Fungi

Question 1

fungi are eukaryotic organisms that possess a ____ cell wall and produce ____ structures and _____

Answer 1

fungi are eukaryotic organisms that possess a chitinous cell wall and produce filamentous structures and spores

Question 2

describe the structure and composition of the fungal cell wall

Answer 2

• chitin, B (1,3) and B(1,6) glucans, mannose modified proteins and glycosylphosphatidylinositol (GPI)-anchored proteins
• the cell membrane has ergosterol instead of cholesterol

Question 3

describe fungal morphology

Answer 3

• fungi grow as unicellular (yeast) or multicellular organisms (mold)
• multicellular fungi connect together in long strands known as hyphae; hyphae grow at their tips and form branches–tangled masses of hyphae are mycelia
  
  • hyphae may have walls between neighboring cells, making them septate or aseptate
  • aseptate hyphae are referred to as coenocytic, in that a single plasma membrane surrounds many nuclei creating a multinucleate cell
• fungal nuclei are haploid except during the generation of a zygote
  
  • a fungal cell with 2 haploid nuclei is known as dikaryon

Question 4

describe the 4 major types of fungal diseases

Answer 4

• hypersensitivity: allergic reaction to molds and spores e.g. farmer’s lung, tea picker’s lung, bark stripper’s disease
• mycotoxicoses: poisoning of man/animal by feeds/products contaminated by toxin producing fungi that colonize crops such as grains, corn, peanuts; lethal to poultry and livestock e.g. aflatoxin, ergot alkaloids
• mycetismus: ingestion of pre-formed toxins (mushroom poisoning)
• infections: mycoses

Question 5

describe fungal classification based off morphology

Answer 5

Question 6

give examples of the different fungal classifications

Answer 6

• ascomycetes form sexual spores in an ascus, an enclosed sac
  
  • largest fungal phylum and includes Candida albicans, Pneumocystis jirovecii and most of the human fungal pathogens
• basidomycetes form sexual spores on the surface, known as a basidium
  
  • Cryptococcus neoformans
• Zygomycetes is the most primitive fungal phylum, forming sexual spres in an enclosed sac known as zygosporangium
  
  • notable pathogens are Mucor spp., Rhizopus spp. and Absidia spp.

Question 7

explain the concept of dimorphism

Answer 7

dimorphism enables switching from one form to another

• primary trigger = environmental conditions
  
  • temperature
  • pH
  • iron levels

Question 8

describe the 3 mechanisms of fungal reproduction

Answer 8

• 1) production of spores: dispersal units
  
  • can germinate > new hyphae
  • formation can be by:
    
    • asexual process (mitosis only)
      
      • sporangiospores
      • conidiospores
    • sexual process
      
      • ascospores
      • basidospores
• 2) budding
• 3) hyphal fragmentation
  
  • germinate from new hyphae

Question 9

which term best describes fungal growth requirements?

Answer 9

• saprophytic: obtain nutrients and energy from dead or decaying organic material
  
  • occassionally human tissue -> damage

Question 10

describe the superficial mycoses

Answer 10

colonization of the outer layers of skin, hair and nails, and rarely invades deeper tissues

• pityriasis (Tinea) versicolor caused by the dimorphic Malassexia furfur, which infects skin and alters color
  
  • M. furfur flouresces under UV light
• Tinea Nigra, caused by Hortaea werneckii, causes skin to darken
• black piedra is a superficial infection of the hair shaft caused by Piedra hortae
• onychomycosis is fungal infection of fingernails and toenails caused by Trichophyon rubrum and T. mentagrophytes

Question 11

describe subcutaneous mycoses

Answer 11

• sporotrichosis is a subacute or chronic granulomatous infection, often following lymphatics, caused by the soil fungus Sporothrix schenckii
• a mycetoma (or eumycetoma) is a granulomatous inflammation that may extend beneath the subcutaneous region to bone
  
  • pigmented nodules may drain through sinuses and produce colored grains which are helpful in identification
• chromoblastomycosis is an infection that forms warty pigmented lesions which grow outward from site of introduction

Question 12

name the 5 common opportunistic mycotic infections

Answer 12

• Candidiasis
• Aspergillosis
• Cryptococcosis
• Zygomycosis/mucormycosis
• Pneumocystis

Question 13

describe Candidiasis (common opportunistic mycotic infxn)

