03/01a Diagnostic Mycology Flashcards
What are the three categories of mycotic afflictions?
Allergic - can be major or minor
Toxicosis - ingestion of mycotoxins
Mycosis - infection by fungi, can range from colonization to disseminated
What are the four portals of entry for mycoses?
Skin - simple contact or implantation by trauma
Inhalation
Mucous membranes (usually normal flora)
Iatrogenic - prolonged broad-spectrum antibiotic treatment, immune suppression, indwelling catheters
What are the four major categories of clinical classifications of mycoses?
Superficial and cutaneous - infect by casual direct contact
Subcutaneous - infect by traumatic implantation
Endemic/systemic - geographically restricted primary pathogens
Opportunistic - often normal flora
What are the characteristics of a superficial mycosis?
Colonizes non-living material
Causes no immune response
Very common, usually non-fatal
What are the characteristics of a cutaneous mycosis?
Colonizes non-living material
Immune responses can be present
What are the characteristics of an endemic mycosis?
Usually have a limited geographic distribution
Infect by inhalation of spores
Low mortality
Some infections can be chronic and serious, especially in immuncompromised patients
What is fungal dimorphism?
Fungi (usually endemic) that can exist in two different forms - a mold in nature, and a non-mold in the host (e.g. yeast)
What genera of fungi exhibit fungal dimorphism? Name six
Blastomyces Coccidioides Histoplasma Paracoccidioides Sporothrix Penicillium marneffei
What are the characteristics of opportunistic mycoses?
Much higher incidence than endemic mycoses (global distribution)
Often normal flora
High mortality
What are six common opportunistic mycoses?
Candidiasis Cryptococcosis Mucormycosis Aspergillosis Hyalohyphomycosis
What are the four laboratory methods used to diagnosis fungal infections?
Microscopy of specimens
Culture
Nucleic acid-based methods
Serologic testing
What are the advantages and disadvantages of microscopy as a diagnostic tool for fungi?
Advantages - fast and immediate results
Disadvantages - less sensitive than culture, consumes part of specimen, definite ID is not possible without a positive culture
What specimens are not usually suitable for microscopy?
Specimens of insufficient quantity
Blood
How does a KOH exam work?
KOH dissolves human material, but has no effect on fungal material - makes fungi easier to see
Disadvantage - preparations are not permanent
How can you improve a KOH exam?
By the addition of calcofluor white, an optical brightener that fluoresces under UV light
Calcofluor white binds to cellulose and chitin and makes them appear much brighter
Highly sensitive, but not entirely specific for fungi
How useful is a Gram stain for detecting fungi? Why?
Not very (not suitable for molds) Gram stain detects cytoplasm, but fungi cells have minimal cytoplasm
For what types of infections is India ink stain useful in diagnosis? Why?
Encapsulated yeasts such as Cryptococcus
Carbon particles of India ink cannot penetrate the extracellular capsule, and thus serve as a negative stain for Crytococcus
Sensitivity and specificity is high
What are the six major histological stains for fungi?
1) Silver stain (GMS) - most sensitive because it precipitates on fungal cell walls
2) Gridley - no longer widely used
3) PAS - less sensitive because it poorly stains dead or senescent cells
4) H&E - least sensitive because it does not stain the fungal cell wall
5) Mucicarmine and Alcian Blue - for Cryptococcus
6) Fontana-Masson - for Cryptococcus, highly sensitive
What does Cryptococcosis look like in a clinical specimen?
Encapsulated
Fairly large budding yeast
What does Candidiasis look like in a clinical specimen?
Budding yeast + pseudohyphae
What does Blastomycosis look like in a clinical specimen?
Budding yeast with NO filaments
Very large
Thick-walled
Broad points of attachment between mother and daughter cells
What does Histoplasmosis look like in a clinical specimen?
Small oval yeast
Often found within macrophages
Looks like it has a capsule on H&E, but it is actually a staining artifact
What does Paracoccidioidomycosis look like in a clinical specimen?
Large yeast
Produces multiple buds in a wheel-like appearance
Grows as a mould in the lab but as a yeast in humans
What does Penicilliosis look like in a clinical specimen
Yeast cells that divide by fission (can be identified by a heavy septum between dividing cells)
What fungal genera cause Hyalohyphomycosis?
Aspergillus
Fusarium
What fungal genus cause most cases of Mucormycosis?
Rhizopus
What structure is characteristic of Coccidioidomycosis?
Spherules - grow in size and release endospores
What are the advantages of culture as a diagnostic tool for fungi?
More sensitive than direct exam
Can provide definitive identification
Antifungal susceptibility testing is possible
What are the three different media used for culturing fungi?
Non-selective (everything can grow)
Selective against bacteria
Selective against certain other fungi
At what temperature should you culture most fungi? Why?
30C - many fungi don’t grow very well at body temperature (37C)
How quickly do clinically relevant fungal isolates appear?
Within 1-5 days
How are yeast isolates identified?
Colony morphology
Primarily biochemical characteristics
What are the quickest ways to screen yeasts?
India ink for capsule detection
Germ tube test - no constriction indicates Candida albicans
What tests can you use to identify Candida albicans?
Germ tube test
Enzyme detection
What test is used to identify a non-Candida organism?
Carbohydrate assimilation - tests whether the yeast can use specific carbohydrates as a carbon source
How are mould infections usually identified?
Mostly through morphology
Also biochemical and nucleic acid sequencing tests
What is a DTH test? What infections is it useful for?
Delayed-type hypersensitivity skin test
Used for endemic mycoses such as Coccidioidin, Hystoplasmin, and Paracoccidioidin
How is a DTH test interpreted?
Must have a history of recent conversion for a meaningful positive test, because once a patient is positive, they are positive for life
Negative skin test can exclude that mycosis in an immunocompetent patient
Describe three serological tests for fungal antigens, and which genera they can detect
Capsular antigen, detected in CSF via latex agglutination; highly sensitive for Cryptococcus
Carbohydrate antigen can detect Histoplasma
Cell wall component beta-glucan can detect Candida, Aspergillus, Fusarium, Pneumocystis, and Coccidioides
