Fungi and Lichens

• There are only ~200 fungal species that are pathogenic to humans and animals of the >100,000 identified ones.
• Over the last 15 years, the incidence of serious fungal infections caused by opportunistic fungal pathogens has been increasing in healthcare settings and in immunocompromised patients.
• The study of fungi is called mycology.
• Benefits of Fungi:

  1. Fungi decompose dead plant material, thereby recycling vital elements.
  2. Nearly all plants depend on mycorrhizae – symbiotic fungi that helps plant roots absorb minerals and water from the soil.
  3. Fungi farming ants cultivate fungi that break down cellulose and lignin from plants.
  4. Humans consume fungi for food and to produce food and drugs
• All fungi are chemoheterotrophs, requiring organic compounds for energy and carbon.
• Fungi are either aerobic or facultatively anaerobic while only a few are anaerobic.
• Fungi are eukaryotic, while bacteria are prokaryotic.
• In cell membrane of fungi, sterols are present while absent in bacteria (except for Mycoplasma).
• Concerning cell wall, glucans; mannans; chitin are present in fungi. On the other, peptidoglycan is present in bacteria.
• The thallus or body of a mold or fleshy fungus consists of long filaments of cells joined together, which are called hyphae.
• Most molds have hyphae which contain cross walls called septa (singular: septum), which divide them into distinct, uninucleate cell-like units (i.e. septate hyphae).
• In some fungal classes, the hyphae contain no septa, and it appears as long, continuous cells with many nuclei (i.e. coenocytic hyphae).
• Vegetative hypha is a portion of a hypha that obtains nutrients while the portion concerned with reproduction is the reproductive or aerial hypha, which often bears reproductive spores.
• Under suitable environmental conditions, the hyphae grow to form a filamentous mass called mycelium.
• Yeasts are nonfilamentous, unicellular fungi; typically spherical or oval.
• Yeasts are widely distributed in nature like molds, frequently found as a white powdery coating on fruits and leaves.
• Budding yeasts divide unevenly (e.g. Saccharomyces spp.).
• In budding, the parent cell forms a protuberance (bud) on its outer surface. As the bud elongates, the parent cell’s nucleus divides, and one nucleus migrates into the bud. Cell wall material is then laid down between the bud and parent cell, and the bud eventually breaks away.
• Some yeasts produce buds that fail to detach themselves, forming a short chain of cells called a pseudohyphae (e.g. Candida albicans).
• Fission yeasts divide evenly to produce two new cells (e.g. Schizosaccharomyces spp.).
• Candida albicans attaches to human epithelial cells as a yeast but usually requires pseudohyphae to invade deeper tissues.
• Binary Fission of Schizosaccharomyces pombe

  During fission, the parent cell elongates, its nucleus divides, and two offspring cells are produced. Increases in the number of yeast cells on a solid medium produce a colony similar to a bacterial colony
• Some fungi, most notably the pathogenic species, exhibit dimorphism - two from of growth.
• Dimorphic fungi grow either mold or yeast.
• The moldlike forms produce vegetative and aerial hyphae, thee yeast like form reproduce by budding.
• Dimorphism in a pathogenic fungi temperature-dependent.

  * At 37 C, the fungus is yeastlike.

  * At 25 C, it is moldlike.
• Nonpathogenic dimorphic fungi changes with CO2 concentration.
• The macroconidia of Histoplasma capsulatum ae especially useful for diagnostic purposes. Microconidia bud off from hyphae and are the infectious form. At 37°C in tssues, the organism

  converts to a yeast phase composed of oval, budding yeasts.
• Filamentous fungi can reproduce asexually by fragmentation of their hyphae.
• Both sexual and asexual reproduction in fungi occurs by the formation of spores, which are useful for fungal identification.
• Fungi produce true endospore, unlike bacteria.
• Histoplasma capsulatum, a dimorphic fungus that causes histoplasmosis
• Fungal spores can be either sexual or asexual.
• Asexual spores are formed by the hyphae of one organism.
• Sexual spores result from the fusion of nuclei from two opposite mating strains of the same species of fungus.
• Asexual spores are produced by an individual fungus through mitosis and subsequent cell division wherein there is no fusion of the nuclei of cells.
• Two types of asexual spores: Conidiospore and Sporangiospore
• Conidiospore - a unicellular or multicellular spore that is not enclosed in a sac.
• Conidiospore is produced in a chain at the end of a conidiophore (e.g. Penicillium and Aspergillus).
• Conidia formed by the fragmentation of a septate hypha into single, slightly thickened cells are called arthroconidia (Coccidioides immitis).