Answer 13

• candidiasis:
  
  • Candida albicans causes superficial skin infection, oral cavity, genitalia, large intestine
  • forms off-white, pasty colony with a yeasty odor
  • causative agent of thrush, vulvovaginal yeast infection and cutaneous candidiasis

Question 14

describe Aspergillosis (common opportunistic mycotic infection)

Answer 14

• aspergillosis:
  
  • very common airborne soil fungus, usually infecting lungs
  • serious opportunistic threat to AIDS, leukemia, and transplant patients
  • invasive aspergillosis can involve many organs

Question 15

describe Cryptococcosis (common opportunistic mycotic infxn)

Answer 15

• cryptococcosis:
  
  • Cryptococcus neoformans inhabits soil around pigeon roosts, usually infecting lungs and other organs, notably, brain
  • common infxn of AIDS, cancer and diabetes patients

Question 16

describe Zygomycosis/mucormycosis (common opportunistic mycotic infxn)

Answer 16

• zygomycosis/mucormycosis
  
  • saprobic fungi found in soil, water, organic debris and food e.g. Rhizopus, Absidia and Mucor
  • usually harmless but can invade nose, eyes, heart and brain of people with diabetes and malnutrition with severe consequences

Question 17

describe Pneumocystis (common opportunistic mycotic infxn)

Answer 17

• Pneumocystis:
  
  • Pneumocystis jiroveci, a small unicellular fungus that causes penumocystis pneumonia (PCP), the most prominent opportunistic infxn in AIDS patients
  • can be rapidly fatal if not controlled with medication

Question 18

describe fungal culture medium

Answer 18

• the medium commonly employed is Emmon’s modification of Sabouraud Dextrose Agar (pH = 5.4)
• the media may be supplemented with antibiotics such as gentamicin and chloramphenicol to minimize bacterial contamination and cyclohexamide to inhibit saprophytic fungi

Question 19

describe hyphae

Answer 19

• hyphal structure reflects the phylogeny of the fungi
  
  • septate fungi taken from lesions are nearly always ascomycetes
  • aseptate fungi are zygomycetes
• pseudohyphae are characteristic of Candida spp. infxns
• low angle branching is characteristic of Aspergillus spp. and right angle branching of Zygomycetes
• coloration may be described as dematiaceous (dark) or hyaline (transparent)

Question 20

describe spores

Answer 20

• spore forming structures are either enclosed (sporangia) or open (conidiophores)
• phialides are flask shaped cells from which conidia bud
• arthrospores are ceclls that become conidia as they break off from the end of a hypha, in contrast to blastospores, which are produced by budding, such as at the ends of phialides or simply from the sides of hyphae
• chlamydospores are thick-walled spores capable of surviving adverse conditions

Question 21

describe fungal stains

Answer 21

• sputum, washing and aspirates may be prepared with 10% KOH
  
  • only the chitinous hyphae survive
• calcofluor white and/or Gram stain are also used
  
  • calcofluor white binds chitin and fluoresces under UV illumination
  • Candida albicans stains Gram positive
• Gomori methenamine silver staining is a popular histochemical stain used to demonstrate hyphae, spores and conidiophores

Question 22

describe fungal serology

Answer 22

• detection of anti-fungal antibody is helpful in diagnosis of sub-cutaneous and systemic mycoses and for the prognosis and response to anti-fungal drugs

Question 23

describe conidial structure

Answer 23

• macroconidia and microconidia refer to the size of spores
  
  • Zygomycetes may form both types as blastospores
• Tuberculate refers to the knobby apperance of a spore
  
  • Histoplasma capsulatum forms tuberculate macroconidia as characteristic spores

Question 24

describe fungal cultures

Answer 24

• fungi are usually easily cultured on general bacterial media or Sabouraud agar
  
  • Sabouraud agar is too acidic to support bacterial growth
• dimorphic fungi are usually demonstrated by the growth of yeast colonies at elevated temperature
  
  • Candida albicans is the reverse however, and the growth of pseudohyphae is favored at elevated temperature

Question 25

describe the control of fungal infections

Answer 25

• control involves use of anti-fungal compounds that target specific metabolic pathways
  
  • inhibition of ergosterol, beta-glucan and chitin biosynthetic pathways, inhibition of DNA synthesis by depleting thymine pools