Lecture 6 The Fungi

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Lecture 6 The Fungi

• Lecture aims
• To understand some basic terms and features of
  fungi
• To be aware of the structure of some disease
  causing fungi
• To be aware of the various types of fungal
  infections that may occur
• To become aware of predisposing factors
  contributing to a fungal infection
• Refs Black Chp 11 p297-304

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Some terms and features of fungi

• Mycology is the study of fungi
• Mycosis is a fungal infection of tissue
• Fungi as a group
• are eucaryotic chemoheterotrophs
• have vegetative and reproductive forms
• are called Fungi Imperfecti if no sexual
  reproductive stage has been defined (which
  includes most pathogens)
• are usually multicellular except for yeasts which
  are unicellular (but may bud)

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Structure of some fungi associated with disease
(Fungi imperfecti)

• Fungi may consists of fine filaments called
  hyphae
• Hyphae may have cross walls separate (septate
  hyphae)
• Or be continuous (nonseptate hyphae)

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Fungal structure contin...

• A mass of hyphe is called a mycelium
• Hyphae may be
• vegetative or
• Reproductive (are often aerial, look thicker and
  have fruiting bodies and spores

Mycelium (thalus)
Reproductive hyphae
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Fungal structures contin...
Sac

• The vegetative hyphae of Fungi imperfecti often
  look much the same
• Most fungi as a result are identified by their
  reproductive hyphae
• Variations may occur with spores which can be in
  sacs, within a segment or naked

Spores
Arthrospore
Reproductive hyphae
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Examples of different fungal sp based on
reproductive bodies

• Eg Aspergillus sp

• Penicillium sp

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Reproduction of asexual spores (conidia)
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Fungal infections (mycosis)

• Most are opportunistic
• Many are saprophytic in origin
• Tend to feed on dead and decaying material
• Are a significant source of antibiotics
• Can be very difficult to treat
• Systemic infections can be lethal

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Mycoses contin...

• There are 3 main types of mycoses
• Superficial mycosis
• Subcutaneous mycosis
• Systemic mycosis
• Some species of fungi may be associated with
  multiple syndromes or restricted to superficial
  infections- can be influenced by hosts status

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Superficial mycosis

• Usually caused by dermatophytes (affect skin,
  hair and nails)
• Main reason for infection in these sites is the
  fungi use the keratin present as a nitrogen
  source
• eg Common ringworm includes species such as
  Trichophyton sp Microsporium sp Epidermophyton
  sp.

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Subcutaneous mycoses

• Involves infection of skin and underlying tissues
  (without further dissemination)
• Humans are often an accidental host eg trauma
  with saprophytic fungi
• ie Fungi usually found in dead vegetation
• Can cause localized infection after a eg
  contaminated splinter trauma
• eg Sporothrix sp

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Systemic mycoses

• Generally results from inhalation of spores, can
  be saprophytic sp
• eg Cryptococcus neoformans a yeast form found in
  soil, pigeon droppings, Koala intestine
• Aspergillus sp (Aspergillosis of lung)
• Candida albicans- the yeast can be localized or
  disseminated

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Systemic infections characteristics

• Usually begin in the respiratory tract- can
  spread to other organs from here
• Usually opportunistic infections of compromised
  hosts
• Can invade CSF brain from blood stream
• Can become disseminated (eg candidiosis)

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Treatment of mycosis

• Superficial mycosis topical agents such as
  Nyastatin (a cotrimazole)
• Systemic or serious subcutaneous mycosis
  amphotericin B is the drug of choice but has
  toxicity problems- imidizoles and some other
  agents may be less toxic (but also less effective)

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Predisposing factors for developing mycosisNote
the large array of mycotic syndromes

• Moisture
• Humid, tropics, dribbling, skin folds
• Hormonal changes
• Pregnancy, diabetes, endocrine disease
• Diseases
• Leukaemia, Lymphomas

• Immunosuppression
• Corticosteroid, Transplant
• Destabilizing normal flora
• Antibiotics
• Invasive devices
• Catheters

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There will be a demonstration of several clinical
slides given during the lecture relevant to
mycoses