Lecture 3: Mycology The World of Fungi

Question 1

What two kingdoms are in the world of fungi

Answer 1

• Yeast (unicellular)
• Mold (multicellular)

Question 2

Fungal charateristics

Answer 2

• Like all eukaryotic organisms: fungi contain membrane bound organelles
• Have distinct organelles including mitochondria (cell generator), endoplasmic reticulum (manages cellular export) and a membrane bound nucleus
• Sexual (molds) and asexual (yeasts generally by budding and also seen in molds) reproduction

Question 3

What are Yeasts?

Answer 3

• Yeast colonies look similar to bacterial colonies BUT they are NOT bacteria
• Over 1500 different species in kingdom Fungi
• They are unicellular EUKARYOTIC organisms
• Unlike bacteria which have a fairly rudimentary cellular structure, yeast have much more sophisticated cellular machinery

Question 4

What is the Fungal Cell Wall

Answer 4

• Cell wall is made of Ergosterol, similar to Cholesterol and is a primary sterol
• Contains cell wall
• No peptidoglycan
• Major structure is Chitin
• Exploited for diagnoses typically
• New drugs use the cell wall as a target

Question 5

What is the importance of Fungal Cell Structures

Answer 5

• Fungal cell wall contains Ergosterol as the primary sterol
• Gives shape to fungi
• Gives them strength by protecting them from organic solvents and UV light
• Secretes enzymes from their walls - able to hydrolyze sugars for nutrition

Question 6

How does fungal reproduction work

Answer 6

• Primary reproductive structures are called spores or conidia
• Classification gets complicated because based on size, shape, arrangement are all used to classify the organism
• All pattern recognition

Question 7

Medically relevant Yeasts

Answer 7

• Candida app (~200 spp)
• Cryptococcus spp (Cryptococcus neoformans): when soil is damaged and people breathe it in, people can get very sick

Question 8

Medically relevant Molds

Answer 8

• Dermatophytes (~50 spp): ringworm, athletes food @24 they are molds but @37 they turn to yeast
• Zygomycetes
• Hyaline Molds
• Dimorphic Fungi (6 spp)

Question 9

4 Infection Classifications

Answer 9

• Superficial or cutaneous mycoses (hair, skin, nails infections)
• Subcutaneous mycoses (blood and tissue infections)
• Systemic mycoses (very aggressive, can have dual infections)
• Opportunistic mycoses

Question 10

Yeast: Candida spp.

Answer 10

• Normal flora of the GI tract and often colonized the skin and the environment
• Leads to infections in immunocompromised individuals (happens mostly in a compromised state)
• Can lead to several infectious aetiologies
• Many risk factors; antibiotics don’t work on fungi, most patients are immunocompromised

Question 11

Yeast: Cryptococcus spp.

Answer 11

• Most medically relevant species are Cryptococcus neoformans and Cryptococcus gattii
• Typically do not cause infections - but can lead to disease/infection in those with altered immune status or underlying morbidity
• Found in soil and in dried animal faeces - specifically associated with bird poop or guano
• Can lead to pneumonia, fungemia (disseminated), meningitis and cutaneous infection
• Contains a large capsule around the yeast cell - helps it evade immune mediated phagocytosis

Question 12

6 Dimorphic Fungi

Answer 12

Epidemiology matters
1. Histoplasma capsulatum (Globally)
2. Blastomyces dermatitidis; likes old moist areas + when dirt is disturbed because these spores are in the ground (Southeastern ON, New England)
3. Coccidioides immitis; in sand most will get sick after a sandstorm (in SW USA)
4. Paracoccidioides brasiliensis ( In South America)
5. Sporothrix schleiferi (“gardeners disease” likes thorns)
6. Talaromyces marneffei (Southeast Asia)

Question 13

Temp dependent dimorphisms (For yeast and mold)

Answer 13

Yeast @ 37 celcius, mold at 25 celcius

Question 14

Dimorphic Fungal Infections: Histoplasma capsulatum

Answer 14

• Clinical presentation include fever, sweats, weight loss, fatigue, and respiratory symptoms
• CNS involvement in 5-20% of cases of acute disseminated histoplasmosis, presenting as chronic meningitis
• Disseminated disease are rare but seen

Question 15

Dimorphic Fungal Infections: Blastomyces dermatitidis

Answer 15

• Can lead to ammonia
• Presents as nonspecific febrile acute or chronic pulmonary disease mimicking CAP and/or malignancy
• Ultimately to disseminated disease involving cutaneous, genitourinary, and bony lesions

Question 16

Dimorphic Fungal Infections: Coccidioides immitis

Answer 16

• Pulmonary involvement and in most cases is asymptomatic
• Symptomatic infection presents with fever, cough, and chest pain and may mimic CAP
• Disseminated disease can occur locally with pleural or pericardial invasion or any organ of the body. Disseminated disease can involves cutaneous, genitourinary, and bony lesions

Question 17

What is Opportunistic Mycoses + most invasive mold

Answer 17

• Fungi considered “non-pathogenic” but can cause significant illness in the immunocompromised, those with prosthetic/intravascular devices and hematologic malignancies
• Commonly found in soil and in the environment
• Opportunistic infections by molds can result in sinus infections, respiratory diseases and fungemia
• Aspergillus: most common invasive mold

Question 18

Aspergillus

Answer 18

• Most common invasive mold
• Has become the major cause of morbidity and mortality in immunocompromised patients, with mortality rates as high as 88%
• Almost 2/3 of patients have underlying hematologic disease and have undergone bone marrow transplantation

Question 19

Examples of Opportunistic Fungi

Answer 19

• Aspergillus
• Penicillium
• Paecilomyces
• Acremonium
• Fusarium

Question 20

What are Mucorales (Zygomycetes) Molds

Answer 20

• Fast growing fungi characterized by primitive coenocytic (mostly aseptate) hyphae
• Environmental fungi in soil, dung, and vegetative matter
• One of the features of this class of fungi is rapid growth, known as “lid-lifters”
• Extremely aggressive disease as the organism does not respect tissue planes including devastating rhino-orbital infections and wound infections

Question 21

Antifungal Therapies (5)

Answer 21

Attacks cell wall
1. Lanosterol Synthesis (Terbinafine)
2. Cell wall synthesis (Echinocandins - Caspofungin, micafungin, anidulafungin)
3. Forms Membrane Pores (Polyenes - Amphotericin B, Nystatin
4. Nucleic Acid Synthesis (5-Flucytosine)
5. Ergosterol Synthesis (Azoles - Fluconazole, Ketoconazole, Clotrimazole, Miconazole, Itraconazole, Voriconazole

Question 22

How does lab detection of fungi work

Answer 22

• Gram stain + additional stains from specimens - may see fungal elements
  
  • Because very thick membrane
• Culture on specialize media - Yeast take 1-3 days, Dermatophytes take 1-3 weeks and systemic fungi can take 3-6 weeks
• Variety of stains used can help identify fungi

Question 23

3 types of Fungal stains

Answer 23

1. KOH
2. Calcoflour stain
3. Silver stain

Question 24

Fungal stains: KOH

Answer 24

Dissolves tissue cells leaving the fungal cells intact (so we can see better, clean and clear)

Question 25

Fungal stains: Calcoflour stain

Answer 25

• Binds to chitin in the fungal cell walls (shows a lot of background)

Question 26

Fungal stains: Silver strain

Answer 26

• Stains fungi black and provides nice contrast. Typically stains proteins in the fungal cell wall (stains them black, produces a lot of background noise)