Fungi

Fungi

  • Fungi are non-motile, eukaryotic organisms that can be:

    • Saprophytes: Live on dead or decaying matter.

    • Parasites: Cause various parasitic diseases.

    • Commensals: Live normally inside hosts without causing harm and sharing mutual benefits.

  • Feature:

    • Lack chlorophyll pigments.

    • Possess differentiated nuclei surrounded by a nuclear membrane.

    • Reproduce by budding, spore formation, or conidia formation (both sexually and asexually).

  • Cell Wall Composition:

    • Rigid cell wall made up of chitin, mannan, and polysaccharides.

    • The cytoplasmic membrane contains sterols.

  • Historical Notes:

    • Identified by Augustino Bassi in 1835 from the muscardine disease of silkworms.

    • Study of fungi is referred to as Mycology (from Greek "Mykos" meaning mushroom).

Classification of Fungi

A. Morphological Classification

  • 1. Yeasts:

    • Unicellular, round to oval fungi.

    • Reproduce asexually via budding, forming creamy mucoid colonies on culture media like Sabouraud's dextrose agar (SDA).

    • Pathogenic yeast of importance: Cryptococcus neoformans.

  • 2. Yeast-like Fungi:

    • Exhibit characteristics of both yeasts and chains of elongated budding cells (pseudomycelium).

    • Example: Candida, which can form pseudohyphae and true hyphae in some species.

  • 3. Molds:

    • Form true mycelium and grow as branching filaments (hyphae).

    • Produce sexual and asexual spores; mycelium can be septate or aseptate

    • Types: Aerial mycelium (above surface) and vegetative mycelium (in medium).

    • Examples include Dermatophytes, Zygomycetes, Aspergillus, and Penicillium.

  • 4. Dimorphic Fungi:

    • Exist in two forms: yeast in host tissues at 37°C (parasitic phase) and filamentous in soil at 22-25°C (saprophytic phase).

    • Includes species like Histoplasma capsulatum, Sporothrix schenckii, Blastomyces dermatitidis, Paracoccidioides brasiliensis, and Coccidioides immitis.

B. Taxonomical Classification

  • Fungi are categorized under the phylum Thallophyta based on sexual spore formation:

    • 1. Zygomycetes:

      • Lower fungi with non-septate hyphae, forming asexual (sporangiospores) and sexual spores (oospores, zygospores).

    • 2. Ascomycetes:

      • Possess septate hyphae, forming sexual spores called ascospores.

    • 3. Basidiomycetes:

      • Also have septate hyphae and form sexual spores known as basidiospores.

    • 4. Deuteromycetes (Fungi Imperfecti):

      • Septate hyphae but do not produce sexual spores; most are clinically important.

Reproduction & Sporulation in Fungi

1. Sexual Reproduction

  • Involves fusion of male and female cells (plasmogamy) followed by fusion of nuclei (karyogamy) leading to the formation of zygotes and various sexual spores.

    • Types include oospores, zygospores, ascospores, and basidiospores.

2. Asexual Reproduction

  • Medically important fungi predominantly reproduce asexually via mitosis producing:

    • Vegetative Spores:

      • Blastospores: Formed by budding from parent cells.

      • Chlamydospores: Thick-walled resting spores formed after dormancy, can be intercalary, lateral, or terminal.

      • Arthrospores: Formed by fragmentation of hyphae, resulting in cuboidal spores.

    • Aerial Spores:

      • Conidiospores: Borne externally on conidiophores, can be microconidia (small) or macroconidia (large).

      • Phialospores: Modified conidia from flask-shaped phialides.

      • Sporangiospores: Formed within a sporangium on sporangiophores.

Cultivation of Fungi

  • Cultivation Media:

    • Natural Media: Made from natural substrates (e.g., corn meal, oatmeal), easy to prepare but composition is unknown.

    • Synthetic Media: Known compositions for precise nutrient control (e.g., Czapek-Dox).

  • Common Agar Cultures:

    • Sabouraud's Dextrose Agar (SDA): General purpose for dermatophytes and yeasts.

    • Inhibitory Mold Agar: Selective for certain pathogenic fungi.

    • Brain Heart Infusion Agar: Nonselective, for saprophytic and dimorphic fungi.

  • Cultivation Conditions: Incubation temperature typically set to 30°C, in humidified environments for up to 21 days.

Sterility Testing of Products

  • Disinfectant Action:

    • Bacteriostatic: Inhibits growth of bacteria.

    • Bacteriocidal: Kills bacteria.

  • Sterility Testing:

    • Tests conducted under aseptic conditions to verify products like injections, implants, and surgical tools are free from microorganisms.

    • Media used include Fluid Thioglycollate for anaerobic bacteria and Soybean Casein Digest for fungi and aerobic bacteria.

  • Testing Procedures:

    • Membrane Filtration: Filters product through a sterile membrane, retains microorganisms for growth testing.

    • Direct Inoculation: Directly transfer samples to culture media for growth.

  • Incubation Conditions: Vary by media type (30-35°C for aerobic, 22-25°C for fungal) with examination for microbial presence.