A 66-year-old woman has received micafungin for 3 weeks for an infected pancreatic collection and now has fever again. Blood cultures grow a small yeast that forms no pseudohyphae on cornmeal agar, and MALDI-TOF identifies Nakaseomyces glabratus (Candida glabrata). Broth microdilution MICs (µg/mL) with CLSI breakpoints for this species: micafungin 0.5 (S ≤0.06, R ≥0.25); anidulafungin 1 (S ≤0.12, R ≥0.5); fluconazole 16 (susceptible-dose dependent ≤32, R ≥64); amphotericin B 1 (no breakpoint; epidemiological cut-off 2). The best interpretation is:

Echinocandin resistant, probably from an FKS mutation

Amphotericin B resistant; echinocandins active

Echinocandin susceptible; fluconazole resistant

All agents susceptible; continue micafungin

Nakaseomyces glabratus (formerly Candida glabrata) differs from most Candida species on cornmeal Tween 80 agar because it:

Forms blastoconidia only, with no pseudohyphae

Forms a capsule seen with India ink

Forms terminal chlamydoconidia on pseudohyphae

Forms arthroconidia that break apart

CSF from a man with advanced HIV shows many encapsulated budding yeasts on India ink. Undiluted CSF is negative by cryptococcal antigen latex agglutination, but a 1:100 dilution is strongly positive. These results are best explained by:

A prozone caused by antigen excess

Cross-reaction with Trichosporon

A capsule-deficient Cryptococcus strain

Yeasts

14 DOH Abu Dhabi (HAAD) lab licensing practice questions on Yeasts, part of Microbiology. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 66-year-old woman has received micafungin for 3 weeks for an infected pancreatic collection and now has fever again. Blood cultures grow a small yeast that forms no pseudohyphae on cornmeal agar, and MALDI-TOF identifies Nakaseomyces glabratus (Candida glabrata). Broth microdilution MICs (µg/mL) with CLSI breakpoints for this species: micafungin 0.5 (S ≤0.06, R ≥0.25); anidulafungin 1 (S ≤0.12, R ≥0.5); fluconazole 16 (susceptible-dose dependent ≤32, R ≥64); amphotericin B 1 (no breakpoint; epidemiological cut-off 2). The best interpretation is:
1. AAmphotericin B resistant; echinocandins active
2. BEchinocandin susceptible; fluconazole resistant
3. CEchinocandin resistant, probably from an FKS mutation
4. DAll agents susceptible; continue micafungin
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Answer: C · Echinocandin resistant, probably from an FKS mutation
Both echinocandin MICs are above their CLSI resistant breakpoints for N. glabratus, and the resistance emerged during prolonged micafungin therapy. In this species, echinocandin resistance is usually caused by hot-spot mutations in the FKS1 or FKS2 genes, which encode the drug target, beta-1,3-glucan synthase; sequencing can confirm them. N. glabratus has no 'susceptible' fluconazole category, so an MIC of 16 µg/mL is susceptible-dose dependent, and azole resistance often develops as well. The amphotericin B MIC is below the epidemiological cut-off, so it is the agent most likely to remain active. The result should be phoned, because the patient is failing current therapy.
- A: Amphotericin B 1 µg/mL is below the cut-off of 2, so no resistance is shown.
- B: Both echinocandin MICs exceed the resistant breakpoints, and fluconazole is only SDD.
- D: Micafungin 0.5 and anidulafungin 1 µg/mL are above the resistant breakpoints.
Question 2 · Hard
A germ tube-negative yeast from a blood culture forms dry, flat, pale-pink colonies with a whitish edge on CHROMagar Candida. Cornmeal agar shows elongated blastoconidia branching like crossed matchsticks. This isolate is intrinsically resistant to which antifungal?
1. AAmphotericin B
2. BVoriconazole
3. CMicafungin
4. DFluconazole
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Answer: D · Fluconazole
Rough, pale-pink colonies on CHROMagar and elongated blastoconidia in a crossed-matchstick pattern point to Pichia kudriavzevii (Candida krusei). This species is intrinsically resistant to fluconazole, because its target enzyme, lanosterol 14-alpha-demethylase, binds the drug poorly, so it is reported as fluconazole resistant whatever the MIC. It is usually susceptible to voriconazole, echinocandins and amphotericin B. Fluconazole prophylaxis can select it in patients with blood cancers.
- A: Amphotericin B is usually active, although MICs can be slightly higher.
- B: Voriconazole is usually active against P. kudriavzevii (C. krusei).
- C: Echinocandins such as micafungin are usually active and often used first.
Question 3 · Easy
Nakaseomyces glabratus (formerly Candida glabrata) differs from most Candida species on cornmeal Tween 80 agar because it:
1. AForms a capsule seen with India ink
2. BForms terminal chlamydoconidia on pseudohyphae
3. CForms arthroconidia that break apart
4. DForms blastoconidia only, with no pseudohyphae
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Answer: D · Forms blastoconidia only, with no pseudohyphae
N. glabratus is a small, haploid yeast that does not make pseudohyphae, so cornmeal agar shows only small oval blastoconidia. It is germ tube negative. It is a common cause of candidemia in many hospitals and often has reduced susceptibility to fluconazole, so identification to species matters. Confirmation is by MALDI-TOF or a biochemical panel.
- A: A capsule seen with India ink is a feature of Cryptococcus.
- B: Terminal chlamydoconidia on pseudohyphae are typical of Candida albicans.
- C: Arthroconidia are made by Trichosporon and Geotrichum, not N. glabratus.
Question 4 · Easy
Cryptococcus neoformans is found mainly in which environmental source?
1. APigeon droppings
2. BBat guano in caves
3. CDesert soil
4. DEucalyptus tree debris
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Answer: A · Pigeon droppings
C. neoformans lives in soil enriched with bird droppings, especially pigeon droppings, and infection follows inhalation of small yeast cells or spores. In people with advanced HIV it spreads from the lung to the meninges. Cryptococcus gattii is linked to eucalyptus and other trees and more often affects otherwise healthy people. Cryptococcosis is not passed from person to person.
- B: Bat guano is the classic source of Histoplasma capsulatum.
- C: Desert soil is the source of Coccidioides species.
- D: Eucalyptus and other trees are linked to Cryptococcus gattii.
Question 5 · Hard
CSF from a man with advanced HIV shows many encapsulated budding yeasts on India ink. Undiluted CSF is negative by cryptococcal antigen latex agglutination, but a 1:100 dilution is strongly positive. These results are best explained by:
1. ACross-reaction with Trichosporon
2. BA capsule-deficient Cryptococcus strain
3. CA prozone caused by antigen excess
4. DRheumatoid factor interference
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Answer: C · A prozone caused by antigen excess
When antigen is in great excess, it saturates the antibody on every latex particle, so the particles cannot be cross-linked into visible clumps. Diluting the specimen restores the right antigen-antibody proportion, and the test turns positive at a high titer. A negative undiluted result in a patient with many yeasts on microscopy should always be repeated on a dilution. Very high titers usually mean a heavy fungal burden and a poorer outlook.
- A: A cross-reaction would not explain a negative undiluted test that turns positive on dilution.
- B: The yeasts are well encapsulated, and a capsule-deficient strain would stay negative on dilution.
- D: Rheumatoid factor causes false-positive, not false-negative, reactions.
Question 6 · Hard
A yeast grows from the urine culture of a patient transferred from an overseas ICU. An API 20C AUX strip identifies it as Rhodotorula glutinis, but the colonies are cream-white rather than coral-red. What should the laboratory do next?
1. ARepeat the API strip and report the second result
2. BReport Rhodotorula glutinis from the strip profile
3. CReport 'yeast, not further identified' and stop
4. DSuspect C. auris; confirm by MALDI-TOF or sequencing
Answer and explanation in LearnFree account
Question 7 · Hard
A 58-year-old woman with rheumatoid arthritis has fever and headache. Her serum cryptococcal antigen latex test is positive at 1:4, but the kit's control latex, coated with normal globulin, also agglutinates. CSF India ink and culture are negative. The most likely explanation is:
1. ARheumatoid factor causing a false positive
2. BEarly cryptococcal infection before meningitis
3. CCross-reaction with Candida albicans antigen
4. DA prozone effect from very high antigen
Answer and explanation in LearnFree account
Question 8 · Easy
Which result is typical of Cryptococcus neoformans and separates it from Candida species?
1. APseudohyphae on cornmeal agar
2. BPositive germ tube test
3. CPositive urease test
4. DNo growth at 37 °C
Answer and explanation in LearnFree account
Question 9 · Medium
In a neonatal intensive care unit, bloodstream infections with Candida parapsilosis are most often linked to:
1. ASpread from the infant's own gut flora
2. BHands of staff and central venous catheters
3. CContaminated infant formula powder
4. DSwallowed maternal vaginal flora at birth
Answer and explanation in LearnFree account
Question 10 · Medium
WHO recommends serum cryptococcal antigen (CrAg) screening for adults with advanced HIV before they start antiretroviral therapy. What is the main purpose?
1. ATo monitor the response to antifungal treatment
2. BTo decide whether HIV therapy is needed at all
3. CTo find early infection for pre-emptive fluconazole
4. DTo confirm that the patient has meningitis
Answer and explanation in LearnFree account
Question 11 · Hard
Candida auris from a blood culture gives these MICs: fluconazole 128 µg/mL, amphotericin B 1 µg/mL and micafungin 0.12 µg/mL. CDC tentative resistance breakpoints are fluconazole ≥32, amphotericin B ≥2 and micafungin ≥4 µg/mL. How should the isolate be interpreted?
1. AResistant to fluconazole and amphotericin B
2. BResistant to fluconazole only
3. CSusceptible to all three agents
4. DResistant to all three agents
Answer and explanation in LearnFree account
Question 12 · Medium
A germ tube-negative yeast from a blood culture forms dark metallic-blue colonies on CHROMagar Candida. On cornmeal Tween 80 agar it shows long pseudohyphae with single blastoconidia along their length, and no chlamydoconidia. What is the most likely identification?
1. ACandida albicans
2. BCandida dubliniensis
3. CCandida tropicalis
4. DCandida parapsilosis
Answer and explanation in LearnFree account
Question 13 · Medium
In culture-based screening for Candida auris, the swab is first incubated in salt Sabouraud dulcitol broth at 40 °C. This step:
1. ATurns C. auris colonies a distinctive pink color
2. BMakes C. auris form germ tubes for quick detection
3. CKills bacteria but lets all yeasts grow equally
4. DFavors C. auris over most other skin yeasts
Answer and explanation in LearnFree account
Question 14 · Medium
An India ink preparation of CSF from a patient with HIV shows encapsulated yeasts. Which test is best for confirming cryptococcal infection?
1. ABeta-D-glucan
2. BCryptococcal antigen (CrAg) test
3. CAspergillus galactomannan antigen
4. DGerm tube test
Answer and explanation in LearnFree account

