Question

A 45 year-old pigeon breeder, who recently received a renal transplant (and thus was on immunosuppressive steroids), complained to his physician of headache, dizziness, blurred vision, and a stiff neck. Specimens of purulent cerebrospinal fluid were sent to your microbiology laboratory. A carefully examined Gram stain of the CSF sediment was negative; however, on brain infusion agar with blood at 30 degrees C, a white, mucoid yeast rapidly grew. Organism was inhibited on brain heart infusion agar with blood gentamicin, cyclohexamide and chloramphenicol.

Which disease do you suspect?
Candidiasis
No diseases -- organism was a laboratory contaminant
Histoplasmosis
Cryptococcosis
Coccidiodomycosis

Give three reasons for choosing that answer?

If the yeast was significant, why was it not observed on the CSF Gram stain?

If you ranswer to number 1 was not B, what three tests would you perform to speciate the yeast?

Answer 1

The suspected disease is cryptococcosis, indicated by the white, mucoid yeast growth in a patient with appropriate risk factors such as immunosuppression and pigeon exposure. Cryptococcus neoformans may not be visible on CSF Gram stain due to its thick capsule. Confirmatory tests include India ink stain, cryptococcal antigen test, and selective fungal cultures.

Step-by-step explanation:

Possible Diagnosis and Justification

Based on the information provided, the disease suspected is cryptococcosis. This is due to three main reasons. Firstly, the white, mucoid yeast organism recovered from the cerebrospinal fluid (CSF) is consistent with the characteristic appearance of Cryptococcus neoformans, a fungus that manifests as a yeast in human infections. Secondly, the patient is an immunocompromised individual due to the use of immunosuppressive steroids post-renal transplant, which puts them at higher risk for opportunistic infections such as cryptococcosis. Lastly, the patient has had exposure to pigeon droppings, which are a known reservoir of Cryptococcus neoformans.

The yeast was not observed on the CSF Gram stain because Cryptococcus neoformans has a thick polysaccharide capsule that doesn't retain the stain well, making it poorly visible by Gram staining. This organism is more easily seen using special stains like India ink, which help visualize the encapsulated yeast cells, or by culturing on appropriate media.

Diagnostic Tests

If the diagnosis was cryptococcosis, three tests that could be used to confirm the species include:

1. India ink stain of the CSF to visualize the capsule of the yeast cells.
2. Cryptococcus antigen test to detect the polysaccharide capsule antigens in the CSF.
3. Fungal cultures on selective media to further confirm the growth of Cryptococcus neoformans.