Intro To Medical Mycology: Yeasts: Candida and Cryptococcus

What is medical mycology

Study of fungi (yeast, mold, dermatophyte, dimorphic fungi)

Important to know many drugs have been derived from fungi as well

Are fungi unicellular or multicellular

Can be unicellular (yeast) or multicellular (mold)

Cell structure of fungi

True nucleus with membrane bound organelles

Genetic material of fungi

Multiple linear chromosomes located in the nucleus

Replication of fungi

Sexual or asexual by spores (mold)

Asexual budding (yeasts)

Metabolism of fungi

Heterotrophs, absorb nutrients after secreting digestive enzymes

Ribosomes of fungi

Have ribosomes but not a drug target (eukaryotic, too similar to humans)

Cell wall of fungi

Made of chitin, beta glucans, and mannoproteins

Cell membrane of fungi

Made of phospholipids and ergosterol (similar to human cholesterol)

Dimorphism

Switching between yeast and invasive filamentous form (pseudohyphae), seen in some C. albicans

Budding

Asexual reproduction of yeast

Bud forms on the parent cell and the nucleus splits between the two buds, the result is a blastoconidium (budding spore)

Sexual reproduction of yeast

Cryptococcus is the only one that does it, forms basidiospores

Natural reservoir of Candida

Normal flora (GIT, oral tract, GU tract, skin folds)

Natural reservoir of Cryptococcus neoformans

Soil, bird poop

Natural reservoir of Cryptococcus gatti

Trees (wood, bark, leaves) especially eucalyptus trees

How are yeast transmitted

- Opportunistic pathogens (breach of barrier, disruption of normal balance)

- Inhalation

- Direct contact (least common)

Pathogenesis of Candida

Opportunistic pathogen, grows when host immune system is compromised or when there is a disruption in normal flora that causes candida overgrowth (ex. antimicrobials)

It can adhere and invade into cells, adhesion triggers it to switch to pseudohyphae form that penetrate host cells and induce engulfment

Can disseminate into other sites (brain, kidneys, heart, joints)

Virulence factors of Candida

- Morphological plasticity

- Adhesins

- Invasin

- Hydrolytic enzymes

- Candidalysin

Morphological plasticity

Virulence factor of Candida

Changing forms from yeast to pseudohyphae

Adhesins of Candida

Allow for biofilm formation and adherence

Invasin

Virulence factor of Candida

Facilitates host entry by endocytosis or active penetration

Hydrolytic enzymes of Candida

Break down host proteins

Candidalysin

Virulence factor of Candida

Forms pores in host cells

How does Candida evade immune system

- Cell wall masking with mannoproteins that are less immunogenic

- Morphological switching

- Biofilm formation

Microscopy of Candida

Appears as gram positive, large, budding

Culture of Candida

Can be grown on fungal media

Beta-D-glucan serology

Shows presence of fungi, not specific to yeast

Fungi have beta glucan on the cell wall

Germ Tube Test

Test specifically for C. albicans, see hyphae and budding within hours in this specific media

Chromogenic media and Candida

C. albicans appears green on chromogenic media

Molecular/proteomic tests to ID Candida

- MALDI TOF

- Nucleotide sequencing

- PCR

Pathogenesis of Cryptococcus

Inhaled from the environment and taken up into the alveolar macrophages which move around, promoting the spread of cryptococcus outside of the lungs

Can disseminate going to skin, CNS, bones, and prostate

Trophism for CNS (prefers CNS), can cross the BBB by staying within phagocytes or breaching the tight junctions of the epithelial cells

Virulence factors of Cryptococcus

- Polysaccharide capsule

- Melanin

- Urease

Polysaccharide capsule of Cryptococcus

Antiphagocytic

Melanin

Virulence factor of cryptococcus

Protects from oxidative stress

Urease

Virulence factor of cryptococcus

Produces ammonia which damages host cell membranes, promotes dissemination, escape from the phagosome, and entry to the BBB (affecting tight junctions)

Titan Cells

Cryptococcus avoids immune system by being too large for macrophages to engulf

Microscopy of Cryptococcus

Seen with india ink stain, halo around the cell as it cannot take up the stain

Cryptococcal antigen test

Tests specifically for cryptococcus capsule, effective

Performed on CSF or serum

Molecular/Proteomic ID for Cryptococcus

- MALDI TOF

- Nucleic acid sequencing

- PCR

What testing methods are used for Candida

Superficial infection is usually a clinical diagnosis, do not need testing