Yeasts

A germ tube-negative yeast from a blood culture forms dry, flat, pale-pink colonies with a whitish edge on CHROMagar Candida. Cornmeal agar shows elongated blastoconidia branching like crossed matchsticks. This isolate is intrinsically resistant to which antifungal?

Nakaseomyces glabratus (formerly Candida glabrata) differs from most Candida species on cornmeal Tween 80 agar because it:

Cryptococcus neoformans is found mainly in which environmental source?

CSF from a man with advanced HIV shows many encapsulated budding yeasts on India ink. Undiluted CSF is negative by cryptococcal antigen latex agglutination, but a 1:100 dilution is strongly positive. These results are best explained by:

A yeast grows from the urine culture of a patient transferred from an overseas ICU. An API 20C AUX strip identifies it as Rhodotorula glutinis, but the colonies are cream-white rather than coral-red. What should the laboratory do next?

A 58-year-old woman with rheumatoid arthritis has fever and headache. Her serum cryptococcal antigen latex test is positive at 1:4, but the kit's control latex, coated with normal globulin, also agglutinates. CSF India ink and culture are negative. The most likely explanation is:

Which result is typical of Cryptococcus neoformans and separates it from Candida species?

In a neonatal intensive care unit, bloodstream infections with Candida parapsilosis are most often linked to:

WHO recommends serum cryptococcal antigen (CrAg) screening for adults with advanced HIV before they start antiretroviral therapy. What is the main purpose?

Candida auris from a blood culture gives these MICs: fluconazole 128 µg/mL, amphotericin B 1 µg/mL and micafungin 0.12 µg/mL. CDC tentative resistance breakpoints are fluconazole ≥32, amphotericin B ≥2 and micafungin ≥4 µg/mL. How should the isolate be interpreted?

A germ tube-negative yeast from a blood culture forms dark metallic-blue colonies on CHROMagar Candida. On cornmeal Tween 80 agar it shows long pseudohyphae with single blastoconidia along their length, and no chlamydoconidia. What is the most likely identification?

In culture-based screening for Candida auris, the swab is first incubated in salt Sabouraud dulcitol broth at 40 °C. This step:

An India ink preparation of CSF from a patient with HIV shows encapsulated yeasts. Which test is best for confirming cryptococcal infection?