Fungi Flashcards
What are fungi
They represent a unique group of organisms which are extremely diverse with respect to their activities, shape and form
Defining characteristics of fungi (1)
Eukaryotes
Cell wall made of chitin and polysaccharides
Reproduction (sexually and asexually - nuclei are usually haploid)
Heterotrophs (obtaining energy by degrading complex organic molecules)
- contain no chlorophyll
- cannot utilise CO2 as a single carbon source
- cannot fix N2
Defining characteristics of fungi (2) - heterotrophs
Heterotrophs absorb nutrients from their environment and live as either:
- Saprophytes (derive nutrients from dead remains)
- Necrophytes (derive nutrients from organisms they have killed - parasitic)
- Biotrophs (derive nutrients from living host - parasitic)
Successful heterotrophs:
- ability to use diverse range of C sources
- provided with suitable C and N sources - impressive array of bio-synthetic powers
- occupy a diverse array of ecological niches
Differences with fungi and plants
Plants are photoautotrophs. Fungi secrete enzymes and digest organic material
Plants are made up of ‘box-like’ cells. Fungi are usually ‘filamentous’ in form
Differences in the cell wall composition
Nuclear mitosis: fungi takes place within the nucleus. Other eukaryotes, nuclear envelope breaks down
Range of fungal growth forms:
: yeast
Eg Saccharomyces cerevisiae
- single cell / single nucleus
- oval or spherical
- reproduces asexually by producing an identical but separate daughter cell
- can aggregate to form a colony or ‘pseudohyphae’
- 3-5 um diameter
Range of fungal growth forms: filamentous fungi (mold)
- vast majority of fungi
- multicellular
- Long ‘thread-like’ filaments (hyphae)
- polarised tip growth
- mycelium (colony or mass of hyphae)
- large complex bodies (interwoven mass of hyphae)
- reproduce via spores
Range of fungal growth forms: dimorphism yeast / fungi
- Environmental cues (eg temperature, CO2) trigger transition between yeast and filamentous forms
- medically important
Reproduction in fungi: asexual
(Yeasts)
- typically bud a daughter cell
Reproduction in fungi: asexual
(Filamentous fungi)
- generate light weight spores for effective dispersion
- colonise food supply rapidly
- haploid
- formed from specialised aerial extensions from mycelia
- produces in many different species-specific ways
Reproduction in fungi: sexual
- union of compatible nuclei to form diploid (2n) state
- usually occurs when nutrients are in poor supply
- sexual spores are usually more hardy structures allowing survival in adverse conditions
Reproduction in fungi: sexual
(Heterothallism)
Results in exchange of genetic material and requires two different mating types (+ , -)
(Don’t need to find opposite mating type, can still reproduce)
Reproduction in fungi: sexual (homothallism)
Some fungi are able to self-fertilise and produce sexual spores
Spores are more likely to survive in adverse conditions than if produced asexually.
(Don’t need to find opposite mating type, can still reproduce)
Three major divisions (phyla) of fungi
Zygomycota (zygomycetes) (1000 species number)
Ascomycota (Ascomycetes) (65,000 species number)
Basidomycota (basidiomycetes) (30,000 species number)
Zygomycetes
(Refer to slide 16-18 L8 for reproduction information)
Aseptate hyphae (haploid)
Smallest phyla
Asexual features: haploid sporangiospores from sporangia
Sexual features: diploid zygospore
Most are saprophytes (eg Rhizopus stolonifer)
Contains Glomus spp - important mycorrhizal fungi
Ascomycetes
(Refer to slide 19-21 L8 for reproduction information)
- Septate hyphae (haploid)
- asexual features: haploid conidospores from conidiophores
- sexual features: haploid ascospores (meiosis followed by mitosis)
- most are saprophytes
- model organisms (eg Aspergillus nidulans)
- many important plant pathogens (eg Calviceps)
Basidomycetes
(Refer to slide 22-24 L8 for reproduction information)
Septate hyphae (dikaryons)
Asexual cycle is uncommon
Sexual features: haploid basidiospores (meiosis)
Various types of fruiting body (basidiocarps) - can be a food source (eg Agaricus bisporus fruiting bodies)
Most are saprophytes (eg Serpula lacrymans)
Many important plant pathogens (eg rusts; Puccinia spp)
Deuteromycetes
Sexual reproduction cycle absent
Asexual spores formed in various ways (eg conidiophores & arthroconidia)
Fungi imperfect
Most were Ascomycetes and did include the important food spoiler and the industrial workhorse (Aspergillus flavin and Aspergillus niger)
Fungi of medical importance
Mycoses
Allergies
Mycotoxins
Fungi as phytopathogens
Primary cause of disease in plants - effects crop efficiency in plants (agricultural effects)
What are true pathogens
Healthy host
Adapted to high temperature and low oxygen tension
Restricted to specific geographical regions
Display thermal dimorphism
(Slide 6 L9)
Usually asymptomatic in healthy host
Not obligate parasites (have a saprophytic lifestyle)
Examples of true pathogens
Histoplasmosis (Histoplasma capsulatum)
Most common
500,000 cases per year in USA
Eg causes pneumonia
Another example: Coccidioidomycosis (Coccidioides immitis)
Opportunistic pathogens
Host defences need to be impaired (immunocompromised host)
Distributed world-wide
Many species and genera / increasing all the time
Show no specific adaptations to the host environment but are hardy to it (can tolerate)
Superficial / benign to chronic systemic infections
Prognosis - poor (limited drug treatment, increasing mortality rate)
Examples of opportunistic pathogens
Candida albicans infection
- widespread
- immunocompetent: non invasive
- immunocompromised: systemic causing organ failure
- most common fungal pathogen causing loss of life
- fourth biggest killer in tertiary care hospitals
- invades wounds / burns
- dimorphism yeast - hyphae form is aggressive
Another example: Aspergillosis
Dermatophytes (skin plants)
Superficial infections in healthy host (just discomfort, not disease)
World-wide distribution
Most common cause ringworm and a theme yes food
Not life threatening
Feeds off of Keratin
