11. 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
• Thousands of fungal diseases afflict economically important plants
• Mycology


  The study of fungi

  
• Benefits of Fungi


  • Fungi decompose dead plant material, thereby recycling vital elements
  • Nearly all plants depend on mycorrhizae – symbiotic fungi that helps plant roots absorb minerals and water from the soil
  • Fungi farming ants cultivate fungi that break down cellulose and lignin from plants
  • Humans consume fungi for food and to produce food and drugs
• Fungi


  • They are chemoheterotrophs, requiring organic compounds for energy and carbon
  • They are either aerobic or facultatively anaerobic while only a few are anaerobic
• Fungi are either aerobic or facultatively anaerobic while only a few are anaerobic
• Molds and fleshy fungi have a thallus or body consisting of long filaments of cells joined together, which are called hyphae
• Hyphae


  • Most molds have hyphae which contain cross walls called septa, 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


  A portion of a hypha that obtains nutrients
• Reproductive or aerial hypha

  The portion concerned with reproduction, which often bears reproductive spores
• Mycelium


  The filamentous mass formed by the growth of hyphae under suitable environmental conditions
• Yeasts are nonfilamentous, unicellular fungi that are 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


  They divide unevenly (e.g. Saccharomyces spp.)
• Budding in yeasts


  The parent cell forms a protuberance (bud) on its outer surface, 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
• Pseudohyphae


  Short chains of cells formed by some yeasts that fail to detach themselves (e.g. Candida albicans)
• Fission yeasts


  They divide evenly to produce two new cells (e.g. Schizosaccharomyces spp.)
• Binary fission in yeasts

  The parent cell elongates, its nucleus divides, and two offspring cells are produced
• Dimorphism


  The ability of some fungi, most notably the pathogenic species, to grow either as mold or as a yeast
• Dimorphism in pathogenic fungi is temperature-dependent - at 37°C, the fungus is yeastlike, at 25ºC, it is moldlike
• Nonpathogenic dimorphic fungi changes with CO2 concentration
• 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 reproductive spores unlike bacterial endospores
• Asexual spore formation


  Spores are formed from aerial hyphae in a number of different ways, depending on the species
• Asexual spores


  They are formed by the hyphae of one organism
• Sexual spores


  They result from the fusion of nuclei from two opposite mating strains of the same species of fungus
• Sexual reproduction in fungi


  1. Plasmogamy - a haploid nucleus of a donor cell (+) penetrates the cytoplasm of a recipient cell (-)

  2. Karyogamy - the (+) and (-) nuclei fuse to form a diploid zygote nucleus

  3. Meiosis - the diploid nucleus gives rise to haploid nuclei (sexual spores), some of which may be genetic recombinants
• Clinical identification is based on microscopic examination of asexual spores, because most fungi exhibit only asexual spores in laboratory settings
• Nutritional adaptations of fungi

  • Fungi usually grow better in an environment with a pH of about 5, which is too acidic for the growth of most common bacteria
  • Almost all molds are aerobic. Most yeasts are facultative anaerobes
  • Most fungi are more resistant to osmotic pressure than bacteria; most can therefore grow in relatively high sugar or salt concentrations
  • Fungi can grow on substances with a very low moisture content, generally too low to support the growth of bacteria
  • Fungi require somewhat less nitrogen than bacteria for an equivalent amount of growth
  • Fungi are often capable of metabolizing complex carbohydrates, such as lignin (a component of wood), that most bacteria can't use for nutrients
• Zygomycota


  Also called as conjugation fungi, members of Zygomycota are saprophytic molds that have coenocytic hyphae
• Zygospore


  A large spore enclosed in a thick wall that forms when the nuclei of two cells that are morphologically similar to each other fuse
• Microsporidia


  Unusual eukaryotes because of lack of mitochondria and microtubules, obligate intracellular parasites previously classified as protists but revealed as fungi by recent genome sequencing
• Microsporidia causes a number of human diseases, including chronic diarrhea and keratoconjunctivitis, most notably in AIDS patients
• Ascomycota


  Also called sac fungi, this group includes molds with septate hyphae and some yeasts
• Ascospore


  A spore formed when the nuclei of two cells can be either morphologically similar or dissimilar fuse, forming a saclike structure called ascus
• Basidiomycota


  Commonly known as club fungi, this group contains members with septate hyphae, includes fungi that produce mushrooms

  
• Basidiospore


  Spores formed externally on a base
• Microsporidiosis


  An emerging opportunistic infection in immunocompromised patients and the elderly