Fungi

• Fungi are in the Fungi kingdom, chemoheterotrophs that acquire food by absorption, and except for yeasts, are mostly multicellular
• Most fungi reproduce with sexual and asexual spores
• Fungal colonies are described as vegetative structures composed of cells involved in catabolism and growth
• Molds and Fleshy Fungi Thallus consist of long filaments of cells joined together called hyphae, which can grow to immense proportions
• In most molds, hyphae contain cross-walls called septa, dividing them into distinct, uninucleate cell-like units
• In a few classes of fungi, hyphae contain no septa and appear as long, continuous cells with many nuclei
• Hyphae grow by elongating at the tips, and each part of a hypha is capable of growth
• Fungi are usually grown from fragments obtained from a fungal thallus in the laboratory
• Yeasts are non-filamentous, unicellular, spherical or oval, capable of facultative anaerobic growth, and can ferment carbohydrates and produce ethanol and CO2
• Budding yeasts like Saccharomyces form a protuberance on their outer surface, divide their nucleus, and eventually break away from the parent cell
• Fission yeasts like Schizosaccharomyces elongate, divide their nucleus, and produce two offspring cells
• Pathogenic dimorphic fungi exhibit two growth forms, either as a mold (37°C) or as a yeast (25°C)
• Filamentous fungi reproduce asexually by fragmentation of their hyphae and sexually by the formation of spores
• Asexual spores are formed by the hyphae of one organism through mitosis and subsequent cell division, resulting in genetically identical organisms
• Two types of asexual spores are Conidiospores (e.g., Penicillium, Aspergillus) and Sporangiospores (e.g., Rhizopus)
• Clinical identification of fungi is based on microscopic examination of asexual spores
• Sexual spores result from the fusion of nuclei from two opposite mating strains of the same species of fungus, resulting in genetic characteristics of both parental strains
• The three phases of sexual spores are Plasmogamy, Karyogamy, and Meiosis
• Fungi are chemoheterotrophs that absorb nutrients, grow better in a pH of about 5, are mostly aerobic, resistant to osmotic pressure, and can grow in high sugar or salt concentrations
• Environmental requirements and nutritional characteristics of fungi:

  • Usually grow better in an environment with a pH of about 5
  • Almost all molds are aerobic; most yeasts are facultative anaerobes
  • Resistant to osmotic pressure; can grow in relatively high sugar or salt concentrations
  • Grow on substances with very low moisture content
  • Require somewhat less nitrogen
  • Metabolize complex carbohydrates, such as lignin
• Medically important fungi:

  • Teleomorphs are fungi that can produce both sexual and asexual spores
  • Zygomycota/Zygomycetes: saprophytic molds with coenocytic hyphae, asexual spores include conidia, chlamydoconidia, sporangiospores, and zygospores
  • Microsporidia: lack mitochondria, obligate intracellular parasites causing chronic diarrhea and keratoconjunctivitis
  • Ascomycota/Ascomycetes: molds with septate hyphae and some yeasts, asexual spores include conidia and ascospores
  • Basidiomycota/Basidiomycetes: club fungi producing mushrooms, asexual spores include conidia and basidiospores
• Fungal diseases:

  • Mycosis: any fungal infection, generally chronic due to slow fungal growth
  • Classification based on tissue involvement and mode of entry into the host:

  1. Systemic mycoses: deep infections not restricted to a specific body region, usually caused by soil fungi

  2. Subcutaneous mycoses: beneath the skin caused by saprophytic fungi, infection occurs by direct implantation of spores or mycelial fragments

  3. Cutaneous mycoses: fungi infecting only the epidermis, hair, and nails

  4. Superficial mycoses: localized in surface epidermal cells and along hair shafts

  5. Opportunistic mycoses: harmless in normal habitat but pathogenic in debilitated hosts
• Beneficial and harmful effects of fungi:

  • Beneficial effects include decomposition, symbiosis with plants, food production, drug production, and biotechnological uses
  • Undesirable effects include mold spoilage, plant diseases, and an increasing incidence of serious fungal infections in healthcare settings and compromised immune systems
• Fungi are in the Fungi kingdom and are chemoheterotrophs that acquire food by absorption
• Except for yeasts, fungi are mostly multicellular and reproduce with sexual and asexual spores
• Fungal colonies are described as vegetative structures composed of cells involved in catabolism and growth
• Molds and fleshy fungi thallus consist of long filaments of cells joined together called hyphae, which can grow to immense proportions
• In most molds, hyphae contain cross-walls called septa, dividing them into distinct, uninucleate cell-like units
• In a few classes of fungi, hyphae contain no septa and appear as long, continuous cells with many nuclei
• Hyphae grow by elongating at the tips, and each part of a hypha is capable of growth
• Fungi are usually grown from fragments obtained from a fungal thallus in the laboratory
• Yeasts are non-filamentous, unicellular, and capable of facultative anaerobic growth
• In budding yeasts like Saccharomyces, the parent cell forms a bud on its outer surface, and the bud eventually breaks away
• Fission yeasts like Schizosaccharomyces produce offspring cells by elongating the parent cell, dividing its nucleus, and producing two offspring cells
• Pathogenic dimorphic fungi exhibit two growth forms, mold at 37°C and yeast at 25°C
• Filamentous fungi reproduce asexually by fragmentation of their hyphae and sexually by the formation of spores
• Asexual spores are formed by the hyphae of one organism through mitosis and subsequent cell division
• Asexual spores can be conidiospores, anthroconidia, blastoconidia, or chlamydoconidia
• Sporangiospores are formed within a sporangium at the end of an aerial hypha called a sporangiophore
• Sexual spores result from the fusion of nuclei from two opposite mating strains of the same species of fungus