions in disseminated disease, and IRIS after ART is started.

Malassezia species

M. furfur complex causes tinea (pityriasis) versicolor: hypo- or hyperpigmented trunk patches that fluoresce dull yellow under a Wood lamp. KOH of scrapings shows short hyphae and round spores — spaghetti and meatballs. The organism is lipophilic. Unsupplemented Sabouraud often yields nothing unless you overlay sterile olive oil or use a lipid medium (Dixon, Leeming-Notman). In the nursery, M. furfur causes catheter-related fungemia in infants receiving lipid TPN; tell the clinician to stop the lipid and pull the line. Direct Gram stain of blood or the catheter may already show tiny oval yeasts — culture without oil is a false-negative trap. M. pachydermatis (dogs) is less lipid-dependent and can grow without overlay; most human M. furfur isolates will not.

Seborrheic dermatitis and folliculitis are additional Malassezia disease states. Do not set up a dermatophyte workup and call it tinea corporis when the KOH is spaghetti and meatballs rather than long septate dermatophyte hyphae.

Bench pitfalls that cost points

Germ-tube negative does not mean "not Candida." It means not C. albicans/C. dubliniensis. Cryptococcus does not make pseudohyphae on cornmeal and is urease positive; Candida is usually urease negative. Malassezia that "won't grow" is usually a missing lipid, not a dead organism. C. auris is identified by an updated library plus infection-control action, not by CHROMagar color. Keep dimorphic tissue yeasts (Histoplasma small intracellular; Blastomyces broad-based; Sporothrix cigar bodies) out of this yeast algorithm — they convert with temperature and are the next